Utilization of apatite in fresh volcanic ash by pigeonpea and chickpea

Abstract

Volcanic ash from Mt. Pinatubo, Philippines (1991) contained 1.7 g P₂O₅ 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 a greenhouse with and without P application. N was applied as NH₄ ⁺ or NO₃ ⁻ to examine the effect of N forms on plant growth and on pH in the volcanic ash (NH₄ plots, NO₃ plots). Lower pH and higher soil solution P concentration were observed in the NH₄ plots than in the NO₃ plots of pigeonpea. In the NH₄ plots, dry matter yield of pigeonpea without P was similar to that with P. Dry matter yield of chickpea grown in the NO₃ plots without P was 75% of that in the P-amended plots. Soil solution pH of the NO₃ plots was maintained between 7 and 8, and the citric acid concentration of the soil solution was mostly higher in the NO₃ plots without P than in the NH₄ or P-amended plots for both plants. It was suggested that plant P uptake in the NO₃ plots without P was caused by organic add exudation by these plants.

Key Words:

• apatite
• chickpea
• organic acid
• pigeonpea
• volcanic ash