Report of the Committee on Soil Acidity

Soil acidity or rather soil reaction continues to be cne of the most important subjects to be considered in soil survey work. The International Society of Soil Science has a committee on soil reaction rather than soil acidity. Both the mineral and organic colloids of soils are largely amphoteric and may be either acid, alkaline or neutral in reaction. Also as there may be replaceable hydrogen ions on either side of the neutral point, it is rather difficult to confine soil acidity studies only to acid soils of pH less than 7. At one time any acid soil was considered inferior to neutral or high lime soils, consequently soil acidity was thought of as a condition that should always be corrected 100 per cent. We now know that a certain degree of soil acidity is often desirable. The question is not, how much is the lime requirement to make an acid poil neutral. It is rather, what is the reaction of the soil. In addition to soil reaction we wish to know something of the character and proportion of the organic and mineral colloids, the percentage of base saturation, the exchange capacity, the buffer capacity, etc. It is desirable to know the pH of a soil, not so much because the H ion may be toxic but because of the company the H ion keeps. For instance, toxic aluminum, manganese, boron, tine, and other substances are found in the soil at certain H ion concentrations and not at others. Very high or very low pH is undesirable, but within the middle range the best pH of a soil depends partly upon the crop and partly upon the texture of the soil and the character of the mineral and organic colloidal fractions. The mineral acidity of soils is confined almost, if riot entirely, to colloidal silicates, principally of aluminum. Sometimes iron and manganese silicates may be present in acid soils. Certain mineralogists contend that these acid mineral substances are alumino-silicates; that is, acid salts of various silicic acids and aluminum. Others claim that they are true acids or rather double acids in which aluminum as well as silica plays an acid role. Silicoaluminic acids would form distinct classes of salts as do phosphomolybdic acid or phosphotungstic acid. The following table is a partial list of some of these alumino-silicates mentioned in the literature and which may occur in soils. All these natural minerals are crystalline. The more acid ones tend to have a higher proportion of silica and water of constitution. A sample of kaolinite containing 41.4% A1203 46% Si02 12.5% H2O was not acid and had no base exchange properties. After digesting in dilute H Cl and washing, it had 38.6% A1203 49.1% SiO2 and 13.1% H20 and was acid with some base exchange properties. Theoretical kaolinite would have 39.5% Al2Os, 46.5% Si02 and 14% H2O and is not acid. Acid substances sold at Kaolinite probably contain halloysite. Kerr reported an acid washed dialized permutite with an acidity pH 3 or less. X-ray analyses of the silicates are rapidly increasing our knowledge of their structure; much is yet to be discovered, however. The exact position in the lattice of hydrogen, upon which depends the acidic or basic nature is most difficult to work out. We do know that soil alumino-silicates, although crystalline, may be colloidal in dimensions, at least in one direction. Their form is often that of sheets or plates of colloidal thickness. Those with replaceable hydrogen are called acidoids rather than acids. There is a possibility that they may change from one silica, alumina, water ratio to another or they may take on or drop potash, calcium or other bases depending upon climatic conditions. • • • • ' • ' • : In tropical countries of high rainfall, silica is washed out of the complex by large amounts of pure rainwater, more rapidly than iron and alumina, forming lateritic soils. Even with lateries, however, some alumino-silicate of high silica, high water ratio remains, as such soils like the Nipe clay are distinctly acid, much of the iron ind alumina remaining as inert oxides. These soils as a whole are quite high in sesquioxide bases and rather low in silica. In humid temperature regions, the slow decomposition of surface organic deposits together with a medium high rainfall allows the soil to be leached with carbonated water. This water in equilbrium with • COz removes the -bases more rapidly than the silica. The alumino-silicates then take an acidic character.