Use of minjingu phosphate rock combined with different organic inputs in improving phosphorus availability and maize yields on a chromic acrisol in Morogoro, Tanzania

Abstract

Phosphorous deficiency in highly weathered soils is one of the causes of low crop yields in Tanzania. Use of Minjingu phosphate rock (MPR) in combination with organic inputs is considered to be a possible option for increasing P availability and crop yields. However, detailed characterization of organic inputs, the effects of organic inputs when used alone or combined with MPR on soil characteristics and, consequently, maize yields, are limited. The objectives of this study were to characterize farmyard manure (FMY), tithonia, lantana and gliricidia and to evaluate the influence of MPR combined with these organic inputs on soil properties, P availability and maize yields from a Chromic Acrisol. The study consisted of incubation, pot and field experiments. Characterization of organic materials showed that manure and tithonia had more P than the critical value for net P mineralization. Tithonia contained the highest amounts of light molecular weight organic acids (LMWOA), oxalic acid being the most dominant. An incubation experiment showed that all organic materials depressed MPR dissolution, and the depressing effect increased with increase in pH, Ca and P content of the materials. Incubation and pot experiments showed that organic materials at 2.27 g kg'1 (5 t ha'1) increased soil pH, exchangeable Ca and decreased exchangeable Al. All organic materials increased labile P pools and reduced P adsorption maximum and P affinity in the incubation and field experiments. This resulted in increase in available P and consequently, in maize dry matter yields (DMY). Under field conditions tithonia increased soil pH, exchangeable Ca and oxalic acid concentration, and decreased exchangeable Al. Tithonia decreased P sorption and increased total labile P, consequently increasing maize yields. Thedecrease in P adsorption increased with tithonia rates, with the rate of 7.5 t ha'1 resulting in the largest agronomic optimum. MPR was superior to TSP (80 kg P ha'1) in increasing pH, Ca, oxalic acid and in reducing exchangeable Al. Both sole MPR and sole TSP reduced P adsorption but the influence of MPR was greater than that of TSP. Tithonia at 7.5 t ha'1 had similar P adsorption maximum to sole MPR at 80 kg P ha'1. Combining MPR at 40 kg P ha’1 with tithonia at 7.5 t ha'1 had adsorption maximum similar to that of sole tithonia at 7.5 t ha'1 or MPR at 80 kg P ha'1. Both MPR and TSP increased Ca and P uptake and maize yields in pot and field experiments. MPR was superior to TSP in supplying Ca, oxalic acid and in increasing pH in all seasons. Combining MPR with tithonia had a higher RAE in two out of the three seasons than that under sole tithonia or MPR. Organic materials alone or combined with MPR improved soil pH, Ca and decreased exchangeable Al, consequently increasing P availability and maize yields. Combining MPR with organic materials resulted in increase in P availability by reducing P adsorption rather than by enhancing MPR dissolution. Co-application of MPR with tithonia on Chromic Acrisol is recommended. Evaluation of the influence of tithonia on some chemical, physical and biological soil properties under various climatic conditions and for longer periods of time is essential so as to be able to better exploit this resource.