Science, Maths & Technology

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Practising science: Reading the rocks and ecology

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# 2.2.1 Soil pH

pH (a measure of acidity or alkalinity) is an important environmental factor, particularly in soils. Soil is derived partly from accumulated decaying vegetation and partly from broken up fragments of the underlying rocks. Soil pH is determined by both these components and also by the water that fills the spaces between solid soil particles.

How might you expect the pH of soil overlying limestone (or chalk, which is a particular form of limestone) to compare with that of soil overlying sandstone? The soils would have different pH values. Limestone is a calcareous rock rich in carbonates (usually calcite), which gives rise to soils of pH of 7 and above. Sandstones contain much silica and so give rise to neutral or slightly acid soils, where the pH can be as low as 3.5.

Figure 22 shows the distribution of earthworm species in soils of different pH. You can see that some species have wide pH tolerances, whereas others have much narrower ranges, being specialist species of either acidic or more neutral soils. Species of plants and animals that are found only in alkaline environments are termed calcicoles (i.e. organisms that like a calcium-rich environment), whereas those that dislike these soils are called calcifuges (i.e. calcium escapers).

Figure 22 Distribution of earthworm species in soils of different pH. (B.=Bimastos; D.=Dendrobaena; L.=Lumbricus; O.=Octolasium; A.=Allobophora.)

pH is a critical factor for plants, if only because it affects the solubility of mineral nutrients and hence their availability to plants. Figure 23 shows, in the form of ‘kite’ diagrams, the relative availability of several minerals at different pH values. A mineral is most available at pH values where its ‘kite’ is widest and least available where its ‘kite’ is narrowest or non-existent.

Figure 23 Availability to plants of some mineral nutrients in soils of different pH

## Question 6

Which minerals are most available, and which are least available, (a) at pH 7.5–8.5 and (b) at pH 3.5–4.5?