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

Abstract

AbstractMost 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 chinensis, which does not form symbiotic mycorrhizal associations, consistently grew better and took up more P from the fumigated than from the non‐fumigated 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 mycorrhizae in P‐deficient 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.