Phosphorus Uptake By Corn and Soybean and Recycling to the Soil from Crop Residue.

Sometimes there is high soil-test P temporal variability within and between seasons, and reasons are not fully understood. Previous research with K by our group showed that the K uptake by corn and soybean and the magnitude and timing of K recycling from crop residue to the soil explained a great deal of short-term soil-test K variability. There is scarce information for P, however. The objective of this study was to assess aboveground P uptake by corn and soybean, removal with grain harvest, and recycling from plant tissues to the soil. Soil and plant samples were collected in two to four years from 13 Iowa field trials (two with continuous corn and 11 with corn-soybean rotations). Plant samples taken from plots receiving non-limiting N, P, and K fertilization were plant and grain at physiological maturity (PM), grain at harvest time, and residue at 45-day intervals from harvest until spring. On average, the grain P concentration was higher in soybean (5.8 g kg^-1) than in corn (2.9 g kg^-1), but the proportion of plant P removed with grain harvest was higher for corn (76%) than for soybean (65%). The P loss from plant vegetative tissue (excluding grain) decreased sharply between PM and grain harvest time for both crops, but the loss was greater for soybean (67%) than for corn (31%). The P loss from crop residue until early spring was much smaller, and was proportionally greater for corn. By April, 53% of the plant P at PM remained in the corn residue, but only 25% remained in the soybean residue. There was no statistically significant relationship between the P lost from residue and soil-test P increase from the fall to the spring (there was a small apparent increase). This finding is in sharp contrast with previous results for K, which showed a strong relationship and a substantial soil-test K increase. Larger loss of the mostly soluble K in plant residue compared with P and differences between P and K concerning reactions with the soil may explain differences between nutrients.