Crop Nutrition Research

Soil Management

Make Rational Decisions About P and K Management

eKonomics News Team

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Below is an excerpt of an article written by Antonio Mallarino, a professor of agronomy at Iowa State University. This article, originally published in Ag Professional, examines several key factors farmers should keep in mind if they are considering a reduction in application rates due to uncertainty in the industry.

Crop prices have been declining and there is considerable uncertainty about the future. Phosphorus (P) and potassium (K) fertilizer prices have remained approximately constant or have declined slightly. Therefore, producers are thinking of reducing application rates.

Consider the following when making decisions with unfavorable crop/fertilizer price ratios:

Reducing fertilization rates across all conditions is not a rational or sound management decision

Do not reduce rates in low-testing soils, where yield increases and profits from fertilization are very likely, even with unfavorable prices. However, consider not fertilizing high-testing soils.

Soil testing is not a perfect diagnostic tool, but is very useful and has become less expensive in recent years. Testing expenses are low compared to overall production costs and they are especially relevant with unfavorable crop prices. Soil analysis is only as good as the samples that are collected, so it is imperative that proper techniques be used to collect them.

Crop yield increases from P and K fertilization are large and highly likely in very low and low-testing soils, but the size and likelihood of yield response decreases as soil test values increase and yield increase becomes very unlikely in high-testing soils.

If your economic condition is particularly bad, you are unsure concerning field tenure, or unfavorable prices continue, you can temporarily reduce the removal-based rate to maintain optimum soil test levels or apply only starter. Keep in mind that this may increase profits in the short term, but higher rates will be needed in the future because soil test values will decline.

Use a good soil sampling method and variable-rate technology to vary, as needed, the P and K application rate within fields

Use of variable-rate P and K fertilization 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 applications 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. 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. Research suggests that either grid sampling or zone sampling methods are superior to the classic sampling by soil type method. With variable-rate application, the key is to follow the soil test interpretations.

Banding of P and K before planting or with the planter does not reduce the application rate needed to optimize crop yield no matter the tillage system

Research in Iowa soils and other soils of the humid Corn Belt has shown that banding of P and K fertilizer seldom is more efficient than broadcasting, even with no-till management. Therefore, cutting the fertilizer rate for low-testing soils when banding is used will increase the risk of yield loss and may reduce profits from crop production, and the need for future fertilization will increase.

Deep placement of K fertilizer (about 5 to 6 inches deep) 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 applied to the corn seed furrow or beside the seed can complement a primary broadcast application. This happens mainly when applying the P and K rate for one crop year in soils testing extremely low and/or with a thick residue cover or cool or wet soil in spring.

Give credit to P and K in animal manure

Iowa research has shown that manure is an excellent P and K source, when used in conjunction with manure analysis and careful application methods. The research results have been used to develop manure nutrient management guidelines as outlined in Using Manure Nutrients for Crop Production (PMR 1003). The K availability of all animal manure is 90 to 100 percent compared to fertilizer (and assuming otherwise similar conditions), whereas the P availability varies from 60 to 100 percent depending on the type of manure.