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
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Date
1993
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Sokoine University of Agriculture
Abstract
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.
Description
Dissertation
Keywords
Phosphate retention, Soils, Soil minerals