Prices for soil fertility inputs have increased greatly in recent months. The bi-weekly “Iowa Production Cost Report” showed a 51% cost increase in anhydrous, a 31% increase for urea, 56% higher cost for MAP (11-52-0), and an increase of 38% for Potash (0-0-60), from the average prices reported last September 22 compared to the March 9, 2021 report. Many who did not book their fertilizer last fall are questioning how to prioritize cash flow needs with these increased costs. Here are some thoughts.
Soil test to know what P, K, and lime applications are really needed, or utilize your most recent soil tests to aid in these decisions. See the ISU Extension publication PM-287, Take a Good Sample to Help Make Good Decisions, for soil sampling suggestions.
Evaluate soil test results to determine P, K, and lime requirements. See ISU Extension publication PM 1688, A General Guide for Crop Nutrient and Limestone Recommendations in Iowa. As stated in that publication, the percentage of P and K applications expected on average to produce a yield response within each soil test category is 80 percent for Very Low, 65 percent for Low, 25 percent for Optimum, 5 percent for High, and less than 1 percent for Very High. This means that as soil test levels increase, the probability of a positive yield response to fertilization and the amount of an expected yield increase decreases. Additionally, net return decreases and usually becomes negative at High and Very High soil test levels. Avoid applications to fields or field areas that do not need P and K or lime.
Phosphorus and Potassium Applications
For making fertilization decisions, P and K application priority should be to areas of fields where the chance of yield increase is large, and the expected yield increase is sufficient to at least pay for the applied nutrient. You could decide to only apply P and K to soils testing Low and Very Low, with optional application when tests are in the Optimum category, since the probability of an economical yield increase for soils in the Optimum category is lower.
It is economically unnecessary to apply P and K rates at amounts to rapidly raise low soil test levels to the Optimum category. The suggested rates in ISU Extension publication PM 1688, A General Guide for Crop Nutrient and Limestone Recommendations in Iowa, maximizes yield in most conditions, not just economic yield.
Due to crop removal, withholding fertilizer or manure applications will result in a gradual soil-test decline. Therefore, if soil tests are in the High and Very High categories, the P and K “banked” in the soil can be used for next year’s crop production and no application is needed. On average for corn-soybean rotations, soil-test P by Bray-1 or Mehhlich-3 tests decrease about 2 ppm per year and soil-test K by the ammonium-acetate of Mehlich-3 tests decrease 4 to 5 ppm per year. However, there is very high annual variability in how these soil test levels change.
Application to maintain soil-test values in the Optimum category is considered a good practice to sustain profitable crop production over time. However, applications can be withheld until the next year especially when product supply is short, funds are needed for other more critical inputs, or land tenure is uncertain. This is because the expectation for economical response to P and K in the year of application is small in the Optimum category and it becomes more uncertain as the price ratio becomes unfavorable.
Therefore, withholding applications may work in some fields or field areas, with the number of skipped years depending on the beginning soil test level, but will not work in other fields or field areas. Soil testing is the only way to know.
Another option instead of not applying any P or K when the soil test is in the Optimum category would be to apply partial crop removal. This would slow the soil test decline and should provide adequate fertilization for the small and occasional first-year yield response. A common starter P and K rate is enough to avoid yield loss in this category but will not will not maintain soil-test values, especially in high yield environments.
For the corn-soybean rotation, many producers apply P and K needs for both crops once, before corn. This is as effective as applying those nutrients ahead of each crop if the fertilizer need for both crops is correct. However, if fertilizer price/availability will be better next fall, money could be saved now by applying the nutrient need of one crop and fertilizing again next year.
Nitrogen applications should be tailored for the crop rotation. First-year corn following well established alfalfa often needs no N fertilization, and when required only needs 30-40 lb. N/acre. Unfortunately, corn in other rotations almost always needs N application, and yield increase to fertilization on the long-term is quite good. There are not many opportunities to eliminate application when N prices are high or in short supply.
Application rates can be adjusted downward somewhat when N fertilizer costs are high relative to corn prices. However, closely observe both N and corn prices before deciding on reducing N applications. Despite the high N prices, corn prices are also high and therefore the ratio between the two has not changed dramatically.
The Corn Nitrogen Rate Calculator uses many sites and years of data to help you determine the “odds chart” for response to nitrogen, and you can choose the fertilizer source and the crop or fertilizer prices. The first screen asks you if want information graphed for a single price scenario or for various at the same time. In the next screen, choose between corn following soybean or corn following corn from the “Select Rotation” menu (since the rates differ greatly), select Iowa from the “Select State” menu, and you will see that a new “Select Region” menu shows-up so you can choose between southeast Iowa (SEIA) or “Main” for the rest of Iowa. Iowa was divided in these two regions because recent research showed that higher N rates are needed for corn in southeast Iowa. These results are developed from more than 300 field trials that evaluated at least four N rates. This data set is used to calculate the most likely optimum N rate for specific price scenarios, which is referred to as the MRTN (maximum return to N). Additionally, the N Rate Calculator will show a range of application rates that would be profitable within plus or minus a dollar of the optimum N rate. With high N costs, high production costs, and perhaps the need to allocate limited funds for N fertilizer purchase, one can consider using rates in the lower part of the range. Those rates should provide similar yields, but risk of N supply shortage to the crop is greater if N losses occur or if there is greater corn N need. More detailed information about N management, including use of the late spring soil nitrate test, can be found in extension publications CROP 3073 Nitrogen Use in Iowa Corn Production and CROP 3140 Use of the Late-Spring Soil Nitrate Test in Iowa Corn Production.
Information for portions of this this article come from ISU ICM News articles Making Fertilization Decisions as Fertilizer Prices Escalate and Production Costs Are High - Part 1, and Making Fertilization Decisions as Fertilizer Prices Escalate and Production Costs Are High - Part 2. Additional nutrient management information can be found in the ISU Extension soil fertility website.