Impact of tilemsi phosphate rock on soil phosphorus availability and crop yield in the sahelian zone of Mali

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Low soil phosphorus availability is a major constraint to crop production in Mali. The usual fertilizer shortage and the high cost of water soluble phosphate make it a less accessible option for resource poor farmers. This study explored the use of locally available Tilemsi phosphate rock (TPR) to optimize soil P availability for sustainable crop yield. A laboratory study was first carried out to define the standard phosphorus requirement (SPR) using phosphorus sorption isotherm method. The standard phosphorus requirements of four representative soils of Mali were evaluated and phosphorus adsorption data fitted to the Langmuir equation. The amounts of phosphorus required to give 0.20 mg L-1 varied with soil ranging from 7.10 to 24 mg kg-1 (i.e. 16 to 54 kg of P ha-1) whilst sorption maxima ranged from 59 to 200 mg kg-1. The study indicated that soil texture (particularly clay content) is a prominent factor influencing standard phosphorus requirement. The values of standard phosphorus requirement were higher compared to the recommended dose of Tilemsi phosphate rock used for all the soils under study. A field experiment was then carried out to assess the effects of Tilemsi phosphate rock powder and pellet forms, and DAP application at different rates under two tillage practices on the growth and yield of maize as well as soil properties and phosphorus use efficiency indices. The experiment consisted of three rates of phosphorus: 0 kg ha-1 (control), 11 kg ha-1 (recommended rate of TPR application), and 16 kg ha-1 (SPR of the study soil), three types of phosphorus: TPR (powder), TPR (pellet) and DAP and two tillage practices: contour ridge and hoe tillage. The contour ridge (CR) out yielded the hoe tillage (HT) with a percentage grain yield increment of 22 % and 16 %, respectively in 2013 and 2014. Comparing CR and HT, maize biomass yield was increased by 32 % and 14 % and phosphorus uptake by 44 % and 65 %, respectively in 2013 and 2014 under contour ridge tillage compared to hoe tillage. Application of TPR and DAP significantly increased grain yield in the decreasing order DAP > Powder > Pellet > Control plot in both seasons of study. Increasing in P application rate from 11 to 16 kg ha-1 increased crop yield by 26 % and 20 %, respectively in 2013 and 2014. Application of TPR and DAP significantly (P < 0.05) influenced soil available phosphorus and pH in both seasons. Soil available phosphorus was not significantly correlated (P > 0.05) with soil moisture in both seasons. The increase in soil available phosphorus (from TPR) under contour ridge seemed more linked to high uptake of phosphorus from soil solution. The relative agronomic effectiveness (RAE) was higher under CR than HT. The powder form of TPR produced significantly higher value (P < 0.05) compared to the pellet form with respect to phosphorus use efficiency indices evaluated in the study. A second field experiment was conducted to evaluate the effects of different forms of Tilemsi phosphate rock: TPR+ (NH4)2SO4, TPR + KCl, TPR + (NH4)2SO4 + KCL and sole application of TPR as pellet on agronomic characteristics of sorghum and soil properties. The experiment consisted of three rates of phosphorus application (P0, P11, and P16) and four amended types of TPR factorially arranged in randomized complete block design (RCBD) with three replications. Combined application of TPR with sulphate of ammonia significantly (P < 0.001) increased grain yield in both seasons relative to the control. The impact of the soil amendments on grain yield followed the decreasing order: P+SAM > P+K+SAM > P+K > Pellet > Control in 2013. A similar trend was observed in 2014 except that pellet TPR produced higher grain yield than TPR + KCl. Increase in P application from 11 to 16 kg ha-1 increased the grain yield with percentage increment of 36 % and 21 %, respectively, in 2013 and 2014. The application of phosphate rock significantly influenced soil available phosphorus, exchangeable Ca, K and effective cation exchange capacity in both seasons. The combined application of TPR with sulphate of ammonia increased the dissolution of TPR and release of phosphorus due to high uptake of P rather than its acidifying effect.
A thesis submitted to the Department of Crop and Soil Sciences, Faculty of Agriculture, College of Agriculture and Natural Resources, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana, in partial fulfillment of the requirements for the award of the degree of Doctor of Philosophy in Soil Science.