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AMES, Iowa – Crop prices have been declining and there is considerable uncertainty about the future. Although phosphorus (P) and potassium (K) fertilizer prices have remained constant or declined slightly, producers are still thinking of reducing application rates for nutrients to balance yield profits and crop production costs.
“With unfavorable crop fertilizer prices, farmers are considering reducing fertilization rates across all crops despite field conditions,” said Antonio Mallarino, professor and extension specialist in agronomy and nutrient management research at Iowa State University. “However, making rational or sound management decisions should begin by testing soil, following the recommended application rates for additional nutrients.”
Soil testing is not a perfect diagnostic tool, but it is useful and has become less expensive in recent years. Testing expenses are low compared to overall production costs and testing is especially relevant with this year’s lower crop prices. Soil sampling guidelines for phosphorus, potassium and other nutrients are available in the Iowa State University Extension and Outreach publication, Take a Good Sample to Help Make Good Decisions (PM 287). Also, P, K and pH soil test interpretations based on Iowa field research are available in the publication, A General Guide for Crop Nutrient and Limestone Recommendations in Iowa (PM 1688).
In very low and low-testing soils, crop yield increases are highly likely from P and K fertilization, but in high-testing soils, the yield increase becomes very unlikely. Therefore, Mallarino suggests not reducing nutrient rates for low-testing soil samples where there is the potential for yield increases and profitability. Instead, consider not fertilizing those high-testing soils. This will reduce the costs associated with inputs and still keep yields somewhat high for a good return.
“Many Iowa producers are choosing to apply the right rate of fertilizer, at the right time,” said Mallarino. “Using a good soil sampling method and variable-rate technology will help determine the phosphorus and potassium application rate within fields. However, the key issue that applies to many Iowa producers is that there is no rational reason to maintain higher than optimum soil test levels.”
If economic conditions are poor, cropland use is in question or crop prices are low farmers may want to temporarily reduce nutrient inputs to reach optimum soil test levels. This may increase profits in the short-term, but soil test values will steadily decline over time and, as a result, higher application rates will be needed throughout the years.
Iowa research has shown that manure is an excellent phosphorus and potassium source, when used in conjunction with manure analysis and careful application methods. The potassium availability of all animal manure is 90 to 100 percent compared to fertilizer (and assuming otherwise similar conditions), whereas the phosphorus availability varies from 60 to 100 percent according to the type of manure
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