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Contribution of Mycorrhizae to P Nutrition of Crops Growing on an Oxisol¹

Most plant species, including the major crops, form symbiotic mycorrhizal associations between their roots and certain fungi which influence nutrient uptake, especially P, from infertile soils. HOW well-supplied the phosphate must be before mycorrhizae cease to enhance P uptake is not known; nor have the effects of mycorrhizae on external P requirements of crops been adequately determined in the field. This study examined the influence of mycorrhizae on the P requirements of crops in a tropical field environment on a Tropeptic Eutrustox. Plant growth and P uptake by non-mycorrhizal and mycorrhizal plants (methyl bromide-fumigated and nonfumigated soil) were measured at 10 levels of soil P using seven plant species. Brassica chinemis, which does not form symbiotic mycorrhizal associations, consistently grew better and took u more P from the fumigated than from the non-fumigateg soil. All other species growing in nonfumigated plots formed associations with mycorrhizae. In general, plants growing on fumigated soil did not become infected with mycorrhizae. In P-deficient situations, plant concentration of P was enhanced by the mycorrhizal associations. The levels of soil P at which fumigation ceased to make a difference in the P percentage in plants of the various species were as follows: Glycine max (L.) Merr. (0.1 µg P/ml), Vigna unguiculata L. (0.2 µg P/ml), Allium cepa L. (0.8 µg P/ml), Leucuenu leucocephala (1.6 µg P/ml), Stylosanthes hamata and Manihot esculenta (1.6 + µg P/ml). We suggest that this listing may be the order in which these species depend on my. corrhizae in Pdeficient soils. As a mean of six species growing on the two lowest soil P levels, P uptake by mycorrhizal plants was 25 times greater than by plants without mycorrhizal associations. Thus, some crops appear to be quite dependent upon a mycorrhizal association for P absorption from a soil of high sorption capacity.

Key Words: Cowpea • Cabbage • Cassava • Leucaena • Onion • Stylosanthes • P requirement

² Assistant soil scientist and professor of agronomy and soil science, respectively, Dep. of Agron. and Soil Sci., Univ. of Hawaii, Honolulu, HI 96822.

Received for publication January 22, 1979.