Comparison of phosphate rock and triple superphosphate on a phosphorus-deficient Kenyan soil

Soil phosphorus (P) deficiency is a constraint to crop production in many r egions of sub-Saharan Africa, which could be overcome through use of either soluble P fertilizer or sufficiently reactive phosphate rock(PR). A field study was conducted with corn (Zea mays L.) for three growing seasons(18 mo nths) on a P-deficient, acid soil in Kenya to compare a soluble P source (t riple superphosphate, TSP) and relatively reactive Minjingu PR from Tanzani a. In the 18 months following application of 250 kg P ha(-1), bicarbonate e xtractable inorganic soil P (P-i) was higher for application of TSP than PR , but Pi extracted with a mixed anion-cation resin was comparable for TSP a nd PR. Inorganic P extracted by 0.1M NaOH, without prior extraction of resi n and bicarbonate P-i, decreased during the 18 months following TSP applica tion, but increased following PR application. After 18 months, about 7% of the added PR-P remained as Ca-bound P that was extracted with 1M HCl. The 1 M HCl extractable Pi, however, underestimated residual PR-P that gradually dissolved and supplied plant-available P, as indicated by recovery of <40% of PR-P added to soil in laboratory incubations even though PR solubility i n HCl was >90%. Minjingu PR was an effective source of P for corn. Corn yie lds were comparable for TSP and PR, and the relative agronomic effectivenes s of PR averaged 107% in Season 1 and 79% in Season 3. Anion resin and mixe d anion-cation resin appeared to be superior to bicarbonate and NaOH as a s oil P test for use with both TSP- and PR-treated soils.