Effects of Soil Properties, Water Content, and Compactive Effort on the Compaction of Selected California Forest and Range Soils

Soil compaction reduces plant growth in a variety of settings. In forest and range sites, amelioration of a compacted soil is usually impractical, therefore, emphasis has been given to minimizing and preventing compaction. To provide information on inherent soil compactibility, important factors related to susceptibility to compaction were identified by multiple linear regression of soil physical and chemical properties on Proctor maximum dry bulk densities. The following equation was derived: Db = 1.91 − 0.0596 C − 0.0076 Wₗ + 0.0019 S + 0.0058 Fe, where Db = maximum dry bulk density in g/cm³, C = organic C content, Wₗ = water content at liquid limit, S = total sand, and Fe = dithionite Fe. The equation has an adjusted R² of 98.6% and Sy,ₓ = 0.0356. With the use of organic C content, Proctor densities, and normalized change in porosity after compaction as criteria, the 14 soils were ranked by relative susceptibility to compaction. The three criteria produced virtually the same groupings of soils, with the four range soils in the study being most compactible. Moisture content and compactive effort, which can be controlled, are important factors influencing soil compaction. To further understand the compaction behavior of these forest and range soils, six representative soils of different textural characteristics were subjected to Proctor compaction, with 30, 50, and 100 blows at three to four moisture contents ranging from field capacity to 15% less than field capacity. Density increased significantly with each increase in compactive effort, and maximum values were reached near the optimum moisture content of the standard compaction treatment. Analysis of moisture characteristic curves for 14 forest and range soils suggests that two range soils would occur in the field at water contents making them susceptible to puddling. About half the soils would remain at near optimum water contents for compaction for a long period of time under field conditions.