Predicting soybean yield in a dry and wet year using a soil productivity index

A soil-based productivity index (PI) has been developed and is being tested as a means of quantitatively assessing potential soil productivity and predicting crop yield. Validation of the PI requires the PI-yield calibration for various soil-crop-climate systems. It is hypothesized that PI predictability and accuracy would be enhanced by inclusion of a soil water balance component. This study aims at developing a sufficiency factor that accounts for dynamics of soil water influenced by weather to improve the PI predictability. Soybeans (Glycine max[L.] Merr.) were grown in 1992 and 1993 on Mexico soil (fine, montmorillonitic, mesic Mollic Endoaqualfs). Test plots had altered A-horizon thicknesses of 0, 12.5, 25, and 37.5 cm over Bt horizons. A range of PI values in the plots resulted due to A-horizon treatment. The PI increased with increasing A-horizon thicknesses or depth to the Bt horizons. The PI was highly correlated with plot yield in 1992, a relatively dry year, in comparison with 1993, a relatively wet year. Inclusion of a factor assessed by daily balance of soil water significantly enhanced PI predictive power by 20% in both years. The factor best improved the PI predictability when based on the number of soil dry-wet cycles for given depth during the growing season. This study illustrates that yearly variation of soil water induced by weather should be considered for assessing crop performance based on soil properties.