PHOSPHORUS SORPTION STUDIES IN THREE TYPICAL SOILS OF NIGERIA

0
55

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.

PHOSPHORUS SORPTION STUDIES IN THREE TYPICAL SOILS OF NIGERIA

DOWNLOAD PROJECT