RELATIVE EFFECTS OF TRACTOR WEIGHT AND WHEEL-SLIP IN CAUSING SOIL COMPACTION

The results presented are from field studies in which soil compaction was measured under tractors working with different wheel loads and levels of wheel-slip. Wheelslip proved to be more important in causing compaction than additional wheel loading, and this effect was more pronounced for more powerful tractors. In most agricultural circumstances the aim is to secure the best compromise between weight and slip as alternative means of achieving greater work output from tractors. The results indicate that in situations where there is a need to avoid compaction, particular attention should be given to providing sufficient wheel loading and the possibility of moving faster to take up more power at lower slip. For the range of soil moistures encountered sinkage was the most convenient and satisfactory method of assessing total reduction in porosity, and differentiated between treatments better than any of the other methods (see below) apart from water-entry rates. Measurement of soil density at three levels to 15 cm depth accounted for less than 50 per cent of the loss of porosity estimated from sinkage and the method did not detect differences due to treatment. Shear strength measured at one depth (3.5 cm) differentiated between treatments, but penetration resistance at 7.5 cm did not. Water-entry rates sensitively detected treatment differences but the method was too time-consuming for comparison of all treatments. The efficiency of traction of four- and two-wheel-drive tractors was measured at intervals during the winter on a clay loam and the results related to surface moisture contents. After a steep reduction in efficiency, when the soil returned to field capacity, subsequent changes in efficiency were small.