EVALUATION OF A MODEL TO PREDICT NUTRIENT-UPTAKE BY FIELD-GROWN RICE

The Barber-Cushman model has not been evaluated under flooded conditions. A field study was conducted on a Crowley silt loam to evaluate the ability of the model to predict NH4+, P, and K uptake by rice (Oryza sativa L.). Total nutrient uptake and root and soil parameters were measured at vegetative (active tillering, maximum tillering, and 1.25-cm internode elongation) and reproductive (booting and heading) stages of development of three cultivars (Katy, Mars, and Lemont). The model accurately predicted nutrient uptake during vegetative growth but underpredicted nutrient uptake during reproductive growth. About 90% of the total NH4+, P, and K uptake and root length of the rice cultivars occurred within the surface 20 cm of soil, suggesting that samples collected for routine soil tests should be from the top 20 cm. Ammonium and P uptake in the 0- to 20-cm soil depth were most sensitive to root competition, root radius, nutrient solution concentration, and buffer power. However, K uptake was most sensitive to maximum influx rate, root radius, Michaelis constant, and root growth rate throughout the top 40-cm soil depth. These findings indicate that NH4+ and P uptake can be enhanced by increasing levels of fertilization in the top 20 cm of soil where these nutrients are found to be Limiting. Fertilization should increase the NH4+ and P solution concentrations, thus reducing root competition for these elements. Potassium soil levels were not Low enough to limit uptake by the cultivars selected for this study.