Measured cone index values were adjusted for soil water content and bu
lk density by normalizing their effects using a covariance analysis as
described by Christensen et al. (1989). Cone indices adjusted by cova
riance analysis allowed the testing of tillage differences at a given
depth and across depths. The procedure showed that double disking a Sh
arpsburg 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 diffe
rences in the deeper zones though no tillage was performed.