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

Abbreviations: C, clay; EC a , bulk soil apparent electrical conductivity; LL 1.2 , lower limit of plantavailable water for a 1.2-m soil profi le; PAW, plant-available water; PAW c , plant-available water capacity; PAW 1.2 , plant-available water for a 1.2-m soil profi le; SIC, silty clay; SICL, silty clay loam; SIL, silt loam; UL 1.2 , upper limit of plant-available water for a 1.2-m soil profi le. Information on plant-available water (PAW) capacity (PAW c ) variation within a fi eld is useful for site-specifi c management. For claypan soils, established relationships between soil apparent electrical conductivity (EC a ) and topsoil thickness suggested the hypothesis that profi le 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 profi le samples were taken from two Missouri claypan fi elds 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 profi le PAW (PAW 1.2 ) was signifi cant, with regression r 2 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 PAW 1.2 were 14 and 16 mm for Fields 1 and 2, respectively, or 7.6 and 8.6% of the respective mean measured PAW 1.2 . With the two-layer approach, some underestimates of PAW 1.2 resulted from underestimation of topsoil thickness, whereas overestimates were attributed to soil horizons being short of ficapacity at sampling due to slow recharge. The resulting fi eld-scale PAW c information is useful in site-specifi c decision making for soil and water management.

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