Figure 1. Amount of potassium contained in soybean vegetative plant tissue at physiological maturity (except grain) and in residue from harvest until the following spring. In many regions, early fall has been characterized by exceptionally dry conditions due to low rainfall in late summer and early fall, especially since corn and soybean physiological maturity. This may affect quality of soil samples and soil-test results.
Soil sampling issues
Sampling under very dry conditions may increase soil sampling error because it is more difficult to control sampling depth and proper soil core collection. This may be a problem for P and K due to nutrient stratification, which usually is more pronounced in reduced till, no-till and pastures. Both P and K tend to concentrate at or near the soil surface and therefore, the depth control for core collection is very important. Also, when the top inch of soil is very dry and powdery it is very easy to lose this soil portion, which will affect the soil-test result significantly.
Soil-test results issues
Very dry soil conditions may result in more acidic soil pH. Differences from of 0.1 to 0.3 pH units are common with very dry conditions. This is because small concentrations of cations or soluble salts present in the soil solution are not leached by rainfall or are not retained by soil particles, which result in higher hydrogen concentration in the soil solution.
Short-term nutrient recycling from plant residues and the equilibrium between soil nutrient pools also may be affected by rainfall, especially for potassium (K). Potassium is present in the soil in water-soluble, easily exchangeable and slowly exchangeable forms, and in mineral (unavailable) K form. Potassium in fertilizer and manure sources is water soluble and application quickly increases the solution and exchangeable K pools, which are readily available for crops. Potassium in plant tissue also is soluble in water, because little or no K combines in organic compounds.
Re-distribution of K among soil pools occurs as K is added to soil with fertilizer, manure or crop residues. Plants take up K from soil solution, which is readily replenished by the easily exchangeable soil K fraction. In moist soils, some slowly exchangeable K can become exchangeable when easily exchangeable K is depleted by plant uptake or leaching. With dry soil at the end of the growing season, this replenishment of the solution and easily exchangeable K fractions, which is what soil tests measure, is limited or does not occur. These processes also occur for phosphorus (P), but to a much lesser extent and through completely different mechanisms.