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.
|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.