Micro-scale spatial variability of crop roots, water content, soil texture and their influence on soil water extraction rates

Water extraction by roots is an important component in crop growth simulation models. The capability of plants to survive in limited conditions of water supply largely depends on the uniformity and depth of its root system. If roots are not present in some parts of the soil, it is likely that areas where water is available will be left behind during root growth in the soil environment. Roots frequently bypass these soil regions as a consequence of root clumping.; Several models simulate water extraction based on the assumption that roots are uniformly distributed in the soil. However, considering the natural heterogeneity of the soil environment and genetic factors inherent to plant species, the assumption of uniformity of root distribution is not valid. In addition, other factors such as anthropogenic action on soil formation, competition among plants, and soil physio-chemical and biological stresses will also contribute to a non-uniform distribution of the root system. Knowing that this non-uniform root distribution should affect simulation of water extraction by roots, the objectives of this study were to describe the micro-scale spatial variability of crop roots, water content, soil texture and their influence on soil water extraction rates.; To describe such variability, a greenhouse experiment was conducted at Michigan State University, MI, and another one in the field, at Maricopa Agricultural Center, AZ. The plants were grown in a terminal drought condition until they were severely impacted by soil water deficits. Measurements of volumetric water content within soil layers in the greenhouse and in the field experiments were done with a neutron-probe gauge. A small TDR probe was used to determine volumetric water contents in 2.5 cc soil samples collected in a grid pattern in each soil layer at the termination of the both experiments. For each soil sample the amount of roots and soil texture was determined. A functional model for root water uptake was used to simulate water depletion for each soil layer in the greenhouse experiment.; The results revealed that roots were non-uniformly distributed within soil layers in both experiments, and that roots bypassed soil areas leaving water without being extracted. Thus plants could show signs of water deficit despite an appreciable amount of available water in the bulk soil.; A critical value constant, K, in a functional water uptake model representing the daily fraction of extractable water was determined for each soil layer to predict the water extraction based on the non-uniform root distribution. Once these values were estimated, the functional model for root water uptake was able to closely predict the water extraction in each soil layer.