STEINHARDT, R., Institute of Soils and Water, Agricultural Research Organization, Israel
TRAFFORD, B.D., Field Drainage Experimental Unit, Ministry of Agriculture, Fisheries, and Food, Cambridge, United Kingdom

In a field experiment to determine the direct and indirect effect on soil structure, of sub‐surface piped drainage as compared with natural surface drainage only, in ploughed and unploughed soil, a factorial systematic design with four replicated blocks was used. Structural changes were monitored during 8 months of natural rain and finally irrigation, by measuring surface heights and soil strength (penetration resistance) in relation to moisture content and matric suction, at plough sole depth (27 cm). A compaction test using a tractor with differentially loaded wheels, was applied at various times after irrigation, measuring the resulting wheel sinkage and wet density of the soil. The effects of the drainage treatments were found to be temporary, except a ‘crusting’ effect during the drying of the unploughed surface drained soil. The ploughed soil with sub‐surface drainage showed greater frost heave than the undrained soil. The soil strength at 7.5‐22.5 cm. depth was linearly related to the matric suction within the range of –3 to 20 cm‐water. The compaction data for the unploughed soil suggested relationships between matric suction, sinkage, and wet density, but complicated interactions prevented any general conclusion. In the ploughed soil, compaction data established the beneficial effects of subsurface drainage in reducing damage from tractor traffic, decreasing wheel sinkage and reducing compaction both below and 16 cm from the track edge. A rise in matric suction of 10 cm‐water, in the range 2‐24 cm‐water was, on the average, as effective in reducing rutting as a wheel load reduction of 670 kg (0.54 kg/cm2 reduction of tyre inflation pressure). It was concluded that for clay soils having a temporary excess moisture, draining the water table to below 50‐60 cm depth should be recommended as a precautionary measure to minimize structural damage. Copyright © 1974, Wiley Blackwell. All rights reserved