Utilization of Apatite in Fresh Volcanic Ash by Pigeonpea and Chickpea

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Volcanic ash from Mt. Pinatubo, Philippines (1991) contained 1.7 g P_2 O_5 kg<-1> of P mostly as apatite. Dissolution of this apatite was significantly increased by (1) low pH (<4.5) and (2) the existence of chelating organic acids in soil. Pigeonpea (Cajanus cajan (L.) Millsp.) and chickpea (Cicer arietinum L.) were selected as test plants to examine the possibility of utilizing apatite in the Pinatubo volcanic ash because of their ability to exude chelating organic acids. These two plants were grown in agreenhouse with and without P application. N was applied as NH_4^+ or NO_3^- to examine the effect of N forms on plant growth and on pH in the volcanic ash (NH_4 plots, NO_3 plots). Lower pH and higher soil solution P concentration were observed in the NH_4 plots than in the NO_3 plots of pigeonpea. In the NH_4 plots, dry matter yield of pigeonpea without P was similar to that with P. Dry matter yield of chickpea grown in the NO_3 plots without P was 75% of that in the P-amended plots. Soil solution pH of the NO_3 plots was maintained between 7 and 8, and the citric acid concentration of the soil solution was mostly higher in the NO_3 plots without P than in the NH_4 or P-amended plots for both plants. It was suggested that plant P uptake in the NO_3 plots without P was caused by organic acid exudation by these plants.
社団法人日本土壌肥料学会の論文