Soil

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Soil	LC0102
David Cummings, Box Hill January, 2001

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It is easy to take soil for granted. But it deserves to be treated with respect, with care and even with awe. It is the life support platform of our planet. Soil is more than something to walk on. It is more than an anchor for our trees and buildings.

It is a vibrant environment, full of life, full of activity. The atmosphere, the seas and the rivers all pall in comparison. Soil is the most biologically active zone in the world.
It is not just worms, ants and plant roots that live in it. Bacteria, fungi, arthropods are there in their millions. Even a single spoonful of healthy surface soil may contain hundreds of millions of organisms.

Why is soil valuable?
Soil makes the earth habitable.

It does this by:

being the interface between the atmosphere (air), the geosphere (rock), the hydrosphere (water) and the biosphere (organisms). This is illustrated in Figure 1.
being a fecund environment for the development and support of life by having a vast interconnecting matrix of pores, channels and surfaces for containing, air, water, nutrients and organic materials.
housing the important life supporting cycles: the energy cycle, the hydrological cycle, the carbon cycle, the nitrogen cycle – in effect a self regulating biological factory.
being the primary medium on earth for cleansing, storing and recycling.

Diagram: Soil is the interface linking the four spheres: atmosphere; hydrosphere; geosphere; and biosphere (Rimmer 1998)

Figure 1. Soil is the interface linking the four spheres: atmosphere; hydrosphere; geosphere; and biosphere (Rimmer 1998)

How is soil built?
Soil forms from the breakdown of rocks and sediments via a vast array of physical, chemical, and biological activities. It is slow, very slow, taking upwards of ten thousand years in most cases.

The silts, the sands and the gravels are all just fragments of the original parent material but the rest of the soil is highly modified. Clays are special types of minerals that develop slowly over time. Different clays result depending on parent materials, soil age, humidity and temperature. They have a strong influence on soil stability, fertility, water holding, drainage and aeration. Organic materials accumulate in soils with plant growth. While they initially come from plants, they are then used by other soil living organisms (as food) who then build-up a whole range of consequent organic materials. These become indispensable to the productivity and stability of the soil.

Diagram: The soil pores are where the action is

Figure 2. The soil pores are where the action is. (#The proportion of solids to pores depends on the structural arrangement of solids as stabilised by clays and organic materials)

How is soil structured?
If we slice carefully through soil we will find that less that half of its volume is solid. The rest is a complex arrangement of pores and channels (see Figures 2 and 3). It is these pores and channels, which enable so much activity to occur.

Water, air, nutrients and organic materials can all accumulate and be stored here, so that bacteria, fungus, and the tiny animals can grow and multiply.
Roots can grow readily into and through the soil
Rain water can be stored and excess rain can be drained to the groundwater systems

It is worthwhile to mention that the better the range of sizes, the condition and the stability of the soil pores, the more productive the soil.

Diagram: An example of a well structured soil (x30)

Figure 3. An example of a well structured soil (x30)

Solids range from sand size particles down to clay sized particles. They are joined together into aggregates by various chemical forces organic fibres and biological glues. Water will drain readily from the large pores and subsequently allow ready air movement. Moisture will remain in the smaller pores for use by micro-organisms and plant roots.

What are desirable characteristics for a soil?
Soils vary from place to place. They are quite variable in form and in composition so it is not easy to list a set of ideal characteristics.

However it is possible to define some overall requirements as follows:

General
A porous friable soft and stable material which readily accepts, stores and transmits water and air to enable biological activity

Biological
Vibrant growth of

soil fauna (worms, ants, etc) to help break-down coarser organic materials and build and stabilise macropores
a vast range of micro-organisms to cycle nutrients and energy
plant roots exploring the soil environment to harvest water and nutrients

Chemical

Good range of nutrients being contained, recycled and readily available for plant growth
Neither too acid nor too alkaline
No excess build up of salts

Physical

Well distributed size range of particles (ie clays, silts, sands and gravels)
Pore space greater than half of the soil volume with a good distribution of pore sizes. Macropore space is adequate and with interconnection through the profile to encourage the internal drainage and aeration of soil
Structure is relatively stable and able to support moderate levels of trampling and occasional disturbance without collapse.

How can a soil be degraded?
Soil can degrade in 3 ways:

Physical, chemical or biological run-down causing a reduction in vigour. This can result from excessive product removal (depleting soil nutrients), reduction in plant growth, lowered organic cycling, increasing soil temperatures, leaching, compaction and surface crusting.
Reduction in mass and volume through erosion. This reduces the physical size of the soil ecosystem.
Accumulation of specific soil chemicals to levels that detrimentally effect plant growth. Such materials include: soluble salts (causing salinity); hydrogen ions (causing acidification); and, some chemicals from industrial, mining and agricultural activities (chemical contamination).

Further information
Hillel, D (1991) Out of the earth: civilization and the life of the soil. University of California Pressl
Rimmer, D (1998) Ultimate interface. Inside Science number 113. New Scientist

This note replaces note number SC0056.

The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.

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