What Is Soil?

By Dr. Patricia M. Fraser
Soil Scientist
New Zealand Institute for Crop & Food Research

What is soil? Soil is many things to many people. It is the underlying foundation of our houses, factories, and motorways as well as a filter for human, industrial and animal wastes. Still, to most people, soil is the natural medium in which plants grow. Since early in the 19th century, however, researchers have recognized that soil is more than just a medium; rather it is an active, ever-changing body which provides the elements essential for plant growth. Dirt, on the otherhand, is simply misplaced soil; that is soil that has been moved, often by human influence, but also by wind or water, to a place where it is not wanted.

What is in soil?

Soil is composed of four major components: minerals, organic matter, water and air. In general, the soil minerals provide physical support for the plants while the organic matter contains many of the nutrients essential for plant growth. Water and air are, of course, required by all living organisms including plants for growth.

Minerals: Solid mineral particles (Sand, Silt and Clay) make up about 45% of the soil volume. The smallest particles are clays (<0.002mm) which forms a sticky mass when wet and hard clods when dry. Silt particles are intermediate in size (0.002-0.05mm) and have a powdery, silky texture when dry. The largest particles are sands (0.05-2 mm in diameter) which do not stick together and feel gritty when rubbed between the fingers. These inorganic minerals are the original source of most essential plant nutrients, though they are only very slowly released in forms that are available to plants.

Organic matter: The smallest component of soil is the soil organic matter, making up only a few percent of the soil volume. It is composed of the partially decayed remains of plants and animals, the organic compounds produced by organisms during decay and the organisms themselves. The organic matter content of a typical well-drained agricultural soil usually does not exceed about 5% by volume.

Water: usually makes up 20-30% of the soil volume and is an important carrier for many of the nutrients essential to plant growth. Still, only a portion of this water is available to plants and this appears to depend most on the amount of water and the size of the pores where it resides. The remainder of the pores are filled with soil air; a mixture of gases similar to that of our atmosphere, though usually containing more carbon dioxide and water vapour and less oxygen.

Life in Soil

Soils harbour a diverse community of living organisms, both animals and plants. These range from burrowing rodents and large tree roots to earthworms and insects down to the tiniest bacteria.

The number and weight of these organisms can vary tremendously, depending on the conditions. A single gram of soil can contain up to several billion bacteria and more than a kilometre of fungal strands. In most cases, microorganisms make up 90-95% of the total weight of organisms in soils.

What do these living things do?

Decomposition: Their activities include the physical breakdown of plant litter (leaves, twigs, roots) by insects and earthworms and the chemical breakdown (decomposition) of these materials by microorganisms.

Create structure: They may also change the physical structure of soils by creating large pores that are capable of transporting water down into the soil. Others produce compounds that stick mineral particles together, changing the pore structure and making it easier for plant roots to penetrate.

Release nutrients: One result of the decay process is the release of essential plant nutrients, such as nitrogen, phosphorus, and sulphur, from their organic forms in litter to mineral forms available to growing plants. Still other microorganisms are able to capture nutrients from the atmosphere or degrade the potentially harmful compounds produced by both plants and animals.

All of these factors have a profound influence on plant growth.

For more information, contact:
Dr. Patricia M. Fraser

While every care has been taken when preparing this document, no liability will be accepted by the New Zealand Institute for Crop & Food Research Limited for any loss or damage suffered as a result of applying the information contained in this document. Copyright © 1999 The New Zealand Institute for Crop & Food Research Limited, Private Bag 4704, Christchurch, New Zealand.