Extraction of Labile Soil Phosphorous

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Extraction of Labile Soil Phosphorous

The labile is that portion of the total soil P that is relatively loosely bound into or associated with soil minerals or amorphous minerals. Usually it is small percent of the total P and is related to the quantity of P that is utilized by the growing plants.

Selection of Extractant:

The extraction should:

1. Rapidly dissolve and or desorbs soil P and be time independent after 30 minutes or less.

2. Maintain organic matter and soil clay in a flocculated stage.

3. Avoid precipitation of dissolved P and or hydrolysis of organic P.

4. Contain no excess salts, buffers or ions that interfere with analytical determinations.

5. Extract meaningful quantities of other nutrient icons as well as P.

6. Be easy to prepare , store and dispose off.

While selecting as an Extractant for a laboratory one should always consider the degree of correlated of the Extractant with plant response to soil and fertilizer P.

Olsen P:

Olsen reagent ( 0.5 N NaHCO3adjusted to PH 8.5) extracts P from Ca-P by lowering the ca concentration by causing precipitation of Ca as CaCO3 and there by increasing the P concentration in solution. The reagent also extract some phosphate from the Al-P and Fe-P fractioner in soil. This method has been found widely applicable in slightly acid, neutral and calcareous soil. In the filtered extract P is complexed by adding ammonium molybdate and thereafter reducing the phosphor- molybdate complex in acidic media. The intensity of the blue colour an reduction provides a measure for the concentration of P in the test solution. The soil extraction ratio is 1: 20 and 30 minutes for shaking.

Bray Method:

This method primarily ment for soils which are moderately to strong acid ( PH around 5.5 or less).In this NH4 F-HCL combination ( 0.025 M HCL in 0.03 M NH4F) is designed as a measure of adsorbed plus acid soluble form of P. Soil extraction ratio 1: 10 shaking time 5 minutes. Comparison of stannous chloride and ascorbic acid methods used for determination of soil P.



Stannous Chloride

Ascorbic Acid






Effect of temperature on colour




Effect of doubling reductant




Effect of time and colour




Final acidity ( N)

0.7 +-0.04



Dilution ratio of 10 ppm solution for 50% T




Allowable range of acidity ( N) in ppm P aliquot



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