Controlled Displacement Technique for Measuring Soil Friability

An important physical property of agricultural soils is their friability or tendency to fracture into small fragments when tilled. Because friability varies greatly with soil water content, simple tests to determine the point of maximum friability are needed. Previous tests have failed to simulate a set of conditions that are necessary to achieve effective soil fragmentation in tillage. The most important of these conditions is a loading path that imposes a compression/shear ratio conducive to fracture and that also provides continuous energy for crack propagation after fracture initiation. Also, many tests do not directly measure the amount of soil fragmentation. The latter is a primary variable of interest when considering soil reaction to tillage. A new procedure is described that provides the necessary fracture conditions and gives a direct measure of the amount of soil fragmentation under any desired compression/shear ratio. Forces, displacements, and energy expenditures during the soil fragmentation process can be measured. The amount of soil fragmentation determined by the method was found to be very sensitive to compression/shear ratio, particularly in highly plastic soils. However, different soils always ranked the same for all ratios. Friability indices were measured with the technique on a set of soil cores sampled in the field at different soil water matric pressures. The matric pressure at which these indices indicated maximum soil friability agreed well with the point of maximum friability determined by tillage of the same soil.