Sandy acidic soils have low phosphorus (P) levels which limit growth of many plant species. Phosphorus in these highly weathered soils is poorly available to plants, primarily because of low total amounts. It has been shown that exudation of organic acids by plant roots can increase the mobilization and hence, bioavailability of soil P. Also, addition of rock phosphate to these soils has been shown to promote a very high crop response and plant organic acid exudates are believed to make this Ca-bound P available to crops.
In a previous experiment, 696 genotypes of cowpea (Vigna ungiculata) from the U.S Core Collection and IITA exhibited a high degree of variability in their response to low-P soils and in their ability to utilize P from added rock phosphate. We hypothesized that exudation of organic acids by these genotypes was the dominant mechanism determining this wide range of responses. The present study was undertaken to verify the nature and quantity of the organic acid exudates in cowpea under P-deficiency stress conditions.
Sixteen genotypes reflecting the observed array of responses were selected for this investigation based on cluster analysis results. Seeds of each genotype were pregerminated in vermiculite and transplanted to a balanced nutrient solution for four days before treatment began. Plants were grown hydroponically with two nutrient solutions (no P and +P). Organic acids were collected from 21-day-old plants in CaCl₂ and KCl solution. The nature and quantity of the collected organic acids was determined by Ion Chromatography.