One mechanism by which plants can mobilize organic and inorganic forms of phosphorus (P) in soils is by exudation of low molecular weight organic acids. Laboratory and field trial were carried out during 2011 and 2012 cropping seasons to study the effects of additions of organic acids (citric, oxalic and tartaric acids) on the mobilization of phosphate of soils from Minna and Mokwa, both in Southern Guinea Savanna of Nigeria. For initial laboratory studies and prior to field cultivation, soil samples were collected from these locations and incubated at 25± 1oC and 40% moisture content for three weeks with citric, tartaric or oxalic acids at 1.0 mmol kg-1 of soil. Soil Olsen P and inorganic P fractions were analyzed. The experimental design used during the field trial was split plot design with organic material sources (orange waste, amaranthus and tamarind pulp) assigned to the main plot while the rates (0, 2.0, 4.0, 6.0 and 8.0 tons ha-1) of application occupied the sub-plots. Each treatment received three replications in each of the locations. Maize was planted during the two cropping seasons as test crop. Both agronomic, Olsen – P and soil inorganic P data were determined. The results indicated that Olsen – P and NH4Cl – P were significantly increased by treating with the three organic acids. Al phosphate (Al – P), Fe phosphate (Fe –P), occluded phosphate (Occl – P) and Ca phosphate (Ca – P) were also mobilized and released in various degrees in each of the locations irrespective of the cropping season. The relative fractions of inorganic P was in the order Occl – P > Fe – P > Al – P > Ca – P. The effect of organic acid sources on maize plant height at 4 and 7 weeks after planting in the two locations were not significant during 2011 cropping season, but significant during 2012 cropping season. However, the effect of sources of organic acid and their rates of application on maize grain yield was significant in each location and the season. It was also observed that the results of soil inorganic P after the field trial followed similar trend with what was obtained from the initial laboratory studies (Occl – P > Fe – P > Al – P > Ca – P), but the effect was much lower. The order of increased mobilization of phosphate by these organic acids was citric acid > tartaric acid > oxalic acid and orange waste ˃ tamarind pulp ˃ amaranthus leaves respectively for both initial laboratory studies and field trial. Also, it could be concluded that hydroxyl acids i.e tricarboxylic acids such as citrate form stronger complexes than those containing single COOH groups. The pattern of P mobilization by addition of organic acids differed from one location to another. The comparison suggested that the mobilization of P was highly soil dependent, and the soil P status such as amount and distributions of P fractions may be important for solubilization of P after the addition of organic acids. These three organic acids therefore have the potentials to increase the availability of available P. The practical implication of these processes is that organic residues could be used as a strategic tool to reduce the rates of fertilizer P required for optimum crop growth on acidic and P-fixing soils of Nigeria.
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