Soil strength and forest operations.

The use of heavy machinery and transport vehicles is an integral part of modern forest operations. This use often causes damage to the standing trees and to the soil. In this study the effects of vehicle traffic on the soil are analysed and the possible consequences for forest management discussed. The study is largely restricted to sandy and loamy soils because of their importance for Dutch forestry.Soil strength, defined as the resistance of soil structure against the impact of forces, can be described in terms of four basic strength factors: cohesion, friction, density, and structure. The experimental work was carried out in the laboratory, using three compaction tests: confined uniaxial compaction, hand compaction (newly developed), and Proctor. The results show the importance of moisture tension, soil structure, and loading type for soil strength. Soil strength is largely related to organic matter content for all sandy soils. The effects on soil structure of soil compaction and soil disturbance are measured as changes of soil water relations, density, and penetration strength. The results are represented in a so-called soil strength diagram. Soil strength is quantitatively modelled as a function of cohesion, density, and load factors. Moreover, a qualitative model of field soil strength and soil stability is presented.The experimental results are interpreted in terms of effects on root growth and functioning, choice of vehicles and operation pattern, and possibilities for soil management. The possibilities for soil classification are explored, but it is concluded that the necessary soil data are poorly represented in standard soil surveys. Moreover, the practical use of such a classification is probably limited. Finally, some examples are described