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

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Mycorrhiza in Organic Farming E8

Joner, E. J.

Department of Biotechnological Sciences, Microbiology Section, Agricultural University of Norway, P.O.Box 5040, N-1432 Aas, Norway.

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In organic plant production the supply of phosphorus (P) is a bottle neck, as P is the only macro nutrient that cannot be obtained through biological fixation or weathering of parent rock minerals. Farmers thus rely upon recycling of nutrients from plant residues and manure, or addition of e.g. rock phosphate to meet plant's demand for P. Irrespective of how P is supplied, the fate of phosphate ions in soil is to a large extent an irreversible adsorption. To maintain a sufficient P supply plants and certain fungi form a symbiosis called mycorrhiza. The mycorrhizal fungi form hyphae in soil that act as an extension of the roots, transporting nutrients from the soil to the plant. Regarding organic nutrients, mycorrhiza has been shown to improve the recycling of both N and P in plant material, as it's wide distribution makes more frequent contact with sites where organic matter is mineralized. This symbiotic scavenging of the soil is the main mechanism for plant supply of P and some other plant nutrients in agroecosystems where these are not added as easily soluble salts. Cultivation practices and plant breeding in conventional farming systems disregard mycorrhiza, and consequently benefit little from this biological resource. In contrast, plant cultivation practices in organic farming often take more advantage of mycorrhiza (consciously or not).

The presentation will explain the functioning of mycorrhiza, and how cultivation practices may be optimized to benefit the most from this symbiosis. Results from recent experiments with cycling of P in organic matter (Joner and Jakobsen, 1994, 1995a, 1995b) will be presented together with perspectives for future research.

Joner, E. J. & Jakobsen, I. (1994): Contribution by two arbuscularmycorrhizal fungi to P uptake by cucumber (Cucumis sativus L.) from 32P-labelled organic matter during mineralization in soil. Plant and Soil 163, 203-209.

Joner, E. J. & Jakobsen, I. (1995a): Growth and extracellular phosphataseactivity of arbuscular mycorrhizal hyphae as influenced by soil organic matter. Soil Biol. Biochem., 27, 1153-1159.

Joner, E. J. & Jakobsen, I. (1995b): Uptake of 32P from labelled organic matter by mycorrhizal and non-mycorrhizal subterranean clover (Trifolium subterraneum L.). Plant and Soil, 172, 221-227.