The moist soil test for potassium, other nutrients
click image to zoomFigure 1. Comparison of amounts of soil K, P, and Mg extracted by the Mehlich-3 test from many soil samples taken from Iowa fields. The dotted diagonal line indicates an exact 1:1 ratio. The amount of extracted K is lower for the moist test than for the dry test at values usually optimum for crops or lower, but the difference decreases as levels increase. At extremely high levels, the moist test values can be higher than the dry test values. However, the moist and dry tests result in approximately similar values for P (by the Bray-1, Olsen, or Mehlich-3 methods), calcium (Ca) and magnesium (Mg) (by the ammonium-acetate or Mehlich-3 methods), pH, and buffer pH (by the SMP or Sikora methods). Figure 1 shows, as an example, comparisons for K and P extracted by the Mehlich-3 method from moist and dry samples.
Results from more than 300 field-response trials for corn and soybean confirmed that the field-moist test for K is better. Figure 2 shows the relationship between soil K measured by moist or dry tests and the relative yield response of corn and soybean to K fertilization. The graph for the dry test shows the current ISU interpretations for K, and further information including recommended fertilization rates is available in Extension publication PM 1688, A General Guide for Crop Nutrient and Limestone Recommendations in Iowa. The graph for the moist test shows the interpretations that ISU suggested for this test in the late 1980s. The fertilizer rate recommended for the Medium class used at the time was the amount to maintain soil-test values based on K removal, which in concept is similar to current Optimum class for the dry test. The old moist test classes relate to the recent yield responses almost exactly as they related to yield responses from research conducted during the 1980s (not shown).
click image to zoomFigure 2. Relationship between the relative yield response of corn and soybean and soil-test K measured from dried and moist soil samples (ammonium-acetate test, 6-inch sampling depth). VL, very low; L, low; O, optimum; M, medium; H, high; VH, very high. Additional information and results will be presented at the North-Central Extension-Industry Soil Fertility Conference in Des Moines (November 14-15, 2012), the ISU Extension Integrated Crop Management Conference in Ames (November 28-29), and other ISU Extension conferences and workshops during the winter.
This fall the NCR-13 regional committee will publish an updated sample preparation chapter of the soil-test methods publication that includes the moist procedure. The ISU interpretations and fertilizer recommendations for the moist test will be developed in the future, as results for several ongoing field trials become available and can be merged with previous results. Interpretations for the moist test for P using Bray, Olsen, and Mehlich-3 (colorimetric or ICP procedures) should be similar to those for the dry test, since data already showed similar test results. The interpretations for the moist test for K likely will be similar to those suggested by ISU in the 1980s.