Update to Iowa P, K and lime recommendations
Testing dried or field-moist soil does not change P soil-test results by any method, but can greatly affect K test results with the ammonium-acetate and Mehlich-3 tests (these two tests give similar results no matter the sample handling procedure). Therefore, different interpretations are provided for K testing of moist or dried samples. The moist K test can be performed either on field-moist samples or on a slurry made with field-moist soil and water, and both versions give similar results. Research has indicated that the moist K test is more reliable than the test based on dried samples and is a better predictor of crop K fertilization need. Improved interpretations are provided for the K test based on dried samples because some laboratories offer both tests, and some still offer only the dried test. Although the new interpretations improve the performance of the dry K test, we encourage use of the moist test for K.
The amounts of K extracted can differ greatly between moist and dried samples, and the differences change greatly across soil series, the soil-test K levels, and soil conditions related to drainage and moisture content cycles. There is no numerical factor that can be used to transform test results between moist and dried testing procedures. Laboratories should not transform soil-test results obtained by one procedure to express values by the other procedure. When switching to the new moist test, the K test results may be lower, approximately similar, or higher than results in previous years based on the dried test. The moist test will tend to be lower at the lower soil K levels and in soils with fine textures and more poor drainage, but may be similar or higher at high soil K levels and in well drained soils or dry sampling seasons.
Suggested crop P and K concentrations and default yields
The amount of P and K removed with harvest is a very important criterion to determine the rate needed to maintain soil-test values in the optimum interpretation category. The probability of crop a yield increase in this category is 25% or less and the magnitude of the expected increase is small. So maintaining this level is recommended based on the amounts of P and K removed, which is determined by the prevailing yield levels (not the yield goal) and the nutrient concentration in the harvested crop portion.