Adjustment of Flat-Tipped Penetrometer Resistance Data to a Common Water Content

ABSTRACT Soil penetration resistance readings that need to be compared are often taken at different soil water contents. Because soil water can significantly effect penetration resistance, it is often difficult to determine whether penetration resistance differences are caused by water content or treatment. Empirical equations that relate soil penetration resistances from a 5-mm diameter, flat-tipped probe to soil water content were evaluated. The purpose of these equations was to adjust soil penetration resistance with changers in soil water content from plot to plot or time to time in the same plot. Several relationships were developed statistically for United States Southeastern Coastal Plain Ultisols from Norfolk loamy sand data and verified by other data of that soil and other soils. Penetration resistance (PR) was related to water content (WC) and bulk density for laboratory samples with R^ ranging from 0. 86 to 0. 96. When the ratio of functions of water contents was related to the ratio of the penetration resistances, in most cases bulk density cancelled out and R^ ranged from 0. 44 to 0. 99. The R^ for the field samples ranged from 0. 78 to 0. 90, with one value at 0. 25 for the relationship between penetration resistance and water content and bulk density, and 0. 53 to 0. 80 for the ratios. Some of the best relationships were developed from equations that considered boundary conditions of PR = 0 for WC = saturation and PR =. for WC = 0. These fit the data as well as or better than more empirical relationships but used fewer parameters.