By D. W. Gilchrist Shirlaw and J. E. Nichols (Auth.)

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Procedure 1. About 20 g of soil are measured out into a 50-ml beaker. 2. Sufficient distilled water to soak the soil is added. 3. T h e soil is now stirred with a glass rod. 4. e. the paste should flow, but should not be so wet that water stands on the surface. 5. T h e pH is set with a buffer solution according to the maker's instructions. 6. T h e reaction of the soil paste is determined by placing the electrodes of the pH meter into the paste in at least four different places. T h e average value of these four determinations is taken as the soil pH.

2-0 X organic matter per cent) + (0-5 X clay per cent). Based on a soil of pH 6-5 having an exchangeable hydrogen percentage of 15. 3 Based on a soil of pH 5-Q having an exchangeable hydrogen percentage of 60. 23. LIME REQUIREMENT AND AVAILABLE CALCIUM T h e fourth method for estimating lime requirement will be discussed later since it is based on the level of available calcium in the soil and the method for this determination is yet to be described (see sect. 28). T H E DETERMINATION OF NUTRIENT STATUS T h e available nutrients represent a variable proportion of the total nutrients in the soil and it is not possible to assign to them particular chemical compounds.

3. T h e flask is attached to the rest of the apparatus, care being exercised that all the rubber connections are gas tight. 4. T h e flask is now tilted so that the acid comes into contact with the soil. T h e carbon dioxide evolved displaces water in the 500-ml flask equivalent to its own volume. 5. Since this equipment does not allow for the effects of changes in atmospheric conditions it is necessary to repeat the determination using 1 g of pure calcium carbonate. This standardization must be repeated regularly during the course of a series of determinations.

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