Estimating Plant-Available Water Capacity for Claypan Landscapes Using Apparent Electrical Conductivity

Information on plant-available water (PAW) capacity (PAW(c)) variation within a field is useful for site-specific management. For claypan soils, established relationships between soil apparent electrical conductivity (EC(a)) and topsoil thickness suggested the hypothesis that profile PAW(c) could be estimated by assuming a two-layer soil composition, a silt loam topsoil layer and a silty clay sublayer, with known PAW fraction values for each layer. Objectives were (i) to investigate the direct relationships between EC(a) and the upper and lower limits of PAW(c), and (ii) to test the previously stated hypothesis. Nineteen and 18 soil profile samples were taken from two Missouri claypan fields in October 2005. The lower limit of PAW(c) was determined at -1500 kPa soil water pressure. Samples were taken again from the same locations in March 2006 to determine the upper limit of PAW(c). Calculations were on a 1.2-m basis. The direct relationship between EC(a)-1 and profile PAW (PAW1.2) was significant, with regression r2 values of 0.67 and 0.87 and RMSEs of 30 and 20 mm for Fields 1 and 2, respectively. The RMSEs for two-layer-estimated PAW1.2 were 14 and 16 mm for Fields 1 and 2, respectively, or 7.6 and 8.6% of the respective mean measured PAW1.2. With the two-layer approach, some underestimates of PAW1.2 resulted from underestimation of topsoil thickness, whereas overestimates were attributed to soil horizons being short of field capacity at sampling due to slow recharge. The resulting field-scale PAW(c) information is useful in site-specific decision making for soil and water management.