What is soil and how does it work?

What is soil and how does it support plant growth?

Several factors make soil a good medium for plant growth, so what is soil?

First lets consider what soil does. It needs to provide anchorage, nutrients, air, water and biological interactions and a balance of all these things is required for a soil to support strong, healthy plant growth. In providing these functions, soil behaves as a living system. To manage this system requires an understanding of its various components and how they are inter-related with each other. In short, what the natural balance is between the.

What is soil balance?

To understand this balance, of course, we need to know what it is comprised of. There are three major components and in addition to the balance relationship that exists between them, each component has its own natural balance as well.

What is soil structure?

Physically, any gardening soil consists mainly of mineral material with a comparatively small amount of organic matter. The mineral matter of soil results from the action of weathering forces on the parent rock from which the soil formed. A large part of the mineral fraction consists of little more than fragments of this original rock, produced by physical processes with little or no chemical change taking place. This makes up most of the sand and silt fractions of soils.

Clay particles, on the other hand, are also products of the weathering of rocks, but their chemical constitution has been altered during the process of weathering.

Soil Colloids

As you will see below, clay particles are very small. Together with the very fine organic particles ('humus'), they are referred to as "colloids". This means that they are small enough to remain suspended in water for several hours without settling to the bottom.

The significance of this is that such small particles have (collectively) an extremely high surface area, which allows them to provide virtually all the cation exchange capacity in the soil.

Mechanical Analysis of soil

One of the most important in the properties of the mineral part of the soil is the size range of particles it contains. Determination of the proportions of various particles is called Mechanical Analysis.

After breaking down all the aggregates in soil to their ultimate particles, these can be separated according to size:

1. Gravel - 2mm or more.

2. Coarse Sand - 2mm to 0.2mm.

3. Fine Sand - 0.2 to 0.02mm.

4. Silt - 0.02 to 0.002mm.

5. Clay - Less than 0.002mm.

Although there are also other factors that contribute to it, the range of particle sizes present will largely determine the soil texture , in general, it can be said that:

Clay confers cohesion, stickiness and plasticity

The type of clay mineral influences firmness.

Silt confers a silky smoothness to the textures as it fills in the particle size range between sand and clay.

Organic Matter confers cohesion to sandy soils, but slightly less cohesion to clay soils. Above 5% organic matter can also produce a 'loamy' feel.

The balance of exchangeable cations can have a significant effect on soil texture.

Of course, there are other factors that determine the physical character of a soil so here is some additional reading on:

What is soil texture
More detailed information on the factors contributing to soil texture and how to do your own texture assessment.

The Cation Exchange Capacity
The balance of exchangeable cations can have a significant effect on soil texture. This page explains how the cation exchange system in the soil works.

Calcium Magnesium ratio
This is an important, but much misunderstood aspect of soil balance

Soil balance and the Mikhail System
This gives a detailed explanation of soil as a living system and how it works.

Soil problems that occur when the balance is not right.
Many problem soils can be fixed once you understand how the balance system works.

Apart from soil physics, there are two other soil components to consider - Nutrients and Biology.

What is balanced plant nutrition?

Once you know what plant nutrients are available in the soil, you can compare these to the nutrient requirements of the plants you grow and see which are in short supply and so need to be made up with fertilizer.

What is soil biology all about?

This is a complex field that has only recently been opened up by science.

You can read a detailed description of how it fits into the overal state of the soil on our soil biology page.