Cylinders of a clay loam soil were adjusted to different bulk density and matric pressure combinations to study soil resistance to a penetrating probe. Regression analysis of the penetrometer data produced no evidence to reject a regression model of the form Y = β0X0 + β1X1 + β2X2 + β3X12 + β4X22 + β5X1X2 (where Y = penetrometer resistance, X1 = bulk density, X2 = matric pressure). A three-dimensional plot of the generated soil resistance values was constructed to illustrate the nature of the interaction. At any one bulk density in the range 1.0–1.5 g/cc, soil resistance values tended to pass through a maximum as soil moisture was removed over the matric pressure range 1.0–8.0 atm. The tendency was most pronounced at the lower bulk densities. Implications on root growth and soil management are briefly discussed.