THE RELATIONSHIP AMONG THE PRECOMPACTION STRESS, VOLUMETRIC WATER-CONTENT AND INITIAL DRY BULK-DENSITY OF SOIL

Pre-compaction can occur as a result of a combination of soil settleme nt and the effect of field machinery operations and, therefore, provid es some insight into the loading history of a soil. By limiting stress to below that which determined the pre-compaction, the risk of furthe r damage to the soil through additional compaction can be minimised. P re-compaction stress can be determined accurately using plate sinkage test results; however, these tests are time-consuming and require spec ialist equipment. An indication of pre-compaction stress, predicted fr om readily determined soil properties, could provide a useful measure of the mechanical state of soils for use by advisory and extension sta ff in the planning and management of agricultural mechanisation system s. The relationship among pre-compaction stress, volumetric water cont ent and initial dry bulk density was investigated in trials conducted initially under laboratory conditions on a sandy loam, and in the fiel d 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 decre asing volumetric water content which is characteristic of the dominanc e of frictional resistance within this soil. For clay soil, however, i ncreases in pre-compaction stress were found to be largely independent of dry bulk density but closely correlated with decreases in volumetr ic water content which influences the cohesive nature of this soil typ e. Relationships among these parameters were statistically significant for clay soil in the field and sandy loam soil under controlled condi tions in a soil bin. Similar relationships, for a range of different s oils 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 us eful management tool to aid selection of the number, size and specific ation of equipment required to complete necessary land work in the tim e available. (C) 1998 Silsoe Research Institute.