The influence of soil pH on plant nutrient availability is demonstrated
in Figure 20.4. It can be seen that for mineral soils the most favourable
level for ensuring the availability of all nutrients is pH 6.5, while peat
soils should be limed to pH 5.8 in order to maximize overall nutrient
availability. In acid conditions some nutrients, such as manganese, and
other soil minerals, such as aluminium, may become toxic.
Other effects on soil
Beneficial soil organisms are affected by soil acidity and
liming. A few soil-borne disease-causing organisms tend to occur more
frequently on lime deficient soils (see clubroot), whereas others are
more prevalent in well-limed soils. Calcium sometimes improves soil
structure and soil stability. It is probable that this is mainly because
it encourages root activity and creates conditions favourable for
decomposition of organic matter, yielding humus. Free lime
in clay soils sometimes, but not always, leads to better crumb formation
on drying and shrinking.