Soil quality aspects of humid sandy loams as influenced by organic and conventional long-term management

The development of low-input farming systems requires knowledge of the extent to which management characteristics influence the soil as a habitat for micro-organisms and a medium for plant growth. This paper presents a study of long-term effects of organically and conventionally cultivated systems on a range of soil physical, chemical and biological characteristics in Denmark. Seven soils were included, falling into three groups (I-III). Each group consists of an organically managed soil (Org) referenced by one or two conventionally managed soils (Conv) with the same soil type (sandy loam) and pedological history. All organically managed soils were dairy farm soils (H), which had been organically managed for 46, 47 and 40 years, respectively. They had forage crop rotations and received animal manure. The conventional soils were either dairy farm soils or soils grown to annual cash crops (P). Undisturbed core samples were used for assessing dry bulk density and water retention. Undisturbed cubic samples were used for assessing wet stability of macro-aggregates, clay dispersibility, microbial biomass C, beta-glucosidase activity, ergosterol, and the abundance of different groups of fungi. Penetration resistance and ease of soil fragmentation were measured in the field. The quantifying methods were supplemented by a visual inspection of soil structure of the top 30 cm soil in the held. Irrespective of agricultural system, the use of tractors and heavy machinery had caused compaction of the subsoil in the form of a dense pan below ploughing depth. In group I, the H-soils were heavily trafficked and this overshadowed the long-term effects of the diversified crop rotations and animal manure applications on the physical parameters. Generally, the results highlight the paramount influence of soil tillage and traffic in agriculture. Further, the results confirm the positive effects of organic manures and diversified crop rotations on soil quality aspects. Microbial biomass C was found to be higher in organically than in conventionally managed dairy farm soils, and appeared to correlate linearly to the volume of 0.2-3.0 mum pores ('protective' pore space). The simple model did, however, not explain a lower microbial biomass for two soils dressed only with synthetic fertilisers. The results further indicated that the contribution of the different biotic mechanisms responsible for macro-aggregation varied from soil to soil. The results revealed promising correlations between quantitative scientific laboratory methods and descriptive methods in the field. (C) 2002 Elsevier Science B.V. All rights reserved.