Measured cone index values were adjusted for soil water content and bulk density by normalizing their effects using a covariance analysis as described by Chris tens en 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. Keywords. Soil strength^ Core penetrometer^ Cone index. I nformation about soil strength is useful to assess its effect on crop root penetration and plant growth. Soil strength can be quickly assessed by using a core penetrometer to measure penetration resistance. Penetration resistance values can be used to characterize soils in terms of crop growing abihty and resistance to root penetration and seedling emergence (Bowen, 1976; Taylor and Gardner, 1963). Cone penetration results represent a composite soil behavior since soil fails by some combination of cutting, shearing, compaction, and flowing (Gill and Vanden Berg, 1968). However, the interpretation of a cone index is difficult even in homogeneous soils, because the proportion of shear, compression and tensile components that the cone index reflects, varies with soil conditions (Mulqueen et al., 1977). Major factors affecting the cone penetration resistance are soil water content, bulk density, soil type, soil strength, base diameter of cone, apex angle, penetration speed, and surface roughness of the cone. Cone penetrometers have been used in soil dynamics (Gill and Vanden Berg, 1968), tillage (Cassel, 1982; Threadgill, 1982), research related to root growth (Taylor et al., 1966), soil compaction (Raghavan and McKyes, 1977), trafficability (WES, 1948; Flores et al., 1990), and soil strength (Kondner, 1960) studies. Various cone penetrometer types such as static, quasi static, dynamic, and their applications were comprehensively reviewed by Perumpral (1987).