Greenhouse evaluation of phosphate fertilizers produced from Togo phosphate rock

Partial acidulation of phosphate rock (PR) or compaction of PR with soluble P fertilizers represents a means of producing P fertilizers from indigenous PR sources that may otherwise be unsuited for use as a P fertilizer. In the present two greenhouse experiments, we compared the agronomic effectiveness using a relative agronomic efficiency (RAE) index for the following P fertilizer sources: (i) ground Togo PR; (ii) partially acidulated phosphate rock (PAPR) at 50% acidulation with H2SO4; (iii) PR compacted with triple superphosphate (TSP), urea, and KCl at P ratio of PR: TSP = 50: 50; and (iv) commercial‐grade single superphosphate (SSP). The soil used was an acid Bladen sandy loam (Typic Albaquult) limed to pH 5.5 prior to cropping with maize for 6 weeks or cowpeas grown to maturity. Rates of P applied were 0, 50, 100, 150, and 200 mg P/kg. All treatments received adequate rates of N and K, as well as secondary and micronutrients. The results showed that ground Togo PR was an ineffective P source for both maize and cowpeas. The RAE values were not significantly different from those for the control (no P added). For the PAPR and compacted (PR + TSP), however, the RAE values with respect to SSP were 72.5% and 84.7%, respectively, for increased dry‐matter yield of maize and 87.7% and 97.1%, respectively, for increased cowpea seed yield.