Greenhouse Evaluation of Agronomic Effectiveness of Unacidulated and Partially Acidulated Phosphate Rock from Kodjari and the Effect of Mixed Crop on Plant P Nutrition

Abstract

Greenhouse study was conducted to determine the agronomic effectiveness of unacidulated (KPR) and partially acidulated (KAPR) Kodjari phosphate rock relative to triple superphosphate (TSP). The available phosphorus of a ferruginous soil (pH 6.2) was labelled with a 32PO4 3– solution, and the soil was amended with 50 mg P kg–1 as ground KPR, KAPR and TSP. Two plants, maize and cowpea, were grown alone or in association. After 60 days of growth period, shoot yield, P uptake and real coefficient of P utilization (RCU%) were determined. Dry matter yields and P in dried tops increased by KAPR and TSP application; however, with KPR, no significant difference with the control was recorded. Also in KPR treatment, the dry matter yield and P uptake for maize and cowpea, grown in association, decreased in all treatments. The agronomic effectiveness of KPR, KAPR and TSP in terms of RCU% was classed in this order: KPR < KAPR < TSP. The average values of RCU% in KPR treatments of pure or mixed culture were very low (0.3%), indicating that KPR was not dissolved in the soil. In contrast, they were high in KAPR and TSP treatments. In KPR treatments planted with maize alone or in association, the RCU% values were higher than those measured with cowpea. This indicated that maize seemed to use KPR more efficiently than did cowpea due to organic acid anions excreted by its roots and higher root densities. At last, the association did not significantly increase the use of KPR by plants.