What Is Soil?
By Dr. Patricia M. Fraser
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
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
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.
© 1999-2003 Planet
Green, Inc. All Rights Reserved
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