The effects of compaction and soil drying on penetrometer resistance

Penetrometer resistance is widely used as a measure of the mechanical impedance that roots experience in either drying or compacted soil. However, there are relatively few models to predict penetrometer resistance that can be applied without detailed knowledge of soil texture, organic matter content, soil water status, density or other soil variables. Few models allow the effects of management on penetrometer resistance to be predicted in a simple way. It would be useful if it were possible to predict the effects of structure, compaction, and soil drying on penetrometer resistance. We designed a laboratory experiment to explore how compaction and subsequent soil drying affected the penetrometer resistance of three soils: a loamy sand and two silty clay soils with very different organic carbon contents. By assuming that penetrometer resistance is proportional to the small strain shear modulus, G, we were able to develop an empirical model to explain the effects of compaction and soil drying on penetrometer resistance. The parameters of the model were determined by fitting it to experimental data collected in the laboratory. The model was tested on field data using sensed matric potential data and measured soil density data. Model predictions were compared with those obtained with an earlier model. Both approaches explained approximately 60% of the variance in the measured penetrometer data. The future application of this approach is discussed.