Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/11/2007
Publication Date: 10/20/2007
Citation: Jiang, P., Anderson, S.H., Kitchen, N.R., Sudduth, K.A., Sadler, E.J. 2007. Estimating plant-available water capacity for claypan landscapes using apparent electrical conductivity. Soil Science Society of America Journal. 71:1902-1908
Interpretive Summary: In many non-irrigated crop production fields, yield is largely determined by how well the soil stores water for plant growth. The characteristic used to describe a soil’s ability to provide water to plants is called “plant available water capacity” or PAW. Soil properties like texture, organic matter, and pore space help determine PAW. Since these properties vary from one place to another within fields, PAW capacity also varies within fields. Because traditional measurements of soil properties needed for PAW calculations are expensive, most farmers have not been able to use PAW in crop management decisions. Consequently, PAW has primarily been used in research settings and for general descriptions of different soil types. In this investigation we proposed that PAW could be estimated from the depth of topsoil above the claypan soil layer or “horizon.” This approach could be used on the 10 million acres of claypan soils found in the U.S. Midwest. This idea is based on the capacity of the topsoil to have about twice the PAW of the claypan. Another key element was using on-the-go soil electrical conductivity (EC) sensor measurements to estimate topsoil depth. The advantage of soil EC sensing is that fields can be mapped quickly and inexpensively. We found that EC could directly be used to predict PAW, with an accuracy of about +/- 1.5 inches of water for claypan soil profiles. This means that the variability of profile PAW within fields can be directly estimated from soil EC. However a second approach we used was even better. When soil EC was used to determine the depth of the boundary between the topsoil and claypan soil layers, in what we called a “two-layer-soil procedure”, we obtained more accurate estimates of profile PAW, with an average error of about +/- 2/3 of an inch of water. The results of this study prove that soil ECa could provide a quick and cost-efficient method to quantify PAW within claypan soil fields. These results will benefit producers who use site-specific information when making soil and water management decisions to improve crop water use efficiency. The general public will benefit as fields are managed to improve water utilization, in turn reducing runoff into lakes and streams.
Technical Abstract: Within-field variability of plant available water (PAW) capacity is useful information for site-specific management, but is expensive to assess using traditional measurements. For Missouri claypan soils, relationships between soil apparent electrical conductivity (ECa) and topsoil thickness have been well documented. This gave rise to our hypothesis that profile PAW capacity can be estimated by a theoretical two-layer soil body, i.e., a silt loam topsoil layer and a silty clay sublayer, with the boundary between the two layers determined using ECa. If proven, PAW capacity could then be mapped for a field, using known or assumed PAW values for these textures. Specific objectives of this study were (1) to investigate direct relationships between ECa and the upper and lower limits of profile available water, and (2) to test the previously stated hypothesis. Two claypan fields near Centralia MO were sampled in October 2005 at 19 sites in Field 1 and 18 sites in Field 2. Lower limits of available water were determined at -1500 kPa soil water pressure. The same sites were sampled again in March 2006, following wintertime profile recharge, to determine field capacity. Calculations were on a 1.2-m profile basis. The relationship between the reciprocal function of ECa (1/ECa) and profile PAW was significant with a regression (r^2) of 0.76 for the two fields combined. A significant relationship between 1/ECa and the lower limit of profile PAW was also found. Using the two-layer-soil procedure, the overall root mean square error (RMSE) for estimated profile PAW was 14 mm and 16 mm for Fields 1 and 2, respectively. These errors were 8% and 9% of the mean profile PAW estimates for Fields 1 and 2, respectively. Large underestimates of profile PAW were a result of underestimation of topsoil thickness, whereas large overestimates were attributed to a few soil horizons at less than field capacity. The results suggest that soil ECa can potentially provide a quick and cost-efficient method to quantify plant available water at a field scale for similar claypan soils. This information can be used in site-specific decision making with regard to soil and water management.