Effects of added biomass, soil pH and calcium on the solubility of Minjingu phosphate rock in a Kenyan Oxisol
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The effect of added biomass (dried leaves) of tithonia (Tithonia diversifolia [Hemsley] A. Gray), soil pH, and soil calcium (Ca) saturation on the dissolution of Minjingu phosphate rock (MPR) was examined in two incubation experiments in a Kenyan Oxisol. Anion exchange resin (AER) and mixed anion-cation exchange resin (ACER) were used to estimate plant-available P. Selected treatments at 2 and 16 weeks of incubation were sequentially extracted with 0.5 M NaHCO3, 0.1 M NaOH and 1 M HCl. AER-P from triple superphosphate (TSP) treated soil was initially higher than that from MPR treated soils (p < 0.001), but by the end of the incubation AER-P from the two P sources were not significantly different. Dried tithonia leaves at 10 tonnes/ha in combination with MPR (150 kg/ha) caused a significant (p < 0.01) reduction in AER-extractable P. Sequentially extracted NaHCO3-P and NaOH-P were lower in MPR + tithonia treatment compared with MPR only treatment, while HCl-P was higher in treatments containing tithonia. ACER-extractable P concentrations were higher than those of AER-P, and ACER-P from MPR was greater than that from TSP at all dates (p < 0.001). Calcium additions, either as CaCO3 or CaCl2, reduced AER-P significantly (p < 0.001) at all sampling dates. However, while CaCl2 addition reduced ACER-P, CaCO3 addition increased ACER-P. Sequential P extraction indicated that CaCO3 reduced MPR solubility more than CaCl2 did. The results indicate that Ca exerts a significant control on PR dissolution, with pH as an additional effect.