Studies on soil reaction: VI. The interaction of acid soils, calcium carbonate and water, in relation to the determination of “lime requirements.”

The variations in Hutchinson-MacLennan “lime requirement” with the amount of soil and calcium bicarbonate solution are shown to be connected with the buffer action of the soil. Indirect titration curves can be derived from the calcium bicarbonate results, and show a systematic divergence from the direct electrometric titration curves, owing to the variable calcium concentration of the final bicarbonate solutions. In the presence of calcium chloride both methods show lower pH values for a given base absorption and yield almost identical titration curves. The Hutchinson-MacLennan “lime requirement” is always less than the equivalent of the amount of calcium hydroxide necessary to give a neutral suspension in the electrometric titrations. The calcium bicarbonate solutions at equilibrium are always more acid than pH 6·2, but the “salt effect” tends to give results corresponding to a somewhat higher degree of neutralisation. “Lime requirements” should be obtained by interpolation to some arbitrary concentration, and an empirical relationship is given by which the interpolation may be made from a single determination. The Hutchinson-MacLennan method can give no indication of the intensity of soil acidity, but serves to estimate the amount of lime necessary to give a considerable reduction of acidity.