Farm Management and the function of field crop root systems

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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 mode! 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 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 hirnltum L), soybean (Glycine max L.) and winter wheat (Triticum aesti,'um L. em Their.) " ere used to test the model. Measured values of root length density and soil water content over depth a 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 1he 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 sods 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 ~km water potential brought about by friction.
Agricultural Water Management, 7, p. 115-141