ESTIMATING SOIL-WATER CONTENT FROM SOIL STRENGTH

Variable rate application technology requires real time estimates of f ield water contents. This study was conducted to evaluate soil strengt h as a possible indicator of soil water content. The study involved me asurement of soil strength with flat tip and cone penetrometers on lab oratory packed soil cores. Packing treatment varied from Proctor Test (PT) bulk densities to densities similar to those of cultivated field soils. Soils used in the study included 18 benchmark soil series repre senting eight soil orders. Soil samples were from the surface 300 mm d epth and from a subsurface layer. Soil strength tests were run at PT d ensities for all 18 soils and at cultivated field densities for four o f the 18 soils. Two equations were fitted to plots of gravimetric wate r content versus soil strength using least squares nonlinear regressio n techniques. Predicted and measured water contents were in better agr eement at the PT densities than at cultivated field densities. Sand co ntent accounted for 36% of the variation in equation parameters at PT densities.