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Influence of Mycorrhizae on the Mineral Contents of Cowpea and Soybean Grown in an Oxisol¹

Cowpea (Vigna unguiculata) and soybean (Glycine max) were grown in the field on a Tropeptic Eutrustox which, for a period of 8 years, had been maintained at 10 different soil P levels by appropriate P fertilizer applications. Effects of methyl bromide fumigation of the soil on mycorrhizal infection and concentrations of P and other nutrients in the plants were compared. Plants grown on nonfumigated soils with soil P levels below 0.025 to 0.05 mg P/liter contained higher Ca and K percentages than plants grown on fumigated, low P-status soils. Differences in Ca and K percentages in the plants were more closely related to crop growth rates than to vesicular arbuscular mycorrhizal infection levels. Differences in Ca and K percentages were small. In contrast, silicon percentages of mycorrhizal soybean plants were 0.5 to 0.9% Si while non-mycorrhizal plants contained 0.2 to 0.3% Si. The pattern of Si percentages at different levels of soil P was different from that of other nutrients. The levels of Si in mycorrhizal soybean plants were greater than in non-mycorrhizal plants at all levels of soil P, while with other nutrients, differences diminished with increasing soil P level. Crop growth rate and P absorption strongly affected the percentage composition of other nutrients with the notable exception of Si. If mycorrhizae consistently enhance Si uptake by plants such as soybean then Si uptake may indicate mycorrhizal activity. Silicon content of cowpea, however, was not altered by the presence or absence of mycorrhizae.

Key Words: Fumigation • Tropics • Phosphorus • Silicon

² Assistant soil scientist and professor of soil science, respectively. Dep. of Agronomy and Soil Science, Univ. of Hawaii, Honolulu, Hawaii 96822.

Received for publication April 2, 1981.