Short-term P and K recycling to soil from maturing standing plant parts (like leaves) or crop residue after grain harvest usually is reduced by drought, especially K, due to reduced nutrient leaching with the low rainfall. Potassium in plant tissue is soluble in water, because little or no K combines in organic compounds, and plant P is mostly organic. Last fall an ICM News article, "Change in phosphorus and potassium contents of cornstalks over time," provided a summary of very recent research for corn. Similar research conducted for soybean was summarized in the 2011 ICM Conference article "Nutrient uptake by corn and soybean, removal, and recycling with crop residue" (pdf).

On average across fields and years, analysis of corn residue collected in late fall showed that 31% of the P and 41% of the K in the aboveground plant parts (except grain) at physiological maturity had been recycled to the soil, whereas analysis of soybean residue showed that 67% of the P and 62% of the K had been recycled. The actual amounts recycled were much higher for K (49 and 105 lb. K2O/acre for corn or soybean) than for P (11 and 12 lb P2O5/acre for corn or soybean), due to the higher amounts of K in vegetative plant parts and residue.

The P and K loss from physiological maturity to harvest was large for both crops. The K concentration of corn residue (after harvest) continued decreasing during the fall and early spring but the P concentration remained approximately constant. The rate of K loss from corn plants and residue increased with increasing rainfall, but not the P loss. The different results for corn P and K are explained by the K being water-soluble and the P mostly organic forms. For soybean, P and K recycling is rapid no matter the amount of rainfall because most P and K are in the leaves, which drop from the plant and are in contact with the soil and decompose even with low rainfall. Therefore, below-normal rainfall from physiological plant maturity until the time of soil sampling will result in lower than normal soil-test K results, but how much lower is difficult to predict.