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

Abstract

Soil phosphorus (P) deficiency is a constraint to crop production in many regions 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 months) on a P‐deficient, acid soil in Kenya to compare a soluble P source (triple superphosphate, TSP) and relatively reactive Minjingu PR from Tanzania. In the 18 months following application of 250 kg P ha‐1, bicarbonate extractable inorganic soil P (Pi) was higher for application of TSP than PR, but Pi extracted with a mixed anion‐cation resin was comparable for TSP and PR. Inorganic P extracted by 0.1M NaOH, without prior extraction of resin and bicarbonate Pi, decreased during the 18 months following TSP application, 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 1M HCl extractable P., 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 in HCl was >90%. Minjingu PR was an effective source of P for corn. Corn yields were comparable for TSP and PR, and the relative agronomic effectiveness of PR averaged 107% in Season 1 and 79% in Season 3. Anion resin and mixed anion‐cation resin appeared to be superior to bicarbonate and NaOH as a soil P test for use with both TSP‐ and PR‐treated soils.