Predicting Soybean Root Growth by Water Depletion in the Soil Profile.

Predicting the effective rooting zone of a crop is a pre-requisite for determining irrigation needs by the water balance approach. A number of factors influence root growth, including genotype, soil physical and chemical properties, and in-season water regime, determined by rainfall, irrigation and climate. The objectives of this study are to evaluate the congruence of soil water depletion patterns measured by soil water sensors at four or six soil depths to 1.2 m and the presence of roots at those same depths, and to use that data to develop an empirical model to predict soybean root depth during the growing season. A second objective is to test the hypothesis that there is a consistent relationship between root growth and phyto-phenology across locations and years. Soil water sensors were installed in field plots under both irrigated and rainfed conditions at experimental sites in Lincoln and Clay Center, NE in two years.  A minirhizotron was used to evaluate the presence of roots with depth during the growing season by image analysis. Initial results suggest that the rate of downward root growth is linear with time until the seed filling period (despite differences in the timing of scheduled irrigation events), and that the relationship is similar across sites. Presence of roots at incremental depletion depths will be evaluated in relation to phenological stage and cumulative degree days based on soil and air temperature to develop an empirical relationship to predict the soil volume for water extraction.