Crop Fertility Findings:
Making The Most Of Fertilizer In Minnesota Soils

As commodity prices fluctuate, and input costs continue to rise, calculating the most economically advantageous fertilizer rates for typical Minnesota crop rotations becomes increasingly complex. Optimal fertilizer rates are those that not only increase the quality and yield of grains and forage, but also balance cost effectively to allow a healthy profit margin for the grower. A 3:1 return ($3 increase in revenue per $1 spent on fertilizer) is considered typical when considering low to medium soil test areas of the field, but this will vary greatly depending on beginning soil conditions and how well plants use the nutrients being applied.

With current market conditions, many farmers could be tempted to cut fertilizer usage across their operation, as it is a large percentage of the overall cost to produce a crop each year. But, it’s important to think critically about crop fertility decisions to make the most of their investment. One thing to consider is that the number of bushels of corn required to pay for a typical Minnesota 180-bushel fertilizer recommendation has not significantly changed in more than a decade (see table below).

Year: Bushels Needed To Pay For Fertilizer:
2006 22.8
2007 22.9
2008 28.2
2009 28.9
2010 24.2
2018 28.3

 

The following calculations were based on a typical Minnesota 180 bushel/acre corn recommendation:

N-P-K = 140-70-55, urea, MAP and potash. And, utilized corn and fertilizer prices at the same time each year.

Overall fertilizer efficiency, then, should be the goal of any program in order to maximize and sustain profitability over time. Research funded by the Agricultural Fertilizer Research and Education Council (AFREC) in cooperation with the University of Minnesota in recent years explores some of the variables Minnesota growers and retailers should consider in making economically sound fertilizer recommendations.

Follow the 4Rs

The International Plant Nutrition Institute (IPNI) has identified four primary strategies for optimizing fertilizer efficiency:

  • Right source– Ensure a balanced supply of essential nutrients, considering both naturally available sources and the characteristics of specific products, in plant available forms.
  • Right rate– Assess and make decisions based on soil nutrient supply and plant demand.
  • Right time– Assess and make decisions based on the dynamics of crop uptake, soil supply, nutrient loss risks, and field operation logistics.
  • Right place– Address root-soil dynamics and nutrient movement and manage spatial variability within the field to meet site-specific crop needs and limit potential losses from the field.

Obviously, fertilizer rates have the greatest impact on cost, yet overall ROI will be determined by how well each of the other strategies has been employed to:

  • Enhance quality of grain or forage
  • Increase yields (volume and percentage, year over year)
  • Increase net returns (after fertilizer cost)
  • Deliver a higher dollar return per dollar spent on fertilizer

While the true effectiveness of a fertilizer program is best judged over many years, an efficient fertilizer program should produce agronomic improvements in the first year that result in measurable economic gains.

Evaluating Economics

For corn and soybean growers in Minnesota, the most relevant agronomic research studies are those conducted close to home. Research throughout the state that examines various strategies to improve crop quality and yield with fertilizers. Many of the studies include an economic analysis or provide enough data to easily calculate the returns from the scenario being evaluated.

The following information compiled from numerous studies completed over the past10-20 years provides examples of how agronomic decisions may impact farm economics.

Impacts of Fertilizer Rates and Timing in Corn-Soybean Rotations In Minnesota
In corn-soybean rotations, most farmers apply fertilizer ahead of corn. However, in cases of high-pH soils, some producers also may apply phosphorus fertilizer before soybeans. Corn yield potentials have dramatically increased in the past 10 years, while soybean yields have remained fairly stable. This has caused some soybean growers to wonder if fertilizer should be directly applied to soybeans.

More research needs to be conducted focusing on phosphorus (P) and potassium (K) rates, as well as management and timing of applications with regard to crop yields and soil test changes. A research study initiated in 2009 focused on a corn and soybean system with multiple rates of P or K applied either before corn, soybeans or as a split application before both crops. Key findings included:

  • The addition of P and K significantly increased corn and soybean yields at selected locations.
  • Starter fertilizer applied to corn followed by broadcast P for soybeans may be a viable option for applying nutrients both years at some locations.
  • The right rate of P or K fertilizer applied within a two-year rotation appears to have a more significant impact on yield results than timing.

Corn-On-Corn Insights to Optimize N Rate and Timing While Minimizing Nitrate Loss
While most of the nitrogen management information available today is specific to a typical corn-soybean rotation, the current market dynamics could see considerable acres being converted to corn-corn-soybean (C-C-S) rotations in Minnesota in 2019. Since the effects of nitrogen rate and application timing on nitrate loss in tile drainage are not known under high-yield conditions, recent studies set out to answer (among other questions): Will recommended N rates be sufficient for maximum profitability in the long run if 200+ bushel-per-acre yields continue?

Key findings:

  • Applying a slightly reduced N rate (85% of recommended) split between pre-plant urea and V3 injected at the urea ammonium nitrate (UAN) growth stage did not perform well in first-year corn, and thus would not be recommended. However, for second-year corn, split application performed well with increased yield, N uptake, and economic return to N, in addition to less nitrate loss to tile drainage water.
  • Based on the data for the C-C-S system, split-applying 85% of recommended N rate resulted in about 10% less nitrate loss to drainage water than from the 100% pre-plant rate in this C-C-S rotation.
  • Nitrate losses from the C-C-S rotation were calculated to be at least 14% greater than from a C-S rotation.

Nutrient Removal and Balanced Crop Nutrition In Alfalfa
Soil fertility affects alfalfa yield and quality. In alfalfa, Potassium (K) plays a critical role in yield by moving sugars from shoots to roots, and then back again during regrowth; and availability is crucial for optimum stress tolerance, including winter survival. In addition, a proper mix of nutrients including nitrogen, phosphorus, sulfur and boron should be considered to maximize fertilizer impact on intensely managed alfalfa.

As with any crop, it’s imperative producers apply the proper rate in order to maximize profit. One important factor to consider with alfalfa, and other forages, is nutrient removal rates. Because the nutrients do not go back into the soil as they do with grain crops, this must be carefully managed.

The table below shows typical nutrient removal rates per ton of alfalfa for potassium, phosphorus, and sulfur.

Nutrient Removal Rate (lbs / 1-ton alfalfa)
N 51
P2O5 12
K2O 49
S 5.4

Source: International Plant Nutrition Institute (IPNI.net)

Following the standard recommended rates seems to have the most benefit for Minnesota farmers. A recent study funded by AFREC showed alfalfa yield response to potassium at several locations across Minnesota. The following table summarizes those results.

Total Forage Yield As Affected By K Fertilization
Becker Lake City Rosemount Stearns
K Rate (lbs / acre) Yield in Ton/Acre
0 5.6 5.4 5.0 8.0
60 6.6 5.7 5.3 8.0
120 7.0 5.8 5.6 7.7
240 7.4 5.9 6.5 7.9
360 7.3 6.2 5.9 8.1
K Rate Response *** *** *** NS

Insights on Primary Nutrient Interactions:
Recent research at the University of Minnesota has provided some evidence for a potential interaction between phosphorus fertilizer application and corn response to nitrogen. The goal of this project was to utilize strip trial methodology to examine crop response, and then compare how different P management strategies may interact with the use of N in different cropping rotations. Utilization of strip trials allowed researchers to analyze different soils to determine if management strategies need to be adjusted based upon factors such as drainage or P-sorption capacity, which determines the ability of soil to retain applied phosphorus.

Key findings:

  • Application of P fertilizer before corn for a two-year corn-soybean rotation significantly increased yields when soil tested “Low” to “Medium.” In areas testing “High,” there was little to no benefit from P application.
  • Net returns to P were the highest when soils tested “Low” to “Medium.”
  • There was no evidence that the optimum nitrogen rate varied between the P management strategies, and optimum N rates were near those recommended currently by the University of Minnesota.

Understanding Critical Soil Test Levels:
Finding the critical soil test level is essential to maximizing return from your fertilizer investment. That is the point where fertilizer application produces little or no measurable or profitable yield increase. Proper phosphorus (P) and potassium (K) management is essential to maintain higher yields in crop rotations, and soils in Minnesota can contain large amounts of P and K. However, only a fraction of the total amount contained in the soil is readily available for plant uptake. To assess this availability, soil tests were developed that provided an index of crop response. These soil tests are generally divided into multiple categories (Very Low to Very High) to indicate the relative potential for a yield response following application of fertilizer.

The tables below represent the anticipated response for fertilizer at corresponding soil test levels for both corn and soybeans. This quick reference provides guidance in making the most economically efficient crop fertility recommendations.

Source: AFREC and U of M “Evaluation of Critical K Levels In MN Soils” research report.

Final Word

The common theme among the research studies referenced in this article and other studies is that incremental investments in proper fertility can pay high dividends for Minnesota growers, but that proper consideration of research and consultation with an agronomist is highly recommended due to variability of the soil fertility from farm to farm and within each field. AFREC advises growers to consistently practice the 4Rs of nutrient stewardship to best manage their specific balanced fertility program.

©2018 Agricultural Fertilizer Research and Education Council of Minnesota