By D. W. Gilchrist Shirlaw and J. E. Nichols (Auth.)
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T h e calculation for calcium carbonate from the volume THE ANALYSIS OF SOILS 29 of gas produced is aided by a special slide rule. T h e recorded temperature is set against the barometric pressure. T h e number of milligrams of calcium carbonate equivalent to 100 ml of carbon dioxide are now read off against the actual volume of hydrochloric acid used. From this figure the milligrams of calcium carbonate in the soil are readily calculated; an example will make this clear: Volume of carbon dioxide produced: 35 ml.
Filtration can be avoided by the use of these tablets since they include a polyelectrolyte for flocculating the soil. 21. THE DETERMINATION OF LIME REQUIREMENT BY THE METHOD OF HUTCHINSON AND MACLENNAN The classical method for determining lime requirement is that of Hutchinson and MacLennan, which depends on the measurement of the quantity of calcium absorbed when an acid soil is shaken with a solution of calcium bicarbonate. Reagents required Approximately 0-02 normal calcium bicarbonate is prepared by placing an excess of calcium carbonate in a Sparklet syphon with distilled water.
A test on the charcoal for phosphate content should be made by shaking about 2*5 g with 100 ml of 0-5 normal acetic acid, filtering and testing for phosphate as below. This is much more charcoal than will be used in the test, but such a quanttity allows a margin of safety. If the charcoal extract is found to contain phosphate, it should be boiled with 50 per cent hydrochloric acid, filtered and washed until free of acid. T h e test for phosphate is then repeated. With some charcoal two or three treatments with acid will be found necessary.