Browsing by Author "Assenga, Rufini Hassan"

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    Phosphate retention and the effects of retained phosphate on the pH, CEC, AEC and ZPC of soils with high contents of variable charge soil minerals from Tanga, Tanzania
    (Sokoine University of Agriculture, 1993) Assenga, Rufini Hassan
    A laboratory study was undertaken to investigate on the phosphate retention and the effects of retained phosphate on the pH, CEC, AEC and ZPC of three soils, namely Rhodic Kandiustox, Rhodic Haplustox and Typic Haplustox from Tanga, Tanzania. These highly weathered and leached soils have high contents of variable charge soil colloids (VCS) dominated by kaolinite and iron and aluminium oxides and hydrous oxides, with low to very low pH (< 5.5), low CEC (< 10 c.mole/kg soil) and aluminium saturation between 15 to 25%. The phosphate retention capacities of the soils ranged from 1000-1400 mg P/kg in the order Rhodic Kandiustox Rhodic Haplustox < Typic Haplustox. There were high and positive correlations between the amounts of phosphate retained and the contents of iron and aluminium oxides and the 1:1 layer silicates. The phosphate adsorption data did not conform to the linear transformations of the Langmuir and Freundlich equation models. Additions of up to 1600 mg P/kg to the soils significantly increased the pH of the Rhodic Kandiustox, Typic Haplustox and Rhodic Haplustox by 0.1, 0.4 and 0.05 pH units, and the CEC by 103%. 66% and 61%, respectively. Phosphate retention by the soils at 1600 mg P/kg soil reduced the AEC and lowered the ZPC of the Rhodic Kandiustox, Rhodic Haplustox andTypic Haplustox by 1.S0, 1.10 and 0.55 pH units, respectively. The effects of added and retained phosphate on the pH, CEC, AEC and ZPC of the soils could be attributed mostly to the charge reversal through specific ligand exchange reactions between the phosphate ions and the aquo and hydroxo groups on the VCS colloids. The resultant increases in pH and CEC with phosphate retention appears to be very small in relation to the large amounts of phosphate added to the soils. However, the amounts of P fertilizers required to effect such changes in pH and CEC could be minimized if the P fertilizer materials are banded.