Comparative effects of minjingu phosphate rock and triple super phosphate on residual p in an ultisol

The use of relatively high rates of phosphorus (P) is known to result in a build up of P in soils and there are frequent claims that the use of phosphate rocks results in higher residual effects than water soluble sources. Thus the objective of this study was to assess residual P in treatments where relatively larger amounts of P from triple super phosphate (TSP) and Minjingu phosphate rock were applied. Total amount of P applied was 480 kg P /ha applied to Kanhaplic Haplustult at the Sokoine University of Agriculture farm. Residual P was assessed by Bray I extraction, sequential P extraction and growth of maize plants on soil samples collected from the treated soils as well as from the control treatment. The assessment of residual P was done seven years after P application was stopped. The results indicated that Bray I-P increased from 3.4 mg /kg in the control to 26.1 and 33.1 mg /kg in the MPR and TSP treatments, respectively indicating that MPR and TSP had comparable effects on residual available P. The total labile P increased by 45.3 and 50.2 mg /kg in TSP and MPR, respectively. The moderately labile P increased by 104.3 mg /kg and 77.0 mg /kg in TSP and MPR, respectively indicating that the residual P fractions from the two sources were different in P being held strongly by chemisorptions to Fe and Al components of soil surfaces. The increase in total recalcitrant P fraction was relatively higher in the MPR (114.4 mg/kg) than TSP (49.5 mg /kg) treatment. The pot experiment results showed that the P uptake from residual P in MPR and TSP treatments were comparable with 123.2 and 121.2 mg /pot, respectively but were significantly higher than the control treatments. Addition of fresh P applications into soil with residual P resulted in higher extractable P and P uptake than residual P alone. Recommended P applications from fresh TSP and MPR were comparable iniii maize P concentration and uptakes. The contributions of total labile P, moderately labile and HCl- P on P uptake were 58 %, 64 %, and 49 %, respectively. It was concluded that application of a relatively large rates of P either as MPR or TSP resulted in relatively high levels of residual available P seven years after P application was stopped but the levels achieved were below optimum under pot conditions.

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