Prediction of Phosphorus and Potassium Uptake by Soybeans with a Mechanistic Mathematical Model

Mechanistic mathematical models have been used to predict P and K uptake by corn ( Zea mays L.) but not soybeans ( Glycine max L. Merr). A model developed by Cushman to account for root competition has not been tested. The objective of this research was to evaluate the Cushman mathematical model for predicting P and K uptake by soybeans. Three soybean cultivars, ‘Century’, ‘Williams‐79’, and ‘Elf’, were grown on two soils in a controlled climate chamber and harvested at intervals from 12 to 28 d. The soils were Raub, an Aquic Argiudoll, and Chalmers, a Typic Haplaquoll, silt loams. The Cushman model was used to simulate P and K uptake from soil by soybeans. Predicted P uptake ( y ) agreed closely with observed P uptake ( y = 1.08 x −0.06, r = 0.96) for uptake by all cultivars on both soils. Predicted K uptake was overestimated on Raub soil ( y = 1.53 x − 0.84, r = 0.97) but was closer for the Chalmers soil ( y = 0.95 x − 0.43, r = 0.95). The Cushman model was as satisfactory for prediction of P as the Claassen‐Barber model, but predicted K uptake more accurately because root‐to‐root competition for soil K is a factor in K supply to the root.