Possibilities for modelling the effect of compression on mechanical and physical properties of various Dutch soil types

The state of compactness of the arable soil layer changes during the growing season as a result of tillage and traction. The aim of this study was to assess and predict some soil mechanical and physical properties governing machine performance and crop response. The following mechanical properties were studied: compressibility, workability and cone index, CI, the latter as indicator of load-bearing capacity or root penetration resistance. Compressibility of the soil could be described as a semi-log function of pressure versus air volume and moisture content, with texture-specific coefficients for three representative soils, in the range of 6–35% air content. The wet workability limit for 16 Dutch soils was reached when the compaction process turned from “dry” into “wet” at 408 kPa of pressure. Soil rebound after pressure release was taken into account and quantified. Semi-log relations were found for CI versus porosity and moisture. Other physical properties were also studied and it was found that the nature of the pF curve of three representative soils (for seven levels of bulk density) was highly affected by the initial seven pressure–moisture combinations. The “effectivity” of the pore system, indicating the effect of tortuosity and discontinuity on the oxygen diffusion rate, turned out to be proportional to air content in the range of 6–25%. Critical machine and plant related limits for aeration and mechanical resistance, CI, are available from the literature. Aeration is associated with minimum values for air volume and oxygen diffusion rate, respectively. Using this information, CI was associated with minimum values for load-bearing capacity and maximum values for root penetration.The applicability of the comprehensive laboratory approach is found in farming practices and evaluations of land management systems. On the operational level, machine performance can be predicted more accurately under fluctuating soil conditions. Also, the effects of modified equipment can be quantified more accurately in the case of unchanged field conditions. The same holds true for the prediction of crop response, as it is influenced by aeration and mechanical limits for plant growth. It was concluded that the approach of predicting the mechanical behaviour of soil, followed by the pF-derived determination of physical properties, will do justice to the dynamic character of the soil structure related input parameters in the present and future models and simulations for machine performance, crop production and soil conservation.