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Optimal Utilization of Phosphorus, Potassium and Sulfur Fertilization in Corn-Soybean Rotations

Study author(s): Daniel Kaiser, University of Minnesota, Department of Soil, Water, and Climate
Years of study: 2011 – 2017
Location(s): Red Wing MN, Rochester MN, Becker MN, and Lamberton MN

Important: for the complete report, including all tables and figures, please download using the link(s) to the right.

2011 – 2017 Research summary points

Treatment impacts on yield summarized by location

Red Wing: Soil test data collected before trial initiation indicated that a yield response was likely from the application of potassium (K) and sulfur (S) at Red Wing, but the response to phosphorus (P) was not likely due to a high soil test. The greatest net return to fertilizer was from S at this site, where yield increases occurred for both corn and soybean plots. Data indicated that application of up to 200 lb K2O per acre resulted in a positive return on investment (ROI) but the greatest return was to 100 lbs applied every other year before the corn. Corn grain yield responded to P fertilizer in 2 of 3 years resulting in the removal based P application returning the cost of the nutrient over the six years of the study.

Rochester: Soil test P and K were Very High at the beginning of the study. Neither the corn or soybean crop responded to the application of P until soil test for the control plots dropped to near the medium classification. The ROI for P and K was not positive. However, the decrease in soil test did appear to increase the chance for a positive return. For the Rochester site, waiting until soil test P and K values dropped to medium then applying fertilizer appears to make the best economic sense. The only nutrient which consistently increase yield and profitability was S but the increase in profitability was not as large as Red Wing where the soil organic matter (SOM) concentration was less. The suggested practice for Rochester would be to apply S before corn the monitor soil test P and K and apply P and K fertilizer when it is more profitable.

Becker: Soil test P was in the Very High classification at Becker at the onset of the trial but has dropped over time to the Low category. The application of P fertilizer is alone has been profitable while both S and K have had negative ROI throughout the study. The lack of an impact to S is not surprising due to the proximity to a coal fired power plant and the relative amount of S applied annually in the irrigation water. The fact that no response to K has occurred is surprising and bears greater consideration.

Lamberton: The application of S fertilizer has not been profitable at Lamberton. Fertilizer P and K were not profitable during the first corn-soybean rotation but have resulted in a positive ROI during the final years of the study. Soil test P and K have dropped to the low and medium classes, respectively, where there is a greater chance for a response to P and K fertilizer. Similar to Rochester, letting soil tests drop into a more profitable range for fertilizer application make sense for Lamberton to maximize ROI.

  • Application of phosphorus, sulfur, and potassium increased the amount of P, S, or K taken up over the six-year rotation. Increased uptake occurred regardless of increases in yield indicating both corn and soybean have a capacity for luxury uptake of all three nutrients. The greatest likelihood of luxury uptake (uptake in excess of crop needs’) came from potassium. The amount of P, K or S taken up typically only accounted for 10-40% of the fertilizer applied.
  • The uptake of other nutrients was increased when yield was impacted by P, K, or S except for Mg uptake which decreased with increasing rate of K, B uptake was less when S was applied, and Zn uptake was less when S or P were applied.
  • Soil samples collected in the Fall of each year showed a capacity for all medium to fine textured soils to carry over sulfate-sulfur within two feet of the soil surface for one or more years following application. The amount of S relative to C in corn stover did increase with the application of sulfur. However, there was no evidence that there was enough sulfur in corn stover to result in mineralization of sulfur for the following soybean crop. It appears that the primary mechanism of sulfur cycling is a direct carryover of sulfate-sulfur from one year to the next.
  • Application of P and K fertilizer increased soil test P and K mainly in the surface 6 inches. There was some capacity for movement of P and K in a loamy sand soil and limited capacity in medium to fine textured soils particularly for K which moved readily through a sandy soil and enriched soil K to a depth of at least 24 inches.
  • Drying of soil samples resulted in fixation of soil test K for medium textured soils and release of K for fine textured soils. Sandy soils were relatively unaffected by soil drying. The amount of K fixed or released increased as more K fertilizer was applied. The primary concern from all sites came at Lamberton where it was more likely that the level of K in the soil was overestimated using air dried soil tests which would result in under-application of K.
  • Application of sulfur fertilizer showed inconsistent effects on soybean grain quality (protein and oil concentration) and the percentage of the total protein that was cysteine and methionine (sulfur containing proteins). Application of potassium fertilizer tended to decrease protein and increase the oil concentration of soybean grain and also decreased the percentage of the protein made up of cysteine and methionine.
  • Multiple nutrient deficiencies can be detected with active sensors. When deficient, differences in application rates of phosphorus, potassium, and sulfur could be detected using a Crop Circle 430 within corn research plots while differences were detected in soybean plots where phosphorus and potassium were applied. The data suggests that care must be taken when interpreting active sensor data when it is unknown what nutrient deficiencies may be within a given field.


Two-year corn-soybean rotations are an important part of Minnesota agriculture. In these rotations fertilizers such as phosphorus and potassium have been crucial in increasing productivity in soils that were historically low in these nutrients. Fertilizer is applied once before corn for both crops within the rotation to achieve maximum profit for the rotation. In many cases research has shown that this type of fertilizer program can work under most circumstances; however, high fertilizer prices and stagnant soybean yields have caused some producers to question this strategy and to look at annual fertilization. The potential for other nutrient deficiencies, previously unseen, could also occur with the change in management.

To achieve optimum yield multiple nutrients may have to be applied. Plant nutrients perform specific and required functions, which are critical for growth and development. While nutrients may be important for specific functions they are not mutually exclusive in terms of uptake and utilization in the plant. A brief review of the literature found little published data on interactions between macronutrients on crop growth, nutrient uptake and yield. In Wisconsin, researchers studied the effects of P, K, and S on alfalfa nitrogen fixation and found that nodule numbers were increased with P and S fertilization on sandy soils while the addition of K increased N fixation on both a sandy and silt loam soil. Numerous studies have shown a benefit for P and K fertilization on crop growth and yield, and some on S, but in many cases all other nutrients are applied at non-limiting rates ignoring the fact that in field conditions interactions may exist between nutrients and that more than one may be limiting in a particular situation. It has been shown that small rates of P banded near the seed can increase the growth and nutrient uptake of crops such as corn. However, an interaction between these and other nutrients has seldom been discussed. Work completed in 2006 by Randall and Vetsch in south central Minnesota (unpublished data) studied the interactive effects of N, P, K, and S in starter fertilizer for corn grown on high testing P and K soils. While they did find a yield increase to an NPKS mixture in high testing P and K soils, yield increases were generally attributed to S in the mixture, any residual effects on the following year’s crop were not investigated.

A research project near Lamberton has examined intensively managed corn-soybean rotations that included manure and additional P and S fertilizers (Potter, unpublished data). Many factors differed between the management systems, although significant fertility differences included manure, fertilizer P and S, and side-dressed N for the intensive management system as compared to management according to the current Minnesota guidelines. In this study all fertilizer sources were applied before corn, and the soybeans relied on residual nutrients remaining in the soil. Interesting results from this work include: a consistent response to sulfur in corn, a large yield in soybean when sulfur was applied before corn, and consistently higher yields with the intensively managed rotations. One main difference between the two management strategies was an apparent decline in soil K levels in the conventional management treatment, likely a result of no K being applied beyond a small amount of starter K in the first year of the study. This difference may have caused some of the effect on yield even though K levels were still borderline high they were significantly higher when manure was applied. However, other variables differed in those treatments so the full effect may not be attributed to K alone. In the case of the plots receiving sulfur there were direct comparisons with and without sulfur in a subset of the treatments. The response in corn was not surprising since other responses to sulfur have been seen in Minnesota, but the response in beans was unexpected. The big question from this research is what, if any, role nutrient management is playing in the yield differences between these two management systems.

A two year study focusing on combinations of N, P, and S was conducted at four corn and four soybean locations comparing crop response to sulfur and interactive effects of N, P, and S applied in different combinations. At two locations corn responded to sulfur application and yield increases were large, around 12-19 bu/ac. Unless another nutrient was limiting corn, response was correlated with sulfur uptake and sulfur content in the grain. When sulfur was applied the next most limiting nutrient was generally P. In comparison the soybean data did show a clear response of 2-3 bu/ac from sulfur application at one location in 2009 and a similar response to a combination of N, P, and S at another in 2008; however, when compared to sulfur concentrations in the plant or grain there was no clear relationship between yield and sulfur uptake measured at either time. In 2009, when K uptake relative to N and P was compared, yields increased linearly as K uptake increased. In general, applied sulfur increased K uptake which could be related to increased plant growth. This was especially apparent with the treatments receiving N, P, and S which consistently had greater growth at V5 at all locations and at R6 at a few locations. It should be noted that soil test K level at the 2009 location was medium to low thus a response would be likely. However, yield and K uptake appeared to be linearly related and yield never appeared to be maximized with the blanket K rate used (which according to current recommendations should have been enough to maximize yields). Early plant growth data from this study indicated that growth of soybeans could easily be enhanced early in the season by applications of N, P, and S, as was previously stated, and this increase could still be seen late in the season with taller plants at two locations. Overall the effect of sulfur increased the amount of K in the plant, especially when combined with other nutrients. At this time we cannot compare the data from 2008 to responses from 2009. There was one additional site in southeastern Minnesota that showed a small yield increase from the NPS treatment combination in 2008, but there was still much variability in the K content. One difference at this location was that it tested higher in K initially and the response in grain uptake was slightly different. However, we can theorize that the small yield increase may be due to a similar difference in uptake of K in-spite of high soil test levels at that location. Thus, further study should be undertaken to determine the effect of P and S on the uptake and utilization of residual soil K in soybean.

While the yield response data for soybeans to S is limited the link between P, K, and S nutrition in the corn-soybean rotation is an intriguing question since it is an important rotation in the Corn Belt and most research conducted focuses on a single year of crop response or a single nutrient.

The two current projects in Minnesota have raised some important questions 1) K nutrition may be an important factor to consider for soybean in the 2nd year of the rotation, and 2) managing for high yields in corn and sulfur fertilization may also be affecting the second year crop more than was previously thought. The data from 2008 and 2009 indicate a large benefit from sulfur application to corn. Therefore it’s logical to study the effects of biennial applications of nutrients (P, K and S) for corn on the growth and yield of the second year soybean crop. Many producers are managing high yield corn rotations with biennial inputs of fertilizer including P, K, and S. The research question is what benefit is there for the corn and if soybean yields are higher the second year. The two unpublished trials from 2009 looking at soybean sulfur response are intriguing since it provides evidence that sulfur may be directly benefitting soybeans either directly or through indirect effects on corn response. However, this needs to be studied in order to determine if individual nutrients work better when applied in unison in corn-soybean rotations.

This proposal is for a project to examine the interaction of P, K, and S application for two-year corn-soybean rotations. The specific objectives are to:

  1. Examine the effects of K rates applied before corn with and without P or S, on grain and stover quality and yield
  2. Determine if K nutrition differs when either P or S is applied to corn.
  3. Examine interactions between P, K, and S when applied in combination by measuring the uptake of macro- and micronutrients and their relationship to corn grain yield.
  4. Determine the impact of previous nutrient applications and varying concentrations of K in soils on soybean growth, nutrient uptake, yield, and grain quality following one cropping year of corn.
  5. Examine impacts of fertilization on sub-soil P and K le vels over long-term corn-soybean rotations
  6. Study how sulfur cycles within a corn-soybean rotation
  7. Evaluate the effect of multiple nutrient deficiencies on remote sensing data such as multi-spectral cameras or ground based active sensors (crop circle)
  8. Study the impact of P, K, and S fertilization on soil test K measured on air dried and field moist samples


Four trials were conducted on fields testing medium to high (80-150 ppm) in soil K and which had not had a recent history of sulfur or manure (Table 1). Research trials are arranged in a split plot design. Main plots consisted of a factorial combination of P and S fertilizer. The P rate was a single P application based on crop removal for an average crop in a two-year corn-soybean rotation [120 lbs P2O5 per acre applied as triple superphospate (0-46-0)]. The sulfur rate was 25 lbs of S per acre applied as ammonium sulfate (21-0-0-24). Each main plot was split into four sub-plots consisting of potassium rates. Multiple rates of potassium are based on rates above and below expected crop removal (0, 100, 200, and 300 lbs K2O/ac) for a two-year corn-soybean rotation. The K fertilizer source was potassium chloride (0-0-60). All fertilizer was applied before corn with the intention of measuring yield and crop response for soybean the following year applying no additional treatments. Nitrogen was applied at a non-limiting rate to all plots (150 lbs of N per acre) as a combination of urea (46-0-0), ammonium nitrate (34-0-0) or ammonium sulfate (only for treatments receiving S). Nitrogen was applied prior to planting except for at Becker where it was split 50% applied at planting and 50% applied between V4-V5.

Due to excessive rainfall early in the growing season, an additional 50 lbs of N per acre were side-dressed at Red Wing and Rochester in 2013. Supplemental irrigation was supplied at the Becker and the total amount of water and P, K, and S applied in the irrigation water are summarized in Table 2. Initial soil samples were collected for determining P, K, pH, and soil organic matter levels from the top six inches and 6-12” and 12-24” for which were analyzed for K and S only. Additional 0-6” samples were taken in the fall and analyzed for P, K and S. Soil samples were collected from the 6-12” and 12-24” depths from each main plot following the first corn year and from each individual plot the fall following the first soybean crop and every fall thereafter to assess movement of P and K and carryover of S for multiple cropping years.

Corn and soybean stover content and quality and yield will be the primary factors studied. Stover samples consisted of the whole above ground portion of 6 plants without the ear taken at R6 from all of the plots in both trials. Samples were cut off at ground level, dried, ground, and analyzed for P, K, and S concentration. Plant and grain P and K were determined by ICP following digestion in HNO3 and H2O2. Total S content was determined by combustion. For soybean locations, 6’ sections of row will be sampled from each plot at R6 (prior to leaf drop), the total biomass will be dried, ground, and then analyzed for nutrient content. Stover uptake in beans will be the difference between the whole plant samples taken at R6 and the nutrient uptake in grain. Soybean grain was run through NIR to determine protein, oil, and amino acid concentration. Corn grain will be sampled and analyzed for P, K, and S concentration and nutrient uptake. Data will be statistically analyzed to study the main effects of P and S application and K rates. Interactions between the main effects will be studied to determine if the effect of any of the elements change based on when the others are applied.


Initial Soil Test Values

Table 1 summarizes soil test data collected in the spring before the initial treatment application. Soil test P levels were in the high classification at three locations which indicates a very small likelihood that a yield response to P would occur, and were medium at one (Lamberton). Sulfur tested higher at Rochester and Lamberton but the relative levels could not be assessed to whether they are high or low in terms of crop availability since no critical soil test levels have been established for sulfur on medium or fine textured soils. Organic matter levels were in the range where responses to sulfur have occurred except for the Lamberton location. Current research has found that sulfur responses are highly likely when organic matter is around 2.0-3.0% in the top six inches. At Red Wing, Becker, and Lamberton K tested in the Medium classification while Rochester was Very High. While a response is not impossible at the Rochester site it is highly unlikely. The standard deviation of the soil test K values indicated more variability in soil K within the plot area at Rochester and less at the three other locations. Both sulfur and potassium were measured on soil samples deeper than 6 inches. Sulfur was consistent with depth and similar at all locations. Soil test P decreased within the soil profile except for at Red Wing. Soil test K was also similar except for the 12-24” fraction at Red Wing which was higher than the top six inch depth. Soil cation exchange capacity was measured for each sampling depth and increased with depth at Red Wing, was similar across depths at Rochester, and decreased with depth at Becker and Lamberton. Higher CEC values would indicate a greater capacity for the soil to hold cations such as K+. Since phosphate (H2PO4-) and sulfate (SO4-) are anions there is no impact of CEC on the soil’s ability to hold either phosphate or sulfate.

Grain Grain Yield

Corn grain harvest data is summarized in Appendix Tables 3a through 3b for the main treatment effects of potassium rate, sulfur, and phosphorus during the first rotational cycle. All three variables affected yield at the Red Wing location in 2011 (Table 3a). Plots receiving sulfur, on average, yielded 7 bu/ac more than when no sulfur was applied. Yield response to phosphorus was greater at 12 bu/ac. This increase was surprising since the soil test level was in the Very High category and the probability that crops will respond to P is generally about 1% of the time at that soil test level. Yield responded to the lowest K rate, 100 lbs K2O, at this site. There was no corn grain yield increase at the Rochester site in 2011. The only significant effect was an interaction between phosphorus and sulfur at this site. This effect was due to slightly higher yields when sulfur was applied without phosphorus (data not shown). The lack of significance for any other interactions indicate that when an effect was significant there did not appear to be any additive effects of any of the nutrients on each other. This indicates that yield will be affected by a deficiency when present and that either potassium, sulfur, or phosphorus will not substitute for one and another if deficient.

Phosphorus increased yield 8 bu/ac at Becker (Table 3a). In contrast to Red Wing, potassium decreased yield at Becker with each increase in K rate. The yield decrease was not expected due to the low soil test K values at this location. It is possible that the high rates of K may have interacted with nitrogen (N). Past research has shown a negative interaction between K and N. Further study of plant tissue data may yield some information on the relationship between N and K at this site. There was no effect of S on corn yield at Becker. Sulfur is typically applied to similar irrigated fields with sandy surface soils. One reason for the lack of response could be due to sulfate applied through the irrigation water. In 2012, approximately 12.4 lbs of sulfate sulfur were applied through irrigation water (Table 1a). The only effect at Lamberton was a 7 bu/ac yield increase to S. While we did not expect an increase in yield form S at this site the previous crop was corn prior to the establishment year which may have increased the likelihood of a yield response.

Soybean grain yield was not affected by any treatment at Red Wing 2012 (Table 3a) in spite of yield responses during the previous corn year. However, average yield levels were limited by moisture stress which may have limited the potential for yield differences to occur. Average yield levels were similar at Rochester. However, there was a significant interaction between P and S at this site with yields being higher for the S treatment applied without P. This was the same effect seen in the corn year. It is curious as to why yield was not increased when S was applied with P. Yield increases and increased profit from fertilizer application before corn support P, K, and S application. However, there does not appear to be much carryover benefit to soybean.

Data from 2013 at Becker and Lamberton indicated no potential increase in soybean grain yield from P, K, or S. The effects that were closest to the accepted significance level were S at Becker, which would have resulted in a yield decrease if significant, and P at Lamberton (slight increase in yield if significant). Soybean grain yield as a whole at Becker were lower than expected. However, parts of the field were affected by disease and the plants died off early in the growing season limiting yield to between 25 and 30 bu/ac while other areas yielded 50-55 bu/ac. The increase in incidence of disease could be a direct result of increases vegetative growth at this location (which will be discussed later in this report). Even though P, K, and S were medium to low at Becker fertilizer did not appear to be cost effective to apply to this location, which was not expected. However, 8.9 lbs of sulfate-S were applied through the irrigation water in 2013 (Table 1). The amount of P or K applied in the irrigation water was negligible both years of the study at Becker.

A second corn-soybean rotational cycle was initiated at the Red Wing and Rochester locations in 2013. Identical rates of fertilizer were applied to plots in 2013 that were applied in 2011. The effect on corn grain yield is summarized in Table 3b. Corn grain yield averages were lower in 2013 due to dry weather conditions during the growing season. Sulfur and K produced greater grain yield at both Red Wing and Rochester in 2013. This is not surprising at Red Wing since similar effects occurred in 2011. However, the response to S and K was not found in 2011 at Rochester. A potential response to S at Rochester is not surprising due to the cool climatic conditions early in the growing season. A response to K would be less likely due to the higher soil test at Rochester. At both locations, the 200 lb K2O rate produced maximum grain yield. The previous response at Red Wing was to the 100 lb rate. Additional K2O may have been required due to a potential decrease in soil supply from under-application for the first two year rotation.

There were two interactions that occurred at Red Wing. The first interaction was between phosphorus and sulfur. This interaction was a result of a response to P only when no S was applied. Grain yield averaged 161 bu per acre when no P or S was applied, was increased to 177 bu/ac when P was applied without S, and was increased to 190 and 192 bu/ac when S was applied without- and with P at Red Wing, respectively. Since this effect has not been previously seen it is unknown whether the effect was real or a potential effect due to random variability within the field. Further analysis will be conducted to follow up on potential interactions. There was an additional interaction between P and K. However, the effect could not be clearly defined.

Soybean yield for the second rotation at Red Wing and Rochester was significantly affected by sulfur at both locations and by K only at Red Wing (Table 3b). The impact of sulfur was large at Red Wing averaging slightly over 5 bushels per acre and slightly smaller at Rochester (nearly 2 bushes per acre). Since no sulfur was applied before the soybean crop the increase in soybean grain yield was a direct result of sulfur applied to the corn crop indicating that sulfur is carried over from one year to the next. Direct carryover in the soil could be impacted by heavy rainfall similar to what occurred in spring of 2014. However, if the yield response to sulfur was a result of sulfur carried over in the soil it appears that the movement of sulfur may not be rapid. The impact of K on soybean grain yield was expected at Red Wing due to the low K soil test values at this site. Soybean yield was increased up to 200 lb K2O applied before the corn. This rate also resulted in the maximum corn yield the previous year in the corn crop.

A second rotation was initiated at Becker and Lamberton in 2014. Corn yield was significantly affected by P at both locations and by K only at Lamberton (Table 3b). The lack of a response to K at Becker is surprising due to the low initial K test values before the trial was initiated in 2012. The exact trend for corn grain yield was a decrease in yield when K was applied. This effect was significant back in 2012 but was not significant in 2014. The response to P at both Becker and Lamberton was expected due to the difference between P soil tests for plots with and without fertilizer applied. The K soil test has dropped considerably at Lamberton where the control plots with no K test low enough to be in the responsive range.

Soybean data was collected in 2015 at Becker and Lamberton to complete the second corn- soybean rotation. Soybean grain yield response to P at Becker and Lamberton (Table 3b). The increase was 4.5 bu/ac at Becker and 5.2 bu/ac at Lamberton. There was a large difference in soil test P between plots where P fertilizer was and was not applied. The control treatment where no P was applied was in the Low classification at both sites. Both sites had no response to S which there was a corn grain yield increase from K in 2014 but no response to K in 2015 for soybean. There still has not been a response to K applied at Becker in spite of low soil test values. The amount of K applied in the irrigation water is very low and cannot be considered a factor contributing to the lack of response to K (Table 2).

A third corn soybean rotation was established at Red Wing and Rochester in 2015 and treatments were reapplied to the same plots that were applied to in 2011 and 2013. Corn grain yield was influenced by the application of P, S, and K at Red Wing and Rochester (Table 3c). The greatest increase in yield occurred with either P or S application at Red Wing where corn grain yield differed by over 30 bu/ac where P or S was applied. The difference in yield was 10 bu/ac at Rochester. There was a significant interaction between P and S at both sites where there was increase grain yield with P or S only were applied but when both were applied together the increase in yield was not additive (i.e. if P and S both increase yield by 20 bu/ac the addition of both P and S did not increase yield by 40 bu/ac). The addition of P and S together at Rochester and Red Wing did increase yield but not as much as was increase by each nutrient alone. What was interesting was the severity of the S deficiency at Red Wing early in 2015. Striping

indicative of S deficiency was present in the plots where only P was applied but the severity of the striping was less severe than where no P or S was applied. This indicates that additional S was taken up in the P only treatment. It is possible that there was some S in the P fertilizer source as an impurity. At time we do not know how much as the P fertilizer source has not been analyzed for S content. It is also possible that increase plant growth early in the growing season where P was applied may have also increased root growth. Increased root growth would allow for the plant to explore a greater volume of soil and take up more S.

Corn grain yield also differed at both locations where K was applied. Yield was increased only by the first rate of applied K (100 lb K2O/ac). There were no positive or negative impacts of rates of K application in excess of 100 lb K2O. There also were no significant interactions between P or S with K. This indicates that the corn plants response to K is separate from a response to P or S.

Field sites were continued for one last year at Red Wing and Rochester in 2016 to measure soybean yield. Soybean grain yield differed in plot which received S at both locations (Table 3c). Yield differed by 2-3 bushels in plots with sulfur versus those without. Similar effects occurred for the soybean crop in the fourth year of the study. Overall, the data indicates S is an important nutrient to maintain crop yield in a two-year corn-soybean rotation. Potassium application before corn only impacted soybean yield at Rochester even though the effect of K was significant for the previous soybean crop in year 4. Phosphorus did not impact soybean yield at either location which was not surprising since soil P concentration tested high at both locations.

A third cycle was initiated by planting corn at Becker and Lamberton. Yield data for corn at these two locations in 2016 is summarized in Table 3c. Plots treated with phosphorus had greater corn grain yield at both locations. In both cases the difference in yield was relatively high at nearly 40 bu/ac at Becker and 22 bu/ac at Lamberton. Plots with K had greater grain yield only at Lamberton but only to the 100 lb K2O rate. Becker soil K concentration for the control plots was low to very low but there was not response to K. Sulfur applied to corn did not lead to greater grain yield at Becker or Lamberton. Sulfur should have been deficient at Becker but yield responses to S have not occurred during the previous study years. Sulfur application through the irrigation water was less than in previous years at near 5 lbs of S per acre. However, the decrease in SO4-S did not lead to a deficiency at the site.

Soybean yield from Becker and Lamberton from 2017 is summarized in Table 3c. Becker was significantly impacted by white mold which greatly reduced yield potential and yield differences due to P. Plots where P were applied had more vegetative growth which saw a greater impact of the white mold which resulted in an inability to detect differences between plots with and without P. The response to P was still close to significance so it could be assumed P would still have had an impact on soybean yield due to the low soil test P values for the no P plots. Both K and S did not impact soybean grain yield at Becker. At Lamberton, P and K both increased grain yield. The K effect though was only to the lowest application rate (100 lbs K2O). Both P and K soil test values were either low or medium-low where no P or K was applied so a response to either nutrient was expected. There was no clear indication of interactions between nutrients other than a P x K interaction at Lamberton which could not be readily explained by a clear enhancement of yield due to P or K when the other was applied.

Net return to fertilizer applied for the over a two and four-year rotation were calculated and are given in the table above. Sulfur fertilizer typically gave the most consistent return across sites with positive responses over four years at Red Wing and Rochester and over two years at Lamberton. Sulfur will not result in a positive four year return at Lamberton due to a lack of response to S across the rotation. Had Lamberton been soybean prior to the 2012 corn it is likely that sulfur would not have increase yield and over the four years would have resulted in a net loss with sulfur. The positive impact for the first year was due to the first corn crop grown following a previous corn crop. The high return to S at Red Wing and Rochester was due to higher soybean yields in the plots where sulfur was applied in 2013 and 2015.

Increased profitability to P and K has typically followed the sites with soil test that started or have decreased to the Low classification. The only exception has been Becker which did show a positive benefit to P but has not shown a positive benefit to K or S in-spite of low soil test K and low soil organic matter concentrations. Since there was a yield decrease to K during the first corn year the response to K has been strongly negative at Becker. More information is needed as to why the decrease occurred but it appears that the suggested rates for K will not result in increased profitability at Becker. We have purposely decided to maintain the rotations over more rotational cycles to determine if the negative effect of K will hold at Becker of if the site will begin to better respond to K after years of crop removal from the soil. Overall, the data collected for P and K does support using soil test to determine where the greatest chance of a profitable return would occur.

Corn Silage Yield

Corn silage yield was calculated based on the whole plant and grain samples collected at R6 (Table 4a). While none of the hybrids used are intended solely for corn silage, having the plant samples allows us to make some generalizations of effects on silage yield. Silage yield was not affected by any treatment at Rochester or Lamberton. The Lamberton location had low yield potential due to dry weather conditions. Potassium affected silage yield at Red Wing and Becker. Silage yield was increased by 1.3 tons per acre from the middle and higher K rates. Silage yield was reduced in the same manner as grain yield at Becker by an average of 2.0 tons per acre. Sulfur also affected silage yield at Red Wing where yields were 0.9 tons per acre higher with sulfur than without. Phosphorus did not affect silage yield. However, the P value at Red Wing was close to the accepted level were silage yields trended higher with phosphorus. There was a two-way interaction between phosphorous and sulfur at Rochester, but this was again due to higher silage yields when sulfur was applied without phosphorus.

Silage yield was calculated for the second year locations (Red Wing and Rochester, 2013) and is summarized in Table 4b. Effects were similar to those that occurred at each location for corn grain yield. This indicates that the main effect of the fertilizer treatments on silage yield is a direct result of increases in grain yield and not in stover. Stover mass was examined but did not show any clear increases due to P, K or S so it is not listed. The only effect that was not significant was a K response that occurred for grain yield at Rochester but the same effect did not occur for corn silage yield. The increase due to sulfur was greater in 2013 at 3.0 t/ac at Red Wing and 1.6 t/ac at Rochester. At Red Wing, the 200 lb K2O rate increased silage yields by 1-2 t/ac, but the effect was again due to an increase in grain yield.

Silage yield was increase by P in 2014 at both Becker and Lamberton (Table 4b). The average increase was 1.5 tons per acre at both locations. Applied K only affected silage yield at Lamberton. At Lamberton there was no difference in yield among the K fertilizer rates (all increased yield over the control). The resulting increase due to K at Lamberton was around the 1.5 ton per acre range similar to P. Since there was no interaction between P and K at Lamberton the resulting increase in silage yield would be additive to a total of 3.0 tons per acre on the plots were P and K has been applied.

The impacts of increased corn grain yield on corn silage yield was still apparent in 2015 with P, K, and S increasing silage tonnage at Red Wing and Rochester (Table 4c). Sulfur had the greatest impact on silage yield at both locations increasing silage yield by nearly 5 tons per acre at Red Wing and 2 tons per acre at Rochester. At Becker and Lamberton in 2016, P increased silage yield at both locations while K increased silage yield and Lamberton and S did not increase silage yield at either location.

Increases in silage yield have occurred when corn grain yield has been increased. In fact, the rate of P, K or S that has increased grain yield has also increased silage yield across sites. The figure shown above contains data from Red Wing in 2013 where K and S both affected yield and the breakdown of the total silage weight from grain, cobs, and the stalk (stover). If grain was impacted by the application of one or more nutrients there typically was a resulting impact on stover. The key point is that there was never an impact on stover from fertilizer application in excess of effects on grain yield. In addition, the percent increase in silage was greater with resulting increases in grain yield rather than stover. Cob weight was never impacted by fertilizer treatment. The data shown is consistent with data from the other locations. While the hybrids used were not necessarily considered to be silage hybrids, the effect should be similar among various corn hybrids.

Corn Grain Moisture at Harvest

Corn grain harvest moisture was lowered by phosphorus application at Red Wing and Lamberton (Table 5a). Moisture was nearly 1% less with phosphorus than without at Red Wing and 0.4% less at Lamberton. This is similar to starter fertilizer effects with seed placed phosphorus. A potential starter effect from the high rate of P applied could explain the response to P at Red Wing. As was stated previously, the yield response to P was not expected at this location. However, the planting date and growing conditions may have been favorable for a starter response to P at that site. At Rochester and Lamberton, grain harvest moisture was affected by K application rate. In this case the higher the K rate the higher the harvest moisture. It is not clear why this result occurred however K fertilizer has been reported to help plants in droughty conditions due to the involvement of K with water relations in the plant. It could be that the excess K caused more water to be retained for longer possibly delaying dry down in the fall. The difference though was small at only 0.1 to 0.2% with the high K rates versus no K at Rochester and 0.6 to 0.7% at Lamberton. In effect this response would be minor and would not have significant economic consequences for a corn grower.

Corn grain harvest moisture for the second rotational cycle data at Red Wing, Rochester, Becker, and Lamberton are summarized in Table 5b. There was no effect due to K at Red Wing and Rochester. At Becker and Lamberton application of K resulted in greater moisture at harvest. The increase in moisture due to K at two of the sites may be due to the year more than any differences among the sites themselves. Application of S had the greatest effect on grain harvest moisture at both locations in 2013 (Red Wing and Rochester) but not in 2014 (Becker and Lamberton). The difference in response due to S was likely a factor of neither corn site responding to S fertilizer in 2014. Application of sulfur decreased grain moisture by 2.7 % and Red Wing and 3.6% at Rochester. This decrease could be a direct effect of a delay in maturity due to a sulfur deficiency. Phosphorus also decreased grain moisture by 4.0% at Red Wing and by 1.0% at Lamberton. There were no significant interactions that occurred at any location in 2013 or 2014.

For the third corn crop at each location, potassium application resulted in a small increase in moisture in the harvested grain at Red Wing and Rochester (Table 5c). Application of S decreased grain moisture by 2% at Rochester but did not affect grain moisture at Red Wing. Application of P resulted in a 1% decrease in grain moisture at Red Wing and no difference at Rochester. There was a significant interaction of P and S at Red Wing and Rochester in 2015. The interaction was a result of a decrease in grain moisture when P or S was applied but no additional decrease when the two nutrients were applied together. There we no significant interactions of P or S with K. Overall, the data indicates that high rates of K and low available P and S are resulting in delayed maturity of corn that affects the moisture content in the grain at harvest.

At Becker and Lamberton in 2016, plots where P was applied had less grain moisture at harvest (Table 5c). Yield was increased by P at both locations indicating greater grain moisture without P was due to a deficiency of P. Plots were K were applied at Lamberton had increased grain moisture similar to effects at other locations in previous years. Plots with S showed a small decrease in grain moisture at Lamberton even though grain yield was not affected. Decreases in grain harvest moisture were not factored into economic data previously presented. It is unlikely that differences in grain moisture at harvest would change what nutrients were economically justified for application at either location.

Nutrient uptake in stover and removal in grain

Nutrient uptake in corn and soybean stover was examined as a potential pool of recyclable nutrients to the next years’ soybean crop. Nutrients removed in grain can be considered a loss since those nutrients are exported off the field with the crop. Both factors need to be considered when studying nutrient cycling within rotations. In this study phosphorus, sulfur, and potassium uptake were examined and the uptake over two growing seasons (for both corn and soybean) are summarized in separate tables for the Red Wing and Rochester locations and the Becker and Lamberton locations. Only data where a two-year rotation has been completed will be discussed. When the second and third corn-soybean rotations were established at each site fertilizer treatments were re-applied. Therefore, effects on P, S, and K uptake would reflect any fertilizer not used during the previous crop rotation(s) and any fertilizer applied for the current crop rotation. Data from a partial rotation are included in tables in the appendix.

Phosphorus application increased total uptake of P (as P2O5) at three of the four locations during the first rotational cycle (Table 6a and b). The smallest increase occurred at Lamberton where P uptake was increased by 7.9 lb/ac followed by Red Wing (16.8 lb/ac), and Becker (18.9 lb/ac). These increases accounted for 7, 14, and 16% of the total P2O5 applied at Lamberton, Red Wing, and Becker, respectively. The P rate applied was chosen to represent the rate of P2O5 removal in a two year rotation. The total removal in grain accounted for an average of 72% of the total applied. Total uptake of P met or exceeded the total amount of P applied. Stover rarely was shown to exhibit luxury uptake of P. The lone exception was corn and soybean stover at Becker and soybean stover at Lamberton. Since stover was returned any P taken up would be added back to the soil and potentially recycled to the following crop. Luxury uptake and increased removal of P when there is no resulting yield increases would represent lost potential productivity. Since the rate applied exceeded removal an increase in soil P would be expected.

Field trials were re-established at Red Wing and Rochester in 2013 and Becker and Lamberton in 2014. Phosphorus uptake and removal in grain is summarized in Table 6c and 6d. Total P uptake over the 2-year rotation was increased by P application at both locations. Total P2O5 utilization was around 14 lbs greater with P fertilizer applied at Red Wing and only 9 lb greater at Rochester. This total represents about 10% of the P applied before the corn crop. Since neither corn nor soybean showed any yield increase the P taken up would be considered to be luxury uptake. Potassium did slightly increase total P uptake at Rochester but not at Red Wing. The increased uptake of P was a result of increased P in the harvested grain due to yield response to P at Rochester. It is interesting that K did not increase P uptake at Red Wing as both the corn and soybean crop responded to K application. There was a slight trend for greater uptake of P with K fertilizer applied at Red Wing but variation among plots and treatments resulted in an inability to detect small differences in uptake.

Data for the third rotation at Red Wing and Rochester are summarized in Table 6e. At Red Wing uptake of P was 18 lb P2O5 greater accounting for 15% of the P applied. At Rochester, uptake of P was slightly less at 15.8 lb P2O5 greater accounting for 13% of the P applied. P uptake was greater with K and less with S a Rochester but was not affected by K or S at Red Wing. Data from the third rotation at Becker and Lamberton are summarized in Table 6f. The application of P only affected the uptake and removal of P in the rotation. Due to grain yield responses to P at both Becker and Lamberton, there was a large difference in total uptake with and without P averaging 60.5 and 42.4 lbs P2O5, respectively. This uptake represents roughly one-third to half of the P applied in fertilizer was used by the crop during the rotation. The application of K or S did not impact the uptake and removal of P.

Over the six years at Red Wing, 45.8 greater lb P2O5 was taken up for plots with P applied. At Rochester, uptake was only 24.8 lb P greater. The overall efficiency over the three rotations was 13% at Red Wing and 7% at Rochester. Since P was not generally required at Rochester it is not likely that P would have a great impact. Soil test P concentration was very high at Red Wing but corn responded to P in two of the three years at Red Wing which increased the efficiency of P.

Removal of P was greater at Becker and Lamberton when P was applied over the six years. Net efficiency of P fertilizer increased from 16% to 50% at Becker and 7% to 33% at Lamberton due to decreases in soil test P values. Over the life of the study average annual P efficiency was 38% at Becker and 20% at Lamberton. The lower efficiency at Lamberton is mostly due to a higher soil test P at this site and lower grain yield. Irrigation at Becker would give a higher potential yield and a potential to remove more P annually and thus, a greater potential to decrease soil P which has occurred at the site. Application of sulfur did not affect the removal of P at either site as yield was not impacted by sulfur fertilization. Potassium impacted the uptake of P at Becker over the second corn-soybean rotation due to increases in P uptake in 2014 when corn was grown.

Sulfur application data was re-run for the 2011 through 2103 growing season. We currently are comparing methods for analysis of sulfur in plant tissue and have noticed differences and significantly lower values for sulfur values analyzed by ICP (which is a routine procedure for most labs). Figure 1 summarizes data from one of the locations in 2012 showing a poor relationship between the ICP measurements at the dry combustion method. Measurements taken with the ICP appear to reach a maximum but the dry combustion method continues to increase. The dry combustion method appears to return values that are in line with previous research (not shown). At this time all of the samples are being re-run for S by dry combustion to compare to the other results. The ICP analysis is included in the package for running phosphorus and potassium therefore no additional cost is being incurred for this analysis. However, this data is important in that it will give a better understanding of the current critical values and if they need to be re-established using ICP.

Total S content in stover and grain was analyzed by dry combustion. Data have been analyzed through the 2015 growing season and are summarized in Tables 7a through 7e. Tables 7a and 7b include data from the first rotation at the four locations. The effect of sulfur on uptake of sulfur in stover and grain was inconsistent across the four locations during the first rotation. The only site which exhibited any difference in the two year uptake was Lamberton where there was an average difference in uptake of 3 lbs of S per acre when sulfur was applied. There was never a difference in uptake of S in stover or grain at Becker which indicates that supply of S was not an issue for either crop. Sulfur fertilizer influenced the uptake of sulfur for corn and Red Wing and for Soybean at Rochester during the first rotation. Total uptake of sulfur was greatest at Red Wing and Becker at nearly 34 lbs of S taken up over the two-year rotation, followed by Rochester at 33 lbs, then Lamberton at 29 lbs. Sulfur in the corn grain accounted for the majority of the S taken up (over 25%) over the two years.

Data from rotation 2 at all locations is listed in Tables 7c and 7d. A second rotation was completed at Red Wing and Rochester in 2014. Total uptake of sulfur differed when sulfur was applied at both sites. At Red Wing there the uptake of S was increased by 9.2 lbs of S (37% of the S applied) while the differences was less at 5.7 lbs at Rochester (23% of the S applied). The only other element that affected total uptake of S was K, but only at Red Wing. The effect of K was likely a result of greater removal of S in the grain due to higher grain yield with K application for corn and soybean at Red Wing. In the case of S, fertilizer S increased both stover and grain uptake of S in corn and soybean. There was no increase in the uptake of S when P, S, or K were applied at Becker and Lamberton considering the total uptake of S over the 2014 and 2015 growing seasons. There was a small trend for greater S uptake over the two years at Lamberton but the difference failed to achieve statistical significance due to within plot variability at the 0.05 probability level, but the effect was significant at the 0.10 probability level. The difference in uptake of S where S was and was not applied was 19% of the total S applied before corn in 2014.

Sulfur uptake data for the third rotation are summarized in Tables 7e and 7f. Sulfur uptake was increased when S was applied at Red Wing and Rochester where yield was affected by S, and at Lamberton were yield was not. Sulfur uptake was greater when P was applied at Red Wing, Becker, and Lamberton due to increased plant mass and yield affecting S uptake. Increased yield due to the application of K did not affect S uptake over the two-year rotation.

Total potassium uptake was increased at Red Wing and Rochester in 2011 (Table 8a) and Becker and Lamberton in 2012 (Table 8b). Increased uptake of K2O was due to greater uptake of K in the plant stover. Grain removal was only affected at Becker of the four first year rotational sites. When accounting for K2O taken up in both the soybean and corn stover, the percentage of total K taken up in the plant by the stover was near 70% at Red Wing, Rochester, and Becker. The relative levels taken up were lower at Lamberton which was only 51%. The less percentage of K in the stover relative to total uptake at Lamberton could be a result of drier conditions at this site limiting the uptake of K. The fact that grain uptake of K was only increased at Becker could be a result of the lower soil test values relative to the other locations. However, grain K2O removal was decreased by increasing rate of K2O as a result of the negative effects on corn grain yield. There was no effect of K application on the removal of K2O in soybean grain at Becker. In spite of the less uptake of K2O in the corn grain, total uptake of K2O increased with increasing rate of K. Greater uptake of K in the stover was expected. Since K is not a component of organic molecules, and K+ in the tissue can be readily leached and made available for the following years’ crop. Recent work in Iowa has shown that at most 60% of the K can be returned from corn stover by June of the following year depending on the amount of rainfall between the time of corn harvest and the following year. In dry years the K can be tied up in the residue and not returned. If part of what was taken up is expected to supply the soybean there could be issues with K deficiency the second year due to excess uptake by corn. This dynamic is very important when it comes to cycling and is one of the factors we want to keep exploring in this study.

Table 8c and 8d summarizes corn grain and stover K uptake from the second rotational cycle at Red Wing and Rochester in 2013-2014 and Becker and Lamberton in 2014-2015. Unlike the previous cycle, K application increased both K uptake in the stover and K removal in the corn grain which had a impact on total uptake of K2O. For the soybean crop, K application only increased the removal of K in the grain at Red Wing and uptake of K in the stover at Rochester. Uptake of K followed a linear uptake pattern at Red Wing and Rochester. Sulfur fertilizer application did impact the total uptake of K at Red Wing. The result was due to increased grain yield for the corn and soybean crop during the second two-year crop rotation.

At Becker, the total uptake by corn and soybean in 2014 and 2015 increased in plots were P or K was applied. At Lamberton, there was greater removal of K in the grain due to increased corn and soybean grain yield but there was no impact of P on increased uptake of K in the corn and soybean stover. The total uptake of K was much higher at Becker than at Lamberton. The difference could be due to limitations in K uptake due to drier soils at Lamberton which is rain- fed versus Becker which is irrigated. However, soil K is less at Becker in the top six inches than at Lamberton. The uptake of K was nearly linear with K rate applied at Becker and was linear at Lamberton. Corn grain yield did response to K application at Lamberton to the lowest rate of K applied. Since there was no increase in grain yield at Becker to applied K and no further increase at Lamberton beyond the 100 lb K2O rate, the increase in K uptake over the two years demonstrates the capacity for luxury uptake of K in both corn and soybean plants. This uptake likely can delay maturity which has been evident by increased harvest moisture concentration in corn grain multiple years of this study.

Potassium application affected K uptake at Red Wing and Rochester during the third rotation (Table 8e) and Lamberton and Becker (Table 8f). Increase in K uptake was linear at both locations. Yield effect due to S resulted in greater K uptake due to increased removal of S in the grain only at Red Wing. Phosphorus did impact total K uptake at Becker and Lamberton but the impact was largely a result of greater K removal through increased crop yield due to P and not an enhancement of K efficiency by P.

Figure 2 summarizes the relationship between total K uptake and K applied over the two-year rotations. The slope of the relationship accounts for the relative efficiency of the fertilizer applied. Potassium uptake efficiency was greatest in 2015-2016 where 44% of the K was taken up at Red Wing and 42% at Rochester. The next greatest efficiency was found at Becker (2016- 2017 where 30% of of the K applied was incorporated into the corn and soybean crop over the third crop rotation, Lamberton (2014-2015) where 29% of the K was taken up, Red Wing in both 2011-2012 and 2013-2014 28% of the applied K was taken up. The efficiency was less at the four other locations averaging 24% at Lamberton (2016-2017), 22% at Becker (2014-2015), 20% at Rochester (2013-2014), 16% at Rochester (2011-2012), 13% and Becker (2012-2013), and 11% at Lamberton (2012-2013).

The difference in overall efficiency is unknown as it is not related to soil test K as the control plots where no K applied has tested the lowest at Becker. Fertilizer efficiency could be related to environmental conditions at a site or to the responsiveness of the site to a particular nutrient. The 2015 and 2015 growing seasons had greater rainfall throughout the growing season which may have led to greater uptake of K throughout the season. Uptake of K was the greatest for the third rotation at Red Wing and Rochester compared to other sites. A sites responsiveness to K may be an explanation for enhanced uptake of K as the site at Red Wing has been the only location that has consistently responded to K fertilizer during the study and the Lamberton site during year 3 and 4 when soil test K declined in to the Low category had differences in corn yield in 2014. At Becker, the combination of irrigation and K weathering from parent materials may be supplying K to the crop. One area of future research is to determine whether the critical K concentration of soils that are high in sand may be less than medium and fine textured soils.

Soybean Trifoliate Samples

Soybean trifoliate samples were collected to evaluate the availability of P, K, and S applied before corn. Data from Red Wing and Rochester are summarized in Table 9a while Becker and Lamberton are summarized in Table 9b. Phosphorus affected trifoliate P concentration only at Becker. Sulfur data are only available for the Red Wing and Rochester locations. There was no impact of sulfur on the concentration of S in trifoliate samples at most locations. At Lamberton, S concentration was slightly higher for the plots where S was applied before corn. I all cases S concentration in the trifoliate samples were above the sufficiency level (0.21% S) Potassium concentration was increased at all locations. Current guidelines indicate minimum sufficiency of K in trifoliate samples to be 1.71%. Trifoliate K concentration was above this level for all K rates at Rochester and Becker, for K rates 100 lbs K2O per acre and above at Red Wing, and all K concentrations were lower than the sufficiency level at Lamberton. Since soybean grain yield was not increased the differences in tissue P, K, and S concentration were not likely resulting in a direct loss of soybean grain yield. The ratio of N:S and N:K were also examined. The N:S ratio did was changed at Lamberton and the N:K ratio differed due to the variation in K concentration in the trifoliate samples across sites.

For the 2014 soybean sites, trifoliate P concentration was increased when fertilizer P was applied at Red Wing and when K was applied at Rochester (Table 9c). The impact that K had on P concentration was odd at Rochester as the 100 lb K2O rate showed higher P concentration than the control or the higher K rates. It is likely that there was an outlier in the data that is affected the results and that K had little impact on the concentration of P in the trifoliate leaves. Trifoliate K concentration was increased with the application of K before corn at Red Wing and Rochester. The increases were consistent with increases in grain yield that were found at each location. Trifoliate S concentration was increased by the application of S before corn only at Red Wing. Both sites showed some grain yield response when S was applied prior to corn but the increase was greater at Red Wing. The concentration of S was not particularly low at either location which questions the utility of established critical levels for S in soybean trifoliate samples.

Trifoliate P and K concentration were greater in plots that received P for soybean grown in Becker in 2015 (Table 9d) but the concentration of S was less when P was applied. Soybean grain yield was limited by P at Becker such that an increase in S concentration may have been due to S that was taken up was not being efficiently utilized by the plant. Trifoliate K was increased by previous applications of K at Becker while S did not affect trifoliate nutrient concentration. At Lamberton in 2015, trifoliate P concentration was greater in plots where P was applied while K was increased only in plots were fertilizer K was previously applied. The ratio of N:K in the trifoliate leaves decreased with increasing K rate at both sites. The increase in K in the trifoliate samples was due to luxury uptake of K in the plant as soybean grain yield did not responds to previous application of K at Becker or Lamberton. Neither location responded to S which supports the lack of difference in trifoliate S concentration at either site. Since soybean grain yield at both sites responded to P a reduction in the trifoliate P concentration was expected.

Data from 2016 at Red Wing and Rochester were fairly consistent with previous years. As has occurred at other sites, the application of K before corn had typically led to increased K concentration in the soybean trifoliate samples collected at R2 (Table 9e). Past application of P led to greater P in the trifoliate samples at both locations but did not result in great grain yield. Sulfur application before corn did lead to increase trifoliate S concentration at Rochester but not at Red Wing. Trifoliate S concentration was slightly decreased when P was applied at Red Wing. The ration or N:S was not affected by fertilization at either locations but the N:K ratio was impacted by K. As K application increased the N:K ratio decreases as a result of the increasing K concentration. Since K did not affect yield the difference in trifoliate K concentration among the K rates and the effect on the N:K ratio had no bearing on soybean grain yield.

Data from 2017 is summarized in Table 9f. Trifoliate P and K were impacted when either nutrient was applied at both Becker and Lamberton in 2017. Trifoliate S concentration was greater at Lamberton when S was applied but was not impacted by S at Becker. The lack of response to S is still a likely result of S applied in the irrigation water (Table 2) as the overall concentration of S was greater at Becker than at Lamberton for the 0 S plots. Trifoliate N:S and N:K ratios were affected by P only at Becker. The N:K ratio was affected by K at both location as a result of enhanced K concentration with increasing rate of K applied.

Effects on Fall Soil Test Values

Fall soil samples were taken from the 0-6” depth and analyzed for P, K and S to study carryover of soil nutrients not used by the crop. Soil K data is summarized for the ammonium acetate test run on air dried samples, which is standard procedure for most labs, and the field moist/slurry (referred to as the moist test hereafter) test for K. Each graph for each location shows the data for the air dried and field moist ammonium acetate K test. Data collected from the Red Wing and Rochester locations in fall 2011 is given in Figure 3a. Figure 3b summarizes data collected from four locations in fall 2012, 3c in 2013, 3d in 2014, 3e in 2015, 3f in 2016, and 3g in 2017. The field moist test tended to result in higher soil test K values for fields following corn when K was applied. When no K was applied the tests tended to result in similar soil test values. The exception was as Rochester where the moist test was always higher than the air dry. This indicates that the fixation of K upon drying of soil is far greater with higher rates of applied K and that the moist test may be more sensitive to application of K than the air dried.

The Moist K was consistently higher than the air dried samples across years at Red Wing and Rochester although the differences were marginal where 0 K was applied at Red Wing in Fall 2016. At Becker, there typically was a slight difference with the moist tests being higher than the dry. The exception was in Fall 2014 where there was little difference between the two test methods at Becker when 0 or 100 lb K2O was applied. The only deviation from the moist test being higher than the air dried test was at Lamberton for the Fall 2013, 2014, 2015, 2016, and 2017 sampling. In Fall 2013 the moist K analysis extracted less soil K for the 0 and 100 lb K2O rates while the high rate (200 and 300 lb K2O) showed little difference in soil K. There was a large response in yield to K at Lamberton in 2014. For samples collected in 2015, 2016, and 2017 the moist K test was less than the dry for only the plots where no K was applied. The air dried test indicated soils to be still in the Medium soil test category at Lamberton in the control plots (no K applied). This would indicate a slight chance for a response to K. The moist K test was about 20-30 ppm lower which would indicate a stronger response to K.

It is interesting that the moist K test is resulting in higher soil test K values when other research has indicated it may result in lower values. Situations such as found at Lamberton in Fall 2013, 2014, 2015, 2016, and 2017 are of interest as they represent a potential for K availability in the soil to be over-estimated. It is interesting that we are not seeing the same effect at Red Wing in- spite of lower soil K than Lamberton. The difference is likely in the mineralogy of the soils at the two locations with Lamberton having more clays and being more poorly drained. While the moist test for K has not been fully calibrated in Minnesota, even if it is higher it may better.

Soil Test P, K, and S by depth

Samples were collected at different soil depths to determine the movement of P, K, and S following application and tillage. Some samples from previous years are being run for P and K. The major focus of the soil test data collected by depth was to track sulfate-S movement. Since P or K are not mobile we would not expect a buildup of these nutrients much below the 6” or 12” depth depending on tillage. Bray soil test P data are summarized in Figure 4a and 4b. Application of P increased soil test at all locations. The increase in soil P only occurred in the top 6” of the soil profile at Red Wing, Rochester, and Lamberton. In addition to increases in the 0-6” sampling depth, there was a slight increase in soil P for the 6-12” depth at Becker according to the Fall 2014, 2016,and 2017 sampling, but the difference was not significant for the Fall 2015 sampling. There also was a difference in soil P at 6-12” in Fall 2016 at Lamberton Tillage was similar across sites but the Becker site is sandy and well drained. Any ortho-phosphate in the soil water may have a greater chance to move at Becker relative to the other location. Overall, the data indicates that any unused P would not increase subsoil P tests and there was no evidence of a depletion of soil test P when none was applied. There also has not been evidence of mining of the subsoil P when no P was applied. We would not expect any significant mining to occur at sites that test medium to high in P until soil tests in the top six inches drop into the Low category (5-10 ppm Bray-P1 P)

Soil test S is summarized in Figure 5a and 5b. The only location where no consistent increase in sulfate-S was at Becker which is not surprising due to the coarse nature of the soils and potential for leaching losses. However, there were elevated differences in sulfate-S for all depths when S was applied at Becker according to samples collected Fall 2016. Less irrigation was applied during 2016 however rainfall was greater thus the potential for leaching should have been greater leading to more leaching potential for the sulfate fertilizer. In all other cases there was a measurable increase in sulfate-S concentration within the deeper soil depths over years with repeated S application. Increased sulfate-S concentration could be detected at Red Wing, Rochester, and Lamberton, in most cases, after two crops were grown. Since no S fertilizer was applied directly in-front of the soybean crop, any sulfate-S remaining would be from the application before the corn. This indicates that while sulfate-S in vulnerable to leaching the rate of leaching is not that rapid. Because of the relatively low movement, carryover of sulfate-S applied in fertilizer can be a source of S cycling within rotations rather than S mineralized from corn stover as the C:S ratio of the corn stover was well above the 200:1 threshold where, below this level, S would mineralize from stover (Table 10). The soil data indicates that a direct application of S prior to soybean is not required in order to maximize soybean grain yield. In addition, carryover of S from high application rates may, over time, limit potential corn response to S due to a buildup of sulfate-S in the profile over time.

There was as significant potential for leaching of S from Fall 2013 through June of 2014, Fall 2015 through Spring 2016, and Fall 2016 through Spring 2017. In spite of the leaching potential we could still detect sulfur in the soil profile at Red Wing and Lamberton. Sulfate at Lamberton in the soil profile would have been left over from what was applied to the corn crop in Spring 2014. At Red Wing some of the sulfur that was applied before the 2013 and 2015 corn crop could still be detected at depths deeper than 6 inches following the proceeding soybean crop. Data showed a general enrichment of the top 2 feet in sulfate-S at Lamberton. Becker and Rochester did not exhibit any S from previous applications in the top 2 feet after the soybean year.

Early in the rotation there was little movement of K below the top six inches in the soil profile (Figure 6a and 6b) at Red Wing. There was evidence of a slight increase in soil test K at the 6- 12” depths at Rochester for samples collected in Fall 2015 and 2016 and Lamberton for samples collected in Fall 2014, 2016, and 2017. The greatest potential for movement of K at depth was at Becker where differences have been seen in the 12-24” depths starting in Fall 2014. Movement of K is not unexpected in sandy soils with low CEC since the topsoil would have less capacity to hold any of the K applied. The highest rate of K applied has doubled the soil test at depth at Becker. Even with the increase there has been no impact of K on corn or soybean grain yield to date. More research needs to be conducted on the mineralogy of the soils at Becker to better understand why no yield response has occurred in spite of soil test K values which have dropped to 40 ppm in the control plots. Moist K values were slightly higher but not high enough to explain the lack of an effect of K application on corn and soybean yield at Becker.

Effects on Soybean Grain Protein, Oil, and Amino Acid Concentration

Analysis of grain quality was not initially planned for soybean in this study. However, the combinations of nutrients give a unique dataset that may provide some information on the historically low protein values seen in Minnesota. Grain quality was not considered for corn since it is not an important factor to determine the commodity price. Data in Table 11a summarizes the main treatment effects on soybean grain quality at the two locations studied in 2012. Greater differences were seen at Red Wing compared to Rochester. The majority of the effects were due to the application of sulfur which increased total protein, cysteine, and methionine concentration and decreased oil. Cysteine and methionine are important as they are two amino acids that contain sulfur. We previously have seen increased protein concentration as a direct effect of sulfur application to soybean, but were not able to determine if the amino acids were increasing as well. While we may not be able to account for the entire amount of S taken up in grain being directly incorporated into cysteine and methionine, this at least gives some idea of where the S is going that is being taken up. The decrease in oil was expected since protein and oil should be inversely related. However, the decrease in oil at Rochester without an increase in protein was an interesting response which we cannot explain at this time.

Phosphorus application did not affect grain protein concentration at either 2012 location. However, the concentration of cysteine and methionine was increased by the application of P and a significant interaction of P and S was likely a result of the two amino acids having higher concentrations when P and S were applied together than when either was applied alone. The only other response to P was in grain oil which was slightly higher with P at Rochester. There were no clear interactions with P and any other nutrient other than the interaction with S seen at Red Wing. Protein levels trended lower with increasing potassium rate at both locations but the main effect of K failed to reach significance at the accepted level. Therefore, we cannot claim there to be a significant effect of K on grain protein at either site. Grain oil was significantly increased by K at Rochester and cysteine concentration was lowered by K at Red Wing. The evidence shows that K is likely not having a positive effect on grain protein concentration. Some of the significant interactions between phosphorus, sulfur, and potassium seen at the locations probably are the result of the positive effects of phosphorus and sulfur and negative effects of K. This has not been studied at this time.

Sulfur and potassium had relatively less effects on grain quality measurements at the two soybean locations conducted in 2013 (Table 11b). Phosphorus did increase grain protein concentration at Becker but had little to no effect on any of the other variables. The only site where there was some effect was at Lamberton where sulfur increased corn yield the previous year (2012). The lack of effects, particularly from S, at Becker may not be surprising considering S applications made through the irrigation water. It appears that even though S has traditionally been suggested for soils similar to those at Becker the need may be overstated. This is confirmed by other research conducted from 2011 through 2012.

There was a greater impact of sulfur on grain quality measurements for the Red Wing and Rochester locations in 2014 (Table 11c). Sulfur increased protein concentration and the % of the total protein as cysteine and methionine. Sulfur decreased the oil concentration of the grain at both sites which is not surprising since protein and oil typically are inversely related. Potassium only affected protein concentration at Red Wing but the effect was not constant with a response to increasing or decreasing K rate. The only other effect was an increase in the percent of the total protein as methionine at Rochester as K rate increased. Phosphorus decreased protein and the concentration of methionine and increased oil at Rochester in 2014.

In 2015, protein was decreased where potassium was applied at Becker and Lamberton (Table 11d). Protein was also less at Becker in plots where phosphorus was applied. The decrease in protein concentration with phosphorus at Becker was likely a dilution of protein in the grain due to increased grain yield. Soybean oil concentration was less when sulfur was previously applied at Becker and Lamberton while protein was greater when sulfur was applied only at Lamberton. Cysteine and Methionine content decreased with increasing rate of potassium at Becker but were increased when sulfur was applied at Lamberton. Cysteine and methionine composed a greater fraction of the total protein at Becker further supporting the lack of a benefit of sulfur at the Becker location. The continued use of sulfur at Lamberton may have some benefits for soybean grain quality but the over impact at this time is low.

In 2016, grain protein was greater in plots with S or with P at Red Wing and Rochester and protein was decreased by K at Red Wing only (Table 11e). Similar to other years, when protein concentration increased oil concentration decreased or when protein decreased oil concentration increased. This inverse relationship between protein and oil is common for soybean production. The makeup of the protein was changed with the application of S where both cysteine and methionine were increase when S was applied, decreased when K was applied and were increased when P was applied at Red Wing. We are in the process of evaluating the S content of our P fertilizer sources. While there has been no direct evidence of a consistent uptake of K with the 0-46-0 used in this study there are smaller effects that do indicate the potential for S being applied in the P source. Sulfur contamination of P fertilizer is possible due to sulfuric acid being used to dissolve rock phosphate in the manufacture process. Most new high analysis P fertilizer have reduced the amount of S contamination in the product but S concentration in the material needs to be addressed.

Phosphorus increased protein concentration at Becker in 2017 and decreased oil concentration at Becker and Lamberton (Table 11f). Sulfur did not affect protein or oil at Becker but decreased oil at Lamberton. Cysteine increased at Becker when S and P were applied but methionine was decreased by P was applied only at Becker. Typically, K had had an impact on protein, oil, and cysteine and methionine content in past year but that was not the case for the 2017 growing season. Some of the lack of impact at Becker could have been the decreased yield due to high incidence of white mold. A similar yield decrease due to white mold occurred in 2013 at Becker and similar effects were found for protein and oil concentration.

Grain protein concentration was converted into pounds of protein produced per acre by multiplying the protein concentration by grain yield. The total protein produced was compared with the harvest index which is calculated at the percentage of grain produced (on a weight basis) compared to the total biomass produced (grain plus stover). High harvest index values would represent more grain being produced per unit of total biomass produced. The relationships are given in Figure 7a and 7b. The analysis of grain protein concentration was not found to differ at the 2012 Rochester location or the Becker 2015 location and there was no relationship between the harvest index value and the total protein produced. This was in contrast to Red Wing 2012 where there was a significant relationship between the two values. The total amount of protein produced increased as the harvest index increased at Red Wing 2012, but decreased at Becker and Lamberton in 2013, Red Wing and Rochester in 2014, Lamberton in 2015, Red Wing and Rochester in 2016, and Becker and Lamberton in 2017.

The results at Red Wing 2012 indicate that the total amount of protein produced increased as the relative percentage of grain versus the total biomass increased. Since yield was not affected at this site the change in harvest index was mainly due to changes in the amount of stover produced. In fact, the amount of stover was related to the total protein and this resulted in a negative relationship between the two (as one increased the other decreased). Since this supports the harvest index data the relationship with plant stover was not shown. While we could not directly measure differences in stover due to any of the main treatments at Red Wing 2012, there did appear to be some differences at this site. If the differences were being caused by effects specific to treatments applied the previous year it would indicate that some care should be taken when applying nutrients prior to soybean. If we are trying to push for higher yields with high fertilizer applications there may be some negative impacts on soybeans that may not be considered, especially on grain quality components.

Impact of phosphorus, sulfur, and potassium on active sensor indices

Crop canopies at all locations were sensed in 2015 using a Crop Circle 430 sensor that collects reflectance values in the red, red-edge, and near infrared areas of the light spectrum. The data generated is used to calculate two crop indices, the normalized difference vegetative index (NDVI) which is a measure of the amount of biomass relative to bare soil, and the normalized difference red-edge (NDRE) which is more sensitive to differences in chlorophyll content (greenness) of the plant. Sensing data are summarized in Table 12a for 2015 corn and soybean plots. Sensor data was collected between V6-V8 for corn and near R1-R2 at soybean sites in 2015.

At Red Wing, the sensors detected differences in rates applied for all of the nutrients using NDVI but the NDRE only detected differences for sulfur or phosphorus. The pattern of significant for NDVI matched the patter for corn yield. However, we have not looked to correlate the NDVI values with corn grain yield at Red Wing. At Rochester, there were differences in NDVI and NDRE only for the plots where phosphorus was applied. For soybean, the only difference was at Becker where there was a difference in NDVI and NDRE for plots that have previously received phosphorus and a slight increase in NDVI from the rates of K applied to corn. What the data indicates is that the indices are not sensitive to specific nutrient deficiencies. Ground truthing to identify specific issues is still needed to ensure that decisions made with data collected with active sensors are accurate to correct the problem.

For 2016, soybean NDVI was affected by K and S application at Red Wing and P application at Rochester (Table 12b). The NDRE index on soybean was impacted by K at Red Wing and Rochester and S at Red Wing. At the corn locations in Becker and Lamberton, NDVI and NDRE were both impacted by P but not by K or S. There was no relationship between corn and soybean grain yield and the NDVI or NDRE values at any corn or soybean location (Figure 8b).

For 2017, NDVI and NDRE values were greater when P was applied at Becker but the NDRE values decreased with increasing rate of K at Lamberton (Table 12c). Fertilizer S did not impact either value at either location. There was no clear relationship between soybean yield and sensor values collected at Becker or Lamberton (Figure 8c).

Uptake of other macro- and micronutrients

In additional to P, K and S uptake, the ICP analysis provided concentrations which were used to calculate the uptake of other macro- and micronutrients. The exception was calcium (Ca) as the concentration of Ca in corn grain was below the detection limit of the ICP thus the data are not summarized. Data were summarized across the six growing seasons and was not summarized for each year and rotation. Data for magnesium (Mg), boron (B), copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn) are summarized in Tables 13 through 18, respectively. Removal of nutrients tended to follow where increases of yield were found. Uptake in stover and removal by grain is listed in the tables but this discussion will primarily focus on total uptake in stover and grain rather than the two individually.

Treatments did not consistently impact Cu, Fe, and Mn uptake. There were some increases due to P and K depending on site and whether yield was impacted. The uptake of Mg tended to follow an inverse relationship to the uptake of K. This relationship was expected and is commonly seen as excessive uptake of K can reduce the uptake of Mg. Application of P did result in greater uptake of Mg which was again due to sites where plants responded to P. The main effect of P could clearly be seen with differences in vegetative biomass production. Plants grown on low P soil tests were smaller affecting the uptake of most nutrients. The total uptake of most micronutrients was still relatively small. The greatest total uptake was for Fe and the least was B.

The surprising effects were found when examining the data for B and Zn uptake. The uptake of B was negatively affected by the application of S at Red Wing and Rochester and K at Becker. Zinc uptake was negatively impacted by S and Red Wing and Rochester and by P at Becker and Lamberton. Past research has shown negative relationships between P and Zn. What is interesting is less B or Zn uptake with S. There have been increasingly wide-spread reports of low tissue B and Zn concentration in corn across Minnesota in the last five years. Over the last ten years, sales of S fertilizers have increased by over three fold. If there is a negative relationship between S and B or Zn could the increasing application of S be resulting in lower concentration of these nutrients. There has been no direct evidence that more B or Zn is needed for corn production based on current research. It begs the question whether if there is a negative relationship then does the lower value for B or Zn really matter? We have not fully examined relationships between S, B, and Zn for plant tissue surveys, but the data for this study indicates that a further look is warranted to determine if specific tissue samples collected for diagnostic purposes show similar effects.


When deficient, the application of P, K or S increased corn yield. It is unlikely that the application of P or S will affect the responsiveness of corn or soybean to K. The only consistent interaction between nutrients was between P and S. Interactions between P and S are likely a result of S impurities in P fertilizer supplying some of the S needs for corn. The removal of all macronutrients are most likely affected by P, K, or S enhancing grain yield. The application of S may reduce the uptake of B or Zn over a two-year rotation. The application of K will reduce the uptake of Mg.

Soybean grain yield was enhanced when P, K, or S was applied ahead of corn. Yield effects were more consistent for the corn crop but when soil P or K test is low soybean will likely respond even if fertilizer is applied ahead of corn. Soybean yield was affected by S applied ahead of corn at two locations. Soil test P, K, or sulfate-S were all increased with the application of the three nutrients.

Soil test P and K will increase in the top six inches when P or K fertilizer is applied. Increased P or K soil test at deeper depths depends on tillage and soil texture. If enrichment occurs it is more likely to happen for the 6-12” depth except for sandy soils where soil test levels at deeper depths may be affected by broadcast incorporated fertilizer application. Potassium is mobile in sandy soils. Soil test sulfate can be affected by S fertilizer application within the top two feet and may be found one or two years following application. Rainfall will push sulfate-S deeper in the profile but it still may be possible to detect fertilizer applied one or two years after application.

Cycling of sulfate-S from one year to the next is likely a results of residual suflate-S in the soil not used by the crop and not fully leached from the crops root zone. It is unlikely that S will cycle from corn residue as the C:S ratio is enough where immobilization is more likely.

Common remote sensing techniques which measure NDVI or NDRE indices will not be able to differentiate between multiple nutrient deficiencies. Deficiencies of P and S will likely result in reduced plant growth which affects biomass measurements like NDVI. Deficient K may also impact biomass and also may impact the index values. Visual scouting will still be required to determine what deficiencies may occur in a field.

Soil test potassium was impacted by the application of K fertilizer. The impact of drying on soil test K varies based on soil properties. For loams and silt loam soils in the SE part of Minnesota drying soil samples results in soil K test values which will underestimate the amount of available K while the opposite will be more likely in loam and clay loam soils in central and southwestern Minnesota. There was little difference between K tests for air dry and field moist sandy soils. Need for the moist K test will vary and should be assessed on a site by site basis. Soil test K can be affected by the application of P or S as a result of less uptake of K due to deficiencies of P or S which results in a faster buildup of soil K.


The authors would like to thank the Minnesota Agricultural Fertilizer Research and Education Council for the support of this project. We would also like to thank our cooperators for their current and future support on the project along with the crop consultants which also were instrumental in helping locate and establish the trials. We also would like to thank the field crew from the Department of Soil, Water, and Climate for their technical support on the research project.

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