A Guide to the Meaning of Soil Analysis
To interpret analytical data fully in terms of crop requirements, many factors must be taken into account, including the type of soil, its climatic environment, and its recent history (this is why the information asked for on the Soil Sampling Information sheet is so important). It is the agricultural chemist’s job to do this when he/she makes their fertilizer recommendations, but the farmer and the agronomists often like to have a broad idea of what the figures mean so that they can compare the nutrients status after a number of seasons or between different lands. The following notes may help them to appreciate the significance of the analysis.
pH values (Calcium Chloride scale)
Soil acidity is described quantitatively by the ‘pH value’. The accurate measurement of soil pH is not nearly as simple as most people think. The value obtained varies considerably with the amount of water that is present under field conditions. The Chemistry and Soil Research Institute has for many years been using a method of determining soil pH which employs a dilute solution of calcium chloride (M/100 CaCl2), instead of distilled water as in some conventional methods. The calcium chloride method gives much more accurate lab results and, more important, it is a truer measure of what the soil acidity will be under field conditions during the growing season. This is because it is not appreciably affected by the amount of water present, or by fluctuations in the salt content of the soil which are brought about by the use of fertilizers and manure.
Values obtained by the calcium chloride method, however, can be directly compared with those obtained by the water method, which are the ones that many people are accustomed to think and which are commonly referred to when optimum or desirable pH ranges are described in agricultural publications from overseas. On an average, the calcium chloride pH is about 0.7 units lower than water pH (1.5 suspensions), but in individual soils the difference may be considerably lower than this figure, and it is in such cases that the calcium chloride value is a much more reliable indication of soil acidity under field conditions.
It is therefore, necessary to draw up a scale of desirable and critical pH ranges by which to interpret the significance of the calcium chloride pH values for Zimbabwean soils.
Local experience shows that the following may be safely used as a broad guide: -
Above 7.5 -Strongly alkaline. Usually unsatisfactory and requiring investigation and special treatment.
6.5 – 7.5 -Alkaline. Usually on account of the presence of free lime. Adequate for most crops, though higher than desirable.
6.0 – 6.5 -Neutral. Highly unsatisfactory for lucerne, clovers, wheat, and barley, satisfactory for most other crops, but higher than is desirable for tobacco, maize etc.
5.5 – 6.0 -Slightly acidic. Highly satisfactory for almost all crops, including tobacco Lime is not required except in special circumstances.
5.0 – 5.5 -Medium acid. Satisfactory for most crops, but to maintain pH in this range under heavy cropping, liming will be necessary at a suitable stage in the rotation, especially in better rainfall areas.
4.5 – 5.0 -Strongly acidic. In the lower part of this range there is a progressive risk of fertility being adversely affected, and lime is therefore required.
Below 4.5 -Very strongly acidic Severe infertility is likely and liming is essential before planting.
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