SOIL STRUCTURAL QUALITY, COMPACTION AND LAND MANAGEMENT

Soil compaction is a concern worldwide, particularly where compactible soils are used for intensive agriculture in a wet climate. We have in vestigated the impact of compaction and the associated changes in soil structural qualities on crop production and environmental pollution. The overall objective was to develop soil management systems that prov ide suitable conditions for crop growth and minimize environmental dam age. We ran large-scale field experiments studying the preservation of structural quality in arable and permanent grassland, using managemen t systems such as the control or elimination of field traffic and the application of conservation tillage and zero tillage. We measured bulk density, shear strength, cone resistance, macroporosity, relative dif fusivity, air permeability and water infiltrability to identify soil q ualities that could be used for selecting suitable soil management. Al ong with crop yield, we measured environmental impacts, such as the em issions of nitrous oxide from the soil, which require the interaction of soil structure and water content near the soil surface. Soil struct ure influenced wetness, which affected trafficability, compaction and nitrogen retention. Measurement of properties that affect fluid storag e and transport, such as macroporosity, provided soil quality indices that helped in recommending suitable soil management systems. Spatial variation of structure associated with wheel-track locations could be estimated rapidly using a cone penetrometer. Variation was particularl y important in determining crop yield consistency. Crop productivity a nd soil structural qualities were preserved best when field traffic wa s eliminated. A reduced ground-pressure system successfully minimized compaction in grassland but was less effective in an arable rotation. Unless traffic is eliminated, good timing of operations is the most ef fective way to preserve soil structural quality.