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Book of Abstracts

11th IFOAM
Scientific Conference
11-15 August 1996
Copenhagen, Denmark

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Soil Structure and Utilization of Plant Nutrients S21

Hansen, S.

Norwegian Centre for Ecological Agriculture, N-6630 Tingvoll, Norway

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The supply of nutrients to plants in ecological agriculture is based on the turnover of organic matter in soil, as manures, crop residues or native soil organic matter. Key processes are the mineralization of organic matter. In a concerted action animals and microorganisms break down complex organic matter to mineral products. In dense soil there will be a physical protection of dead organic material, bacteria and fungi against grazing of fauna, and in wet climates also large risks of anaerobic conditions. These factors lead to retarded mineralization.

In a pot experiment with a sandy loam the nitrogen mineralization 98 days after addition of white clover material was retarded from 18% mineralized nitrogen in uncompacted soil (bulk density 1.1 g cm-3) to 14% in compacted soil (bulk density 1.4 g cm-3). Soil compaction reduced the volume of pores available to nematodes (diameter larger than 30 micrometer) from 30% to 15% of total volume. The reduced nitrogen mineralization in the compacted soil are probably due to increased physical protection of organic material including microbial biomass, against grazing nematodes.

In a field trial (1985-1995) on a sandy loam in an ecological milk-production system, soil compaction and plant damage through tractor traffic reduced the yields about 25%. The reductions were largest in ley years. In the wet year 1988, the ley yields were decreased with 40%. The average pore volume in soil were in 1989, 54% and 47% in uncompacted and compacted soil, respectively and in 1995, 51% and 46%. The corresponding number of earthworms were 810 and 170 earthworms m-2 in 1989 and 270 and 100 earthworms m-2 in 1995. Nitrogen-balance (manure-N minus harvested-N) was -13 kg N per ha and year in uncompacted and +15 kg N in compacted treatment in 1985-1989, and corresponding -46 and -11 kg N per ha and year, in 1991-1995.

Breland, T. A. and Hansen,S. (1996): Nitrogen mineralization and microbialbiomass as affected by soil compaction. Soil Biol. Biochem. In press.

Hansen, S. (1996): Effects of manure treatment and soil compaction on plantproduction of a dairy farm system converting to organic farming. Agric. Ecosystems Environ. In press.

Hansen, S.(1996): Effects of soil compaction and manure level on utilization of nitrogen in cattle slurry. In: Samuelsen, R. et. al. (eds.) EUR 16757 -Crop Development for the Cool and Wet Regions of Europe. EuropeanCommission, Luxembourg, p.135-142.