PHOSPHORUS SORPTION STUDIES IN THREE TYPICAL SOILS OF NIGERIA
PHOSPHORUS SORPTION STUDIES IN THREE TYPICAL SOILS OF NIGERIA
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Date
1994-11
Authors
UYOVBISERE, EDWARD O.
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Abstract
Phosphorus deficiency is widespread in tropical soils due
to intense weathering. Soil solution phosphates are often low
because Fe and Al oxides sorb P strongly. Phosphorus
availability can be improved by good management practices, but
requires good understanding of soil phosphate reaction
processes. A study was initiated to test the application of
phosphorus sorption isotherms for evaluating the phosphorus
requirement and supplying capacity of soils, and the lability
of P fractions. The objective was to generate prediction
indices, and determine the influence of OM on P sorption and
release to plants.
The study was carried out using three soils on basement
complex from Nigeria (two savanna Alfisols and a forest
Oxisol), some Fe oxide modified soils and two Danish soils.
The soils were acid to neutral in reaction and kaolinitic.
The first section considered P sorption reactions and
relationships with soil properties. Total P (PT ) content of
the surface soils ranged from 251 - 737 yg P/g with P0
constituting 25 - 40 % of PT . Phosphate reactions were largely
controlled by amounts and forms of soil oxides and OM. Though
OM seemed to enhance adsorption in amorphous systems, the
effect of Feox was greater than OM by 72 % to 30 % relative
effects. P sorbed decreased with OM and oxide crystallinity.
The Freundlich equation was superior, and modelled the
sorption data better over the P concentration range tested.
The Langmuir equation was however effective at low solution P
levels. The standard phosphate requirement (SPR) of the soils
ranged from 2 - 693 yg/g. The Alfisols sorbed less P, and were very low - medium sorbing, relative to the Oxisol (which
was medium - high sorbing).
The second section examined plant response to adjusted
solution P levels and the efficiency of the soil test methods
in the greenhouse. The usefulness of adsorption isotherm for
determining the P needs of soils was demonstrated. Critical P
levels for 95 % yield maximum ranged from 0.45 - 1.00 yg/ml,
and was in excess of the standard concept of 0.2 yg/ml. OM
enhanced yield, and the efficiency of P utilization was 2.5
times higher in soils with crystalline Fe oxide dominant.
The efficiency of the soil-test methods was influenced by
the buffer index. The resin method correlated best with plant
uptake and yield and was superior to other methods tested.
The critical and threshold soil-test P values are suggested.
The third section considered the depletion profiles and
the lability of the Hedley P fractions. The P depletion zone
exceeded 5.00 mm, and most of the P depleted at distances in
excess of 3.00 mm from the root was the P fraction. The P
o
acquisitive power of rape was greater than maize and both
plants seemed to differ in fractions preferentially depleted
under conditions of P stress. The results also confirmed the
high lability of the bicarbonate fraction, and P availability
in P deficient soils was dictated by the Po fraction.
The study reveals that most of the tropical soils from
Nigerian are low to medium sorbing, implying plant response to
moderate amounts of applied P. Organic matter management is
fundamental to improving the availability of P in the highly
weathered tropical soils, and estimating P needs of the soils
should be based on the Q/I concept.
Description
A Thesis Submitted to the Postgraduate School in Partial
Fulfillment of the Requirements for the Degree of
Doctor of Philosophy
in the
Department of Soil Science
Faculty of Agriculture
Ahmadu Bello University, Zaria
Nigeria
Keywords
PHOSPHORUS,, SORPTION,, SOILS,, NIGERIA.