Farm Progress

With high yields harvested in 2016 and low grain prices continuing in 2017, you need to manage phosphorus and potassium wisely

Rod Swoboda 1, Editor, Wallaces Farmer

November 21, 2016

6 Min Read

Bumper grain yields have been harvested from most Iowa fields this fall. However, due to low crop prices many producers are thinking of reducing phosphorus (P) and potassium (K) fertilizer application rates. “There are some important management options farmers should consider when making decisions in situations like they now face, with unfavorable crop/fertilizer price ratios,” says Antonio Mallarino, Iowa State University Extension agronomist and crop fertility specialist. He offers the following information and guidelines for deciding how much fertilizer to apply per acre.

1. Use soil sampling and testing to make rational decisions


Soil testing is not a perfect diagnostic tool, but it is very useful. Its use is even more relevant with unfavorable crop prices. Iowa field research results have been used to develop soil sampling guidelines in Extension publication PM-287 (“Take a Good Sample to Help Make Good Decisions”) and P and K soil-test interpretations in publication PM 1688 (“A General Guide for Crop Nutrient and Limestone Recommendations in Iowa”).

Crop yield increases from fertilization are large and very likely in soils testing very low and low, but the size and likelihood of the response sharply decreases as soil-test values increase into the optimum category. ISU research shows very large and likely economic benefits from fertilization of corn and soybean in soils testing very low, but benefits bounce around the breakeven point in the optimum category (due to small or no yield increase), and become mostly negative in high-testing soils.

Thus, farmers should not reduce P or K fertilizer application rate in soils testing very low. They may slightly reduce the application rate in soils testing low and optimum, and only a low starter fertilizer rate might be justified in high-testing soils. ISU Extension Publication PM 1688 indicates recommended rates for low-testing soils. Only removal-based rates are recommended for the optimum category to maintain soil test levels over time.

For corn and soybean, the optimum category is 16 to 20 parts per million. or ppm, P for the Bray-1 and colorimetric Mehlich-3 methods, 26 to 35 ppm for the Mehlich-ICP method, and 10 to 13 ppm for the Olsen method. The optimum category for K by both the ammonium-acetate and Mehlich-3 methods is 161 to 200 ppm for testing laboratory dried soil samples and 86 to 120 for the field-moist or slurry laboratory sample processing and extraction.

2. Watch yield level and P and K removal to maintain Optimum values

The yield level and P or K removal should be used to determine the application rate needed to maintain soil-test levels in the optimum category. Publication PM 1688 includes suggested average P and K concentrations in common harvested crop parts.

As economic returns show, there is high response variation within the optimum category for various reasons. A removal-based rate will maximize yield, but often the small yield increase does not offset application costs, especially with current low corn and soybean prices. Thus, if the farmer’s economic condition is particularly bad or there is uncertain land tenure, a fraction of the estimated removal-based rate or only starter could be applied when prices are low. This may increase profits in the short-term, but higher nutrient rates will be needed in the future because soil-test values will decline.

The decline in soil-test values is much slower than many believe, and farmers who have high-testing fields or field areas could save money by withholding or reducing P and K fertilizer application rates until soil test levels for these nutrients decrease to the optimum category. Farmers should be aware that there is a good long-term relationship between P and K removal with harvest and soil-test values, but not necessarily consistent from year to year, especially for K. ISU research shows average results across five long-term experiments in a corn-soybean rotation. Results from these and other long-term experiments show that, on average, and depending on the yield level, about 10 years are needed for soil-test P or K values borderline between the high and very high categories to decrease to the optimum level, where maintenance based on removal is recommended.

3. Use a good soil sampling method and variable-rate technology application

Use of variable-rate P and K fertilizer application is a good option to improve P and K management in fields that have significant variation in soil test or yield levels. This technology can be used to target application to the most deficient field areas to get the highest possible return when price ratios are unfavorable and also to improve maintenance fertilization by considering yield variability.

Research suggests that either grid sampling or zone sampling methods are superior to the common sampling by soil type (see publication PM 287). However, grid soil sampling often is much better than zone sampling, and allows for easier and safer decisions because the many soil samples allow you to see when large portions of a field or an entire field consistently test low or high. Also, the impact of sampling or testing errors is diminished when several samples are taken. Yield maps from the past two to four years (not just the last one) should be used together with soil-test values to help define P and K application rates.

4. Do not reduce recommended P and K rate for low-testing soils by banding

Research conducted in many Iowa fields during several decades has shown that applying P and K fertilizer in bands in the soil seldom is more efficient than broadcasting, even with no-till or strip-till management. Therefore, reducing the fertilizer application rate for low-testing soils when banding will increase the risk of yield loss, may reduce profits from crop production, and future fertilization rates will need to be increased.

Recent research has confirmed these results. ISU studies show typical results for no-till corn yield response to broadcast or planter-band P or K, and similar results have been observed with tillage and for soybean. Deep placement of K fertilizer (about 5- to 6- inches deep) for corn often is beneficial in ridge till and sometimes in no-till or strip till, but reducing the application rate is not recommended. In some conditions, starter P fertilizer applied to the corn seed furrow or beside the seed can complement a primary broadcast application. This happens mainly when applying the P rate for one crop year in soils testing very low, and/or with a thick residue cover and cool or wet spring weather, or the broadcast rate is much less than recommended for the test category.

For information visit ISU Soil Fertility website

Related articles are: “Research Shows a Role for Phosphorus and Potassium Tissue Testing in Corn”; and “Value of Soybean Tissue Testing for Phosphorus and Potassium”.

About the Author(s)

Rod Swoboda 1

Editor, Wallaces Farmer

Rod, who has been a member of the editorial staff of Wallaces Farmer magazine since 1976, was appointed editor of the magazine in April 2003. He is widely recognized around the state, especially for his articles on crop production and soil conservation topics, and has won several writing awards, in addition to honors from farm, commodity and conservation organizations.

"As only the tenth person to hold the position of Wallaces Farmer editor in the past 100 years, I take seriously my responsibility to provide readers with timely articles useful to them in their farming operations," Rod says.

Raised on a farm that is still owned and operated by his family, Rod enjoys writing and interviewing farmers and others involved in agriculture, as well as planning and editing the magazine. You can also find Rod at other Farm Progress Company activities where he has responsibilities associated with the magazine, including hosting the Farm Progress Show, Farm Progress Hay Expo and the Iowa Master Farmer program.

A University of Illinois grad with a Bachelors of Science degree in agriculture (ag journalism major), Rod joined Wallaces Farmer after working several years in Washington D.C. as a writer for Farm Business Incorporated.

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