SOIL-STRUCTURE AND MECHANICAL RESISTANCE DIRECTLY BELOW THE SEEDBED IN A FIELD EXPERIMENT ON A SANDY CLAY LOAM IN THE NETHERLANDS

In the period 1985-1989, measurements were made on the cumulative effe cts of high (H), low (L) and zero (Z) ground-pressure field traffic sy stems on soil structure in the 2-7-cm layer below the seedbed of sugar beet (Beta vulgaris), onions (Allium cepa) and potatoes (Solanum tube rosum) in a field experiment on a typical, heterogeneous, sandy clay l oam soil in the northwest of the Netherlands. In this layer, in the co urse of 5 years, no systematic differences in soil structure caused by low and high ground-pressure field traffic systems developed. In the zero-traffic system, total pore space was significantly higher than on trafficked plots. On light soil the difference was only small (1%, v/ v), but on heavy soil it was substantial (3%, v/v). In the spring of 1 988 and 1989, the mechanical resistance of the seedbed bottom was meas ured with a hand penetrometer, a shear annulus and a falling-weight ed ge, which proved to be equally useful. However, probably the simple, s pring-loaded hand penetrometer is the best-suited instrument to easily and quickly determine soil strength and its spatial variation directl y below the seedbed. On L and H plots, soil strength of the seedbed bo ttom was similar and always much higher than on Z plots, especially on heavy soil.