Pre-compaction can occur as a result of a combination of soil settlement and the effect of field machinery operations and, therefore, provides some insight into the loading history of a soil. By limiting stress to below that which determined the pre-compaction, the risk of further damage to the soil through additional compaction can be minimised. Pre-compaction stress can be determined accurately using plate sinkage test results; however, these tests are time-consuming and require specialist equipment. An indication of pre-compaction stress, predicted from readily determined soil properties, could provide a useful measure of the mechanical state of soils for use by advisory and extension staff in the planning and management of agricultural mechanisation systems. The relationship among pre-compaction stress, volumetric water content and initial dry bulk density was investigated in trials conducted initially under laboratory conditions on a sandy loam, and in the field on a sandy loam and a clay soil. Pre-compaction stress for sandy loam soil was found to increase with increasing dry bulk density and decreasing volumetric water content which is characteristic of the dominance of frictional resistance within this soil. For clay soil, however, increases in pre-compaction stress were found to be largely independent of dry bulk density but closely correlated with decreases in volumetric water content which influences the cohesive nature of this soil type. Relationships among these parameters were statistically significant for clay soil in the field and sandy loam soil under controlled conditions in a soil bin. Similar relationships, for a range of different soils and conditions could complement existing soil survey databases to provide indications of the susceptibility of a particular soil series, in a given condition, to further compaction. These data provide a useful management tool to aid selection of the number, size and specification of equipment required to complete necessary land work in the time available. (C) 1998 Silsoe Research Institute.
