Using Cone Index Data to Explain Yield Variation Within a Field

Three cone index (CI) penetration probes recorded to a depth of 76.2 cm (30 in.) and a soil sample was collected and analyzed for moisture content at 81 locations along a transect measuring approximately 323 m (1060 ft) in length. An average maize plant yield was assigned to each location based on hand harvested ears. The three probes at each location were averaged to obtain a single profile of CI versus depth. The first and second derivatives of CI with respect to depth were determined to indicate rapid changes in soil density and peaks in the CI data, which are indicative of hardpans. Average yield was compared to mean CI, maximum CI, depth of maximum CI, maximum CI rate of change, depth of maximum CI rate of change, CI peaks, depth to CI peaks, and established CI limits of 0.7, 1.4, and 2.0 MPa (100, 200, and 300 psi). The mean CI throughout the 76.2 cm (30 in.) profile best explained yield variation (r=-0.83). The maximum CI also correlated with yield, but to a lesser extent (r=-0.71). Maximum CI readings at or above 1.4 MPa (200 psi) resulted in below average yields for 89-percent of the readings exceeding this limit. No correlation was established between yield and soil density changes and depth of density changes or between yield and CI peaks and their depth of occurrence.