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Be cautious when interpreting fall soil-test results following drought

Antonio P. Mallarino, Professor, Department of Agronomy, Iowa State University
John E. Sawyer, Professor, Department of Agronomy, Iowa State Universit
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October 16, 2020

Sampling soil this fall following the dry conditions this past summer, and in some places continuing up to this time, may result in lower than expected soil-test results for phosphorus (P), potassium (K), and pH. Especially if soil samples are collected before any significant rainfall. Therefore, farmers and crop consultants should interpret those soil-test results with caution.

P and K removal with crop harvest
Estimates of P and K removal are used to decide fertilizer application to maintain soil-test P and K levels within the Optimum soil-test interpretation category. Prolonged drought can reduce crop grain yield and, consequently, P and K removed with harvest, so the planned removal-based rates may be reduced accordingly. However, a large yield reduction is not likely if below normal rainfall was only from late August, so in these fields the planned removal-based rate should not be reduced. Removal-based rates in fields with low grain harvest recovery from badly lodged corn due to the August or recent windstorms should not be reduced because P and K in unharvested grain will become available for next year crop. Although sampling harvested plant parts for analysis is an option, an easier and effective approach to estimate P and K concentrations per unit of yield is to use information provided in Iowa State University Extension and Outreach publication PM 1688 (A general guide for crop nutrient and limestone recommendations in Iowa). Concentration values in that publication are adjusted from a dry matter basis so they can be directly multiplied by the yield at the standard moisture concentration. For example, values from PM 1688 for corn grain at 15% moisture are 0.32 lb P2O5/bu and 0.22 lb K2O/bu; and for soybean grain at 13% moisture values are 0.72 lb P2O5/bu and 1.2 lb K2O/bu. It must be remembered that the yield level variation is by far much more important at determining nutrient removal than variation in P or K concentrations.

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