Determination of lime requirement

Definition. The lime requirement of a mineral soil is the number of tonnes of calcium carbonate calculated to raise the pH of a hectare of soil 200 mm deep, under field conditions, to, and maintain at, 6.5. 

A low pH indicates that lime is required, but not the quantity. Excess is not only wasteful, but may render certain elements (e.g. Fe, Mn, and B) unavailable to plants. There are several methods for determining the lime requirement, including adding excess calcium bicarbonate and back-titrating the excess adding increasing amounts of calcium hydroxide and monitoring the pH and the use of a buffer solution (MAFF/ADAS, 1986, pp. 150-151), which will be described below. 

Calculation for mineral soils of pH 5.0-6.4. Subtract the indicated pH from 7.00 and multiply by 11.2. The result gives the lime requirement as tonnes ha calcium carbonate. 

Procedure for mineral soils of pH less than 5.0. Add 20 ml of double strength buffer solution to the soil suspension retained from the pH determination, and stir for 5 min. Mix 25 ml water and 20 ml double strength buffer with pH adjusted to between 6.9 and 7.1, and use to recalibrate the pH meter to read 7.00. Read the pH of the stirred sample. 

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DETERMINATION OF LIME REQUIREMENT OF SOIL (Schoemaker etal. 1961)... 

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