Water and nutrients

Most trees do better when there is adequate soil water replenished at regular intervals. Most trees do not like extremes unless they are particularly adapted to them. There have been many experiments showing that trees growing in soils at field capacity, have a much healthier rooting structure, particularly in terms of root length, than in dry soils. Maximum root growth therefore occurs in soils which are regularly at field capacity. Some soils become flooded from time to time. Although a few tree species are adapted to such conditions, long periods of flooding and high soil water tables will affect growth. Problems arise when the soil is so wet that there are insufficient air-filled pores to transfer oxygen from the air down to respiring roots, as well as to take toxic gases away from the roots. For the purposes of most trees a well structured soil allowing ready root gowth (roots 'prospect' the soil), combined with adequate water supply are ideal conditions for most trees.

Forests rely on soils to 'manufacture' the nutrients that they need. In all forest soils there will be a thin, darkish topsoil in which nutrients are extracted from dead plant material and transformed into the nutrients used to maintain the living forest above. In the topsoil, and the deeper subsoils, weathering of minerals will also release nutrients. Trees rely on both sources of nutrients to supply what is necessary for their growth. Unlike in agricultural soils where fertiliser is added regularly to top up nutrient levels by the farmer, in most forest situations nutrients for the trees have to come from stocks in the soil. In some soils the demand for nutrients from the tree population is huge, such as in the tropics. In other climatic situations, such as in the cold Boreal areas, the requirements of the tree population are much smaller and the prevailing climatic conditions are such that the turnover over of plant remains and the weathering of rocks to release nutrients can be very slow indeed.

In recent years, there have been big expansions in commercial forestry. Here, forestry managers can be more selective than nature as to where trees are planted and on what soils. Forest managers can also modify soils before planting, for example by deep ploughing and by breaking through any hardpans that may occur. Also, soil maps are an important aid to the commercial forester, providing information about the type of soils, their depth and other characteristics, enabling the forester to be selective about what species is planted, thereby creating better possibilities for the successful generation of woodland.