SOIL-PHOSPHORUS LEVELS NEEDED FOR EQUAL P-UPTAKE FROM 4 SOILS WITH DIFFERENT WATER CONTENTS AT THE SAME WATER POTENTIAL

Soil volumetric water contents, phi(v), at -33 kPa potential may vary with soil from 0.06 to 0.70. Because P diffusion depends on phi(v), most economic P fertilizer rates required for different soils may require adjusting according to their soil-water relationships. The objective of this study was, after experimentally verifying a mechanistic nutrient uptake model on a series of soils varying in theta(v) at -33 kPa potential, to use the model to predict labile P levels needed for each of these soils to achieve equal P uptake by maize (Zae mays L.) and verify these predictions. Maize was grown in a pot experiment using four soils having theta(v) of 0. 13, 0.20, 0.26, and 0.40 at -33 kPa each at 0, 200, and 400 mg kg-1 of added P. When root parameters obtained experimentally were used, predicted P uptake with the uptake model agreed with observed P uptake, y = 0.99x + 9.08 (r2 = 0.98). When P uptake was plotted vs. soil solution P, C(li), the relation varied with soil. The higher the theta(v) the lower the C(li) needed for equal P uptake. A similar relation was found between P uptake and diffusible soil P, C(si). Differences between the two plots occurred because of differences among soils in buffer power, DELTA-C(si)/DELTA-C(li). The C(si) vs. P added relation was used to calculate differences among soils in the C(si) needed to obtain equal P uptake. The C(si) values ranged from 1.3 to 4.0 mmol kg-1. The calculated values were used in a second pot experiments to verify the predictions. No significant difference (alpha = 0.05) in P uptake occurred. The results of this research indicate that the mechanistic nutrient uptake model can be used to predict the degree of adjustments in C(si) needed to obtain the most economic P fertilizer rates among soils varying in theta(v).