Soil supports all life on our planet, including the abundant wildlife found in tropical rainforests.

Weathering

Rocks are turned into soil through weathering.

Mechanical weathering is the process by which large rocks are broken down into small particles.

Water falling on rock can eventually break off fragments. Flowing water, charged with abrasive rock particles, is extremely destructive. The sun, as it heats and cools rocks, causes particles to become detached from the surface layers. Winds bearing sand grains erode rock.

In the tropics, chemical weathering plays an important role in soil formation.

Water slowly dissolves rock. If water slightly acidic because it contains dissolved carbon dioxide, it dissolves rock more easily.

Some soil minerals are much more difficult to dissolve than others. In the temperate regions, for example, silica (quartz or silicon dioxide) is the most resistant mineral. Because of its great abundance, it is the most common residual mineral in soil and forms the bulk of sand deposits.

Silica is insoluble in cold water, but as the temperature of water rises, it dissolves more easily.

In the tropics, as water percolates through the soils, a great deal of silica is removed. However, since it is present in such vast quantities in the original rocks, it is still common in the soils.

Other minerals, such as aluminum oxide and iron oxide, which are plentiful in soils, do not become much more soluble as temperatures rise. Therefore, they can be found in substantial amounts in tropical soils.

This accumulation of aluminum oxides helps explain why so many of the world's reserves of aluminum ore (bauxite) are in the tropics. Pure aluminum oxide is white, but iron oxide is reddish or yellowish, and gives the tropical soils their distinctive red or yellow hue.

Composition of Soil

Soil is part solid, part liquid and part gas.

The solid fraction, which makes up between 40 percent and 60 per cent of the volume of the soil is composed mainly of fragments of rock, ranging in size from boulders and pebbles to the minute pieces that make up clays.

The rest, the pore space, contains either gas or water, the relative amounts varying from time to time.

The gas, or soil air, is like the atmosphere in composition, but contains more carbon dioxide because of the respiration of roots and other living things.

The liquid, or soil water, is not pure. It is a solution that contains minerals and organic substances, some of which can be taken as nutrients by the plant.

Nutrients

Most tropical soils have developed on ancient land surfaces that have been exposed to weathering for millions of years.

By temperate standards, they are extremely deep. The unweathered rock may be covered by 1 to 200 feet (30cm to 60m) of weathered material, from which at least some of the micronutrients have been removed by percolating rainwater.

Of those micronutrients which are left, a high proportion are not in the soil, but held in the plants. This has obvious dangers. If man clears the forest by cutting the trees and burning them, a sudden rainstorm can wash away the ash and with it, the mineral nutrients which were stored in the plants.

Some tropical forest soils are not poor in nutrients. Forests in Indonesia, and in parts of Central and South America, are occasionally sprinkled with volcanic ash. This finely divided unweathered rock is a source of mineral nutrients.

In temperate regions, large areas of soil have developed since the last Ice Age, either on material eroded out and deposited by glaciers, or on older soils that were thoroughly mixed by the freezing and thawing of ice within them. Therefore, they are relatively rich in nutrients.

The most important nutrients found in soil are nitrogen, phosphorus and potassium

Of the mineral nutrients that plants need from the soil, the most important is probably nitrogen fixed as ions such as nitrate. Elemental nitrogen occurs abundantly in the air but is unreactive, and so is often in short supply in forms that plants can use.

In tropical rainforests, nitrogen comes from the decomposition of organic matter and from the air, either through the activities of blue-green algae and bacteria, some of which live on leaf surfaces, or from the combination of nitrogen and oxygen during the intense heat of lightning flashes.

When plant material is burned, much of the nitrogen is lost to the air. Within the plant, nitrogen forms part of many essential chemical compounds, including proteins.

Another vital element needed by plants is phosphorus. It originates in the soil as a result of the weathering of rocks, but is always scarce. Within the plant, it is used in many vital compounds including the nucleic acids DNA and RNA and the vital energy transfer molecule, ATP.

The third essential element, which also results from rock weathering, is potassium. Plants require a great deal of for normal, rapid growth and enzyme activity.

In addition to these elements, there are about twenty others which are also vital to a plant but which are not usually in short supply.

Soil Decomposition

In the tropical rainforest, soil animals and plants quickly decompose all of the material that falls onto the surface of the soil

Termites and earthworms play a valuable part in initially breaking up litter and incorporating it into the soil.

Bacteria and fungi then break down most of the organic matter.