By D. W. Gilchrist Shirlaw and J. E. Nichols (Auth.)
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3. Approximately normal potassium permanganate prepared by dissolving 3 g of potassium permanganate ,in 100 ml distilled water. 4. Hydrogen peroxide 20 vols. T H E ANALYSIS O F SOILS 49 Procedure 1. 10 ml of the soil extract are pipetted into a 15-ml centrifuge tube and 1 ml of saturated ammonium oxalate added. This precipitates the calcium as the oxalate. 2. T h e tube and contents are heated in a water bath at 90°C for 30 min; this increases the crystal size of the calcium oxalate and ensures complete separation.
T h e pH of the soil can now be read off from an indicator window in the front of the comparator. 19. THE DETERMINATION OF LIME REQUIREMENT BY WOODRUFFS BUFFER SOLUTION Most agricultural crops prefer a soil reaction of about pH 6-5 and, if the soil is more acid than this, some form of liming material is added. T h e weight of lime needed to bring the soil to pH 6-5 is termed the lime requirement. T H E ANALYSIS OF SOILS 35 A relationship exists between the pH of a soil and its lime requirement.
Clearly soils of the same exchange capacity with the same reactions will have the same lime requirements. /100 g (0-5 X clay percentage) and the percentage exchangeable hydrogen exchangeable hydrogen exchange capacity It has been shown that an increase in percentage hydrogen saturation of 15 will result in a drop in pH value of about 0-5 units. Thus at pH 6-5 the level of exchangeable hydrogen will be about 15 per cent; at a pH of 5-5 it will be about 45 per cent. 1 m-equiv. of exchangeable hydrogen will require 1 mequiv.
A Practical Course in Agricultural Chemistry by D. W. Gilchrist Shirlaw and J. E. Nichols (Auth.)