NORMALIZING CONE RESISTANCE VALUES BY CovARiANCE ANALYSIS
Measured cone index values were adjusted for soil water content and bulk density by normalizing their effects using a covariance analysis as described by Christensen et al. (1989). Cone indices adjusted by covariance analysis allowed the testing of tillage differences at a given depth and across depths. The procedure showed that double disking a Sharpsburg silty clay loam soil reduced the cone penetration resistance to 152 mm depth. The penetration resistance was similar in the deeper undisturbed zones. Unadjusted field measured cone indices showed differences in the deeper zones though no tillage was performed.