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The Use of Sodium Edta as an Extractant for Determining Available Phosphate in Soil

B. AHMED
Department of Soil Science, University of Dacca, Dacca

and A. ISLAM
Department of Soil Science, University of Dacca, Dacca

Abstract/ Executive Summary:

The use of neutral solution of 0.0025M Na₂ EDTA as an extractant in the assessment of soil phosphate available to plants is described. In the soil types used, a high correlation was found between EDTA extractable soil phosphate and plant responses, as measured by phosphate content of dry matter. The dry-matter yield of plants was also closely correlated with PEDTA values. The method provides a satisfactory estimate of the availability of phosphate to wheat plants in the soils under investigation.

Keywords: Chloromolybdate, Phosphate, EDTA

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Program Area: Crop-Soil-Water Management

Commodity: Fertilizer

Objectives:

The present paper reports a simple procedure for determining available phosphate in soil samples using neutral 0.0025M Na₂ EDTA as an extractant. Several other chemical methods were also compared with this method to assess their relative efficiency

Methodology:

Samples of surface soils (0-15 cm) differing in pH and texture were collected from ten different sites in Bangladesh. These were air-dried, ground and passed through a 2-mm sieve. Soil samples were extracted for available phosphate using neutral 0.0025M Na₂ EDTA solution as well as the extractants described by Truog (1930), Morgan (1941), Bray and Kurtz (1945), Olsen et al. (1954) and Hanna et al. (1962).A quantity of 20-ml neutral 0.0025M Na₂ EDTA solution was added to 1 g soil sample in a 50-ml centrifuge tube and shaken for 30 min in an end-over-end shaker. The tube was centrifuged and phosphate determined in the supernatant. In all the methods phosphate was estimated by the “Molybdenum Blue” method (Jackson, 1958). A chloromolybdate system was used for all methods except Truog’s which requires a sulphomolybdate system. Stannous chloride was thr reducing agent. For each soil type, triplicate 500-g samples were weighed into plastic pots (10×9 cm) and each received a basal dressing of 40 mg N and 110 mg K as KNO₃. Moisture content was adjusted to 60% field capacity using deionized distilled water, and held at this level throughout the experiment. In each pot 0.25 g og wheat seeds (variety: Mexi-Pak) was sown. Plants were harvested after 21 and 37 days; dried for 24 h at 70^oC, and milled to pass a 1-mm screen. Following digestion with nitric and perchloric acids, the residue was taken up in HCL and phosphate determined colorimetrically using the chloromolybdic acid system (Jackson, 1958).

Source of Information: Geoderma, 14(1975)261-265, Elsevier Scientific Publishing Company, Amsterdam – Printed in The Netherlands

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Achievement/Findings:

PEDTA values and the phosphate contents of dried plant material were highly correlated. The correlation of PEDTA value with dry-matter yield is somewhat lower but still significant at the 0.1% level. Significant correlations (0.1%-2.0%) were obtained between PEDTA values and P extracted by all other methods. This suggested a similarity in selectivity of dissolution of inorganic phosphate fractions by these methods. The efficiency of Na₂EDTA as an extractant might be due to the chelation of Fe and Al in acid soils or Ca and Mg in alkaline soils by these ions. It has been showed that the ability of an ion to prevent phosphate fixation or release of fixed phosphate depends on the stability of the complex or precipitate formed by the anions and the specific cation involved. Na₂EDTA extraction of soil phosphate thus seems to be useful in predicting uptake of phosphate as well as dry-matter yields of wheat plants for the range of soils studied.

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