Soils and trees

Soils are often given superficial treatment, and yet without them forests would quickly cease to function. As well as physically supporting plants, soils act as refuse collectors, processing organic waste and thereby recycling nutrients, a major influence on the productivity of forests. Without functioning soils, forests would rapidly be choked with dead wood and other material, and the bulk of nutrients needed by plants and animals would be locked up and unavailable.

Moreover, soil is not simply a loose collection of ‘dirt’, it is a complex mix of living and non-living components, consisting of air (soil gases: typically 25% by volume), water (25%), mineral particles (45%) and organic matter (5%); the last can be subdivided by weight into around 10% organisms, 10% roots and 80% humus. As described further in Chapters 1 and 7, various soil animals, such as earthworms and arthropods and the micro-organisms, including fungi and bacteria, decompose dead material to release nutrients and form the left-over, rather inert black humus of the soil.

Soil takes a long time to form, usually thousands of years, and its quality is one of the most important conditions governing the growth of trees, smaller plants and associated organisms in any site. As Fig. 2.1 demonstrates, soils have distinct morphologies each with a characteristic profile (a sequence of horizontal layers or horizons from ground surface to unaltered bedrock or sediment).