PRODUCTIVITY OF A CLAYPAN SOIL UNDER RAIN-FED AND IRRIGATED CONDITIONS

The productive capacity of a soil is frequently related to topsoil thickness. This study investigated the relationships among a soil-based Productivity Index (PI) and corn (Zea mays L.) grain yield for rain-fed and irrigated water management of plots having topsoil treatments of 0, 125, 250, and 3 75 mm (0, 5, 10 and 15 inches). A field experiment at Columbia, MO, was conducted for five years on a desurfaced Mexico silt loam soil (fine, montmorillionitic, mesic, Udollic Ochraqualf) having a distinct claypan at about 250 mm (10 inches). The experimental design was a split-plot with topsoil ( whole-plot) and water management (sub-plot) as main effect treatments. All plots were fertilized according to soil test recommendations. A productivity index (PI; 0.0 to 1.0) was determined for each plot based on potential available water content, bulk density and pH. Corn grain yield increased linearly for both rain-fed and irrigated treatments as PI and depth to clay increased. Irrigation increased average corn field over years by 24%. The yield increase due to irrigation was equivalent to any additional depth of soil above the claypan of 0.38 m (1.2 feet) for corn grown without irrigation. On average. irrigation was equivalent to a PI-fraction of 0.30 for corn grown without irrigation. A regression, including a term to account,for differences in seasonal available water with the PI, explained 66% of the variation in yield for all years. This information would be useful to a producer in evaluating the potential for crop production in a given year. The relationships for irrigation benefits and equivalent depth of soil above the claypan would be useful in evaluating potential management decisions for irrigation, in justifying adequate soil conservation measures to minimize loss of topsoil, and estimating lost productivity as a function of soil erosion.