4. CHEMICAL PROPERTIES OF SOIL
4.0 The chemical qualities of soils change with time
The weathering of the parent material by water determines, to a large extent, the chemical composition of the soil which has ultimately been produced. Some chemicals are leached* into the lower soil layers where they accumulate. Other chemicals, more insoluble, are left in the upper layers of the soil. The most rapid removed chemicals are chlorides and sulphates, followed by calcium, sodium, magnesium and potassium.
The silicates and oxides of iron and aluminium decompose very slowly and are rarely leached*. When some of these products come into contact with the air in the soil, chemical reactions occur, such as oxidation in particular, which results in the formation of chemicals either more soluble or more fragile than the original ones. This results in an acceleration of the weathering processes, increased leaching* of chemicals, and further changes in the soil chemical composition.
A freshwater acid sulphate soil (see Section 1.8) may develop when the waterlogged soil containing iron sulphurs (pyrites) is exposed to the air, for example, when constructing ponds. Oxidation of the pyrites takes place and acidification of the soil occurs. The pond water may then become too acid for fish culture (see Section 4.2).
The air present in the soil also contains carbon dioxide. This gas combined with water can form a weak acid (carbonic acid) which will then react with some of the soil chemicals to form new ones.
4.1 The chemical reaction of the soil (pH)
What does pH mean?
Soils may have either an acid or an alkaline reaction, or may be neutral. The measure of the chemical reaction of the soil is expressed by its pH value. The pH value varies from 0 to 14, with pH = 7 indicating that the soil has a neutral reaction. Values smaller than 7 indicate acidity and values greater than 7 indicate alkalinity. The further from the neutral point, the greater the acidity or the alkalinity.
How do you measure pH?
The most accurate method of determining soil pH is by means of an electrical pH meter which gives a direct reading of the pH value when glass electrodes are inserted in a solution obtained by mixing one part of the soil sample with two parts of distilled water. Such equipment is available in soil testing laboratories.
As a general indication of the soil pH, litmus paper and colour indicators can be used in the field. Litmus paper which turns red in acid conditions and blue in alkaline conditions is relatively cheap and may usually be bought from a chemist. It is partly dipped in a soil suspension made by mixing one part of soil with two parts of distilled water or, if necessary, pure rainwater directly collected in a clean container. Soil-testing field kits are also available including a number of colour indicators. Normally, a small sample of soil is mixed with some distilled water and a chemical, as indicated in the instructions. A few drops of a colour indicator are added. The colour of the solution changes and this new colour is compared with a chart supplied with the testing kit, from which the pH value is obtained.
4.2 A particular case: freshwater acid sulphate soils
Actual and potential acid sulphate soils
Actual acid sulphate soils are not common. They may be easily identified in a soil profile by two important characteristics:
Potential acid sulphate soils are far more common. They are defined as waterlogged, unconsolidated soil material which will become acid sulphate upon drainage and exposure to the air. Their pH varies around 5 to 6. However, oxidation through chemical and biological processes will result in the acidification of the soil, the pH reaching 4 or even less within a few months.
Note: if kept submerged, potential acid sulphate soil will never become acid sulphate. It is the exposure to the air which causes such a development.
How to identify a potential acid sulphate soil
It is important to identify a potential acid sulphate soil during a pond site soil survey. It may then be possible to plan the pond construction so as not to expose such a type of soil to the air and thus avoid the strong acidification of pond dikes and pond water.
To identify a potential acid sulphate soil, proceed as follows:
Note: it is important to keep the soil sample moist to ensure a high bacterial activity and a more rapid acidification. In dry samples, minimum pH will be reached only after several months.