Farm management and the function of field crop root systems

Abstract

Because most farmer-controlled factors influence the growth and function of plant roots, we need to develop root-centered models to describe below-ground plant behavior. A root-centered water extraction model is described. This model estimates water extraction rates from a soil layer in terms of the length of root present, the permeability of the soil-root system, and the difference in water potential between bulk soil and the root xylem in the mid-plane of the soil layer. Changes in xylem water potential resulting from frictional losses as water passes through small vessels is estimated using the Poiseuille-Hagen equation. These potential changes are used to calculate the root xylem water potential at any depth in the profile from the xylem water potential at the land surface. Field data for cotton ( Gossypium hirsutum L.), soybean ( Glycine max L.) and winter wheat ( Triticum aestivum L. em. Thell.) were used to test the model. Measured values of root length density and soil water content over depth and time, field estimates of root-soil system permeability, measured diurnal changes in plant water potential and assumed values of numbers and radii of root xylem vessels were used to estimate water extraction patterns for these three crops for one time in the growing season. Estimates of the total water extraction and its distribution in the profile were in fair agreement with measured values. Principle uncertainties were absence of measurements for numbers and radii of root xylem vessels, questions of lack of homogeneity in field soils for application of a single water release curve to all sampling points in a large plot, and absence of confirming measurements for assessing changes in xylem water potential brought about by friction. When farm managers make decisions on dryland farms, they impact the root system by changing soil physical and chemical properties, the activities of microbes and other organisms, water and nutrient concentrations and distributions. These may influence root growth and function and have an impact on yield. Before models can be used to help farmers make cost-effective decisions, we must learn more about crop root system phenology, environmental effects on root growth and function, and relationships between roots and shoots and between crop roots and their neighboring crop or weed roots. We must also develop models which deal properly with heterogeneity in the landscape, in the crop, and in the soil profile.