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Extractable Phosphate in Relation to the Forms of Phosphate Fractions in Humid Tropical Soils to the Humid Tropical Soils

B. Ahmed
Department of soil science, University of Dacca

A. Islam
Department of soil science, University of Dacca

Abstract/ Executive Summary:

An anion exchange resin methods of determining available phosphate were compared with the amounts of inorganic phosphate in different forms in ten Bangladesh soils. Correlation studies between the soil test values obtained by any two methods revealed that the correlation coefficient was usually higher when both test values were well correlated with a common phosphate fraction. This indicated a similarity of methods in selective dissolution of specific phosphate forms. Based on significant positive correlations between methods as well as their similarity in selectivity of dissolution of Ca-P,Al-P and Fe-P, the methods were classified into three groups, Truog, North Carolina and 0.3N HCl methods were grouped together, their extractants being capable of bringing Ca-P more effectively into solution and Al-P into solution satisfactorily.

Keywords: Extractants, Inorganic phosphate, Physico-chemical properties

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Program Area: Crop-Soil-Water Management

Commodity: Land management

Objectives:

This paper reports on the correlation coefficients calculated between the phosphate extractable by different methods and certain phosphate fractions and also on those between the extractable phosphate values determined by pairs of methods.

Methodology:

Samples from the upper 15 cm of ten Bangladesh soils, representing ten soil series and three important soil tracts of Bangladesh, were air dried and ground to pass through a 2mm screen. The soils, described previously (Islam and Ahmed,in press), provide a wide range of parent material, texture, drainage, colour, and other physico-chemical properties.Fractionation of inorganic phosphate was carried out following the procedure outlined by Chang and Jackson (1957) Soils were extracted for available phosphate by 0.002N H₂SO₄ buffered at pH 3 (Troug, 1930), sodium acetate and acetic acid (Morgan, 1937), NH₄F and HCI (Bray and Kurtz, 1945), 0.3N HCl (Baver and BRuneR, 1939), 0.025N H₂SO₄ and 0.05N HCl (North Carolina method, as used by the Soil Testing Division, North Carolina Department of Agriculture, Raleigh, North Carolina, U.S.A.), 0.5M NaHCO₃ adjusted to pH 8.5 (OlseN_{^{et al.}}, 1954), 0.01M CaCl₂(Schofield,1955), 0.1N ammonium acetate and 0.4N acetic acid (Egner et al.,1960) ,0.02N Na₂EDTA (HaNNa et al.,1962) and Deacidite FF SRA-66 anion exchange resin (Cooke and Hislop, 1963).

Source of Information: Trop. Agric. (Trinidad) Vol. 52, No. 2, April 1975

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Achievement/Findings:

The findings of this investigation are of practical value in relation to the choice of a chemical method of determining available soil phosphate for fertilizer recommendations. In the first place, if a certain method is used on a group of soils with the same phosphate distribution pattern, the correlation between available phosphate and crop response to fertilizer will be better than on a group of soils differing in their phosphate distribution patterns. Secondly, when the correlation between crop response and available phosphate determined by one method on a particular group of soils is established, other methods whose extractants are similar in selectivity of dissolution of phosphates can also be applied to this particular group of soils. Thirdly, in each soil, there may be one of two forms of inorganic phosphates which are the main sources of supply of available phosphate to crops. If the extractant used can dissolve these forms selectively, the available phosphate determined by this method will best represent the available phosphate status of the soil. For example, if A1-P is the main source of available phosphate in a rice soil, the 0.2N Na₂EDTA method might better reveal the available phosphate status of the soil. If Ca-P is the main source of available phosphate, acid extractants might afford a better measure, while if Fe-P is the main source, alkaline extractants might be preferable . If all the three forms of inorganic phosphates are equally the sources of available phosphate , the acid fluoride extractant of Bray and Kurtz might be the best choice.

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