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

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.

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:

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.

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.

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.