SIMULATING SOYBEAN ROOT GROWTH AND SOILWATER EXTRACTION USING A FUNCTIONAL CROP MODEL

Abstract

The current soil water component of the CROPGRO-Soybean model was structured for deep, well-drainedsoils. It did not have mechanisms for restricting root growth and root water uptake in soil layers with high mechanicalimpedance or soil layers with non-uniform root distribution. In this research, the model was modified to consider rootgrowth and root water uptake under those conditions. Experimental data sets from Castana, Iowa, and Gainesville,Florida, with two contrasting soils were used for model validation. An empirical soil impedance factor was incorporatedto influence the ability of roots to grow and proliferate in a layer. A root hospitality or clumping factor was incorporatedto limit root water uptake in soil layers where roots are not uniformly distributed. A sensitivity analysis was conducted todemonstrate the effects of these two factors and rate of root depth increase on root growth, soil water extraction, seedyield, and crop biomass. Root growth and soil factors were adjusted so that simulated soil water extraction from each soillayer correctly matched observed data. A maximum rate of vertical root growth was estimated to be 4.0 cm d1 for bothlocations. Root distribution with depth, soil water extraction pattern, and simulated crop responses were highly sensitiveto this parameter. In both soils, the root hospitality factor was 1.0 (implying uniform root distribution) from the soilsurface to layers near the bottom of the root zone below which roots extracted water at lower rates. Both soils required asoil impedance factor at 30 to 45 cm of 0.2 or less to match the apparent vertical rate of root growth, root lengthdistribution with depth, and observed soil water extraction patterns. After confirming that the model accurately simulated soil water extraction patterns in both locations, the modelpredicted aboveground growth and yield very well at both locations. In the Castana experiment predicted seed yield was3127 kg ha1 and mean measured seed yield was 2644 kg ha1. In Gainesville final seed yield predicted by the model was1299 kg ha1 and measured seed yield was 1178 kg ha1. This research confirms the importance of root growth and rootproliferation on soil water extraction, crop growth and yield, and it demonstrates the need for additional research todevelop relationships between soil properties that are routinely measured and relatively simple root growth anddistribution characteristics.