Plant distribution

There are a number of soil conditions and factors that affect which plants will grow where. The most important ones are as follows:

Nutrients

There are a number of nutrients essential for plant growth. These have been described in Soils and health. macronutrients that are needed in relatively large amounts and the micronutrients, including the trace elements, needed much smaller amounts. The nutrient supply in soils varies from one soil to another.

Depending on the nature of the parent rock, some soils will be more deficient in nutrients than others. Furthermore, in soils in high rainfall areas the nutrients tend to be leached out leaving soils capable of supporting only plants that can make do with relatively few nutrients plant growth can be retarded by the low availability of one or more nutrients and this will affect their ability to compete with plants more suited to the conditions. In other situations, such as low rainfall areas there may be accumulations of salts leading to salinisation, a superabundance of a particular element, for example sodium, and this again can be tolerated by only a small range of plants.

Water supply

There are two situations in which soils can be affected by too much water, at least seasonally. Some low lying soils are subject to variable heights of water table and particularly in wet seasons the water table can rise into the upper parts of the soil which then becomes waterlogged.There are also many soils which are relatively impermeable, particularly those developed in clay strata. In the wet season these develop what is known as a perched water table, due to high rainfall but low permeability, i.e. rainfall reaches the soil faster than it can flow through it and it is held up in the soil.

Oxygen supply is depleted in waterlogged soils and carbon dioxide concentrations increase. Under these conditions the waterlogged parts of the soil can accumulate potentially toxic amounts of substances such as aluminium, iron, manganese and sulphur. There are plants that have special techniques that enable them to exist in these situation and such plants will make up the flora of soils subject to waterlogging but most plants are not adapted to these conditions.

Of course, there are also many areas of the world, particularly the arid zone, where there is a deficiency of water. Nutrients may be present in abundance but there is insufficient water for plants to survive but even here some plants are able to adapt to the conditions. Some species survive by developing a highly branched, near-surface rooting system which intercepts the water on the few occasions that it rains.

Other plants have long, deep-penetrating roots which reach down to deep water tables.

Acidity

Plants that can tolerate acid conditions are known as calcifuges.The major effects of soil acidity on plant life are twofold. In acid soils, i.e. those with a pH below 5.0, the availability of nutrients to plants can be restricted, particularly nutrients such as calcium, magnesium, potassium, phosphorus and boron, nitrogen, sulphur, copper and zinc.

Many plants find it difficult to exist at this low pH level because of the low level of availability. When acidity is coupled with poor drainage and periodic anaerobic conditions, some elements are most available, e.g. aluminium, iron and manganese, and may be present in amounts that are toxic to plants, especially if the soil is waterlogged.

High levels of exchangeable aluminium in these conditions has been shown to restrict root growth of some plants and hence their ability to compete with other more adaptable flora. The pH of the soil also influences the capacity of organisms to undertake their roles in breaking down organic remains and releasing nutrients for uptake by plants. Below pH5 the activity of both fungi and bacteria is reduced and the suite of fungi and hacteria more limited. In this situation the breakdown of organic matter is slower and therefore fewer nutrients become available to plants in more acid conditions.

With less acidity many nutrients become more available and this suits a large number of plant species. The pH range 5.5-7.0 is the most suitable for most plants.

Calcareous Soils

These are soils that occur on limestone, chalk and other deposits rich in calcium carbonate. The pH of the soil is usually above 7.0. Some species of plants, known as calcicoles are able to tolerate a high pH but they are usually species particularly adapted to high pH conditions. Although there can be nutrient deficiencies on calcareous soils, conditions are rarely toxic. A number of plants are well adapted to calcareous soils.

Saline Soils

Compared with calcareous soils, saline soils are much more restrictive with respect to plant growth and tolerance of species. Saline soils contain accumulations of salts including chlorides and sulphates of sodium, calcium and magnesium, the amounts of which are sufficient to affect plant growth. The pH of saline soils is usually above 8.5.

At this pH there can be deficiencies in copper, manganese, iron and zinc, which limit plant growth. Given the high pH and the deficiences which result, the range of species that can tolerate the saline conditions is very limited.