DEGRADATION OF SOIL-STRUCTURE BY WELDING - A MICROMORPHOLOGICAL STUDY

In 1985 a plot study was started on sloping cultivated loess soils in South-Limbourg (The Netherlands) to evaluate the effects of various cropping systems of fodder maize on runoff, soil loss and crop yield under natural rainfall. To monitor degradation of soil structure including crust formation, undisturbed vertical samples (15 x 7 cm) for soil micromorphological analysis were taken of the top 7 cm of soil at ten dates chosen in relation to times of tillage and sowing, crop stage, harvest and winter frost. Wheel marks, if present, were also sampled. Contrary to accepted views, no disintegration of structural elements by slaking or dispersion was observed. Freshly tilled soil exhibited a loss of its original granular structure by coalescence of the aggregates into larger units, eventually leading to a structureless state of part of the top soil. Processes held responsible for this are mainly welding of wet aggregates and seldom fusion by silt infillings in vertical inter-aggregate pores. The two layer model of soil crust formation of McIntyre could not be confirmed, as no important vertical translocation of fines or washed-in layers were observed. Horizontal cracks developed in the structureless soil, giving rise to a platy structure in part of the top soil. Transitional stages of loss of soil structure were observed, characterized by the progressive reduction of compound packing voids to vughs, which ultimately disappeared completely and sometimes were replaced by planar voids. Besides structural crusts, sedimentary crusts including splash deposits were observed.