Calmon, M. A., W. D. Batchelor, J. W. Jones, J. T. Ritchie, K. J. Boote, and L. C. Hammond. 1999. Simulating soybean root growth and soil water extraction using a functional crop model. Transactions American Society Agricultural Engineering 42:1868-1877.

The current soil water component of the CROPGRO-Soybean model was structured for deep, well-drained soils. It did not have mechanisms for restricting root growth and root water uptake in soil layers with high mechanical impedance or soil layers with non-uniform root distribution. In this research, the model was modified to consider root growth 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 incorporated to influence the ability of roots to grow and proliferate in a layer. A root hospitality or clumping factor was incorporated to limit root water uptake in soil layers where roots are not uniformly distributed. A sensitivity analysis was conducted to demonstrate the effects of these two factors and rate of root depth increase on root growth, soil water extraction, seed yield, and crop biomass. Root growth and soil factors were adjusted so that simulated soil water extraction from each soil layer correctly matched observed data. A maximum rate of vertical root growth was estimated to be 4.0 cm d–1 for both locations. Root distribution with depth, soil water extraction pattern, and simulated crop responses were highly sensitive to this parameter. In both soils, the root hospitality factor was 1.0 (implying uniform root distribution) from the soil surface to layers near the bottom of the root zone below which roots extracted water at lower rates. Both soils required a soil impedance factor at 30 to 45 cm of 0.2 or less to match the apparent vertical rate of root growth, root length distribution with depth, and observed soil water extraction patterns.

After confirming that the model accurately simulated soil water extraction patterns in both locations, the model predicted aboveground growth and yield very well at both locations. In the Castana experiment predicted seed yield was 3127 kg ha–1 and mean measured seed yield was 2644 kg ha–1. In Gainesville final seed yield predicted by the model was 1299 kg ha–1 and measured seed yield was 1178 kg ha–1. This research confirms the importance of root growth and root proliferation on soil water extraction, crop growth and yield, and it demonstrates the need for additional research to develop relationships between soil properties that are routinely measured and relatively simple root growth and distribution characteristics.

DOI: 10.13031/2013.13352

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