The composition of soil refers to the material that is in it. For the most part, when we consider the make-up of soil we are looking at the percentages of three different components, sand, clay, and silt. In order to be considered to be a fertile loam soil, there should be some of all these components. Differing levels of each of the components will affect the way that the soil absorbs water, retains water, and drains water. The composition also affects the compaction characteristics of soil.

When we speak of soil composition, we are generally referring to the A horizon of the soil, that layer that is just beneath the organic material on top (mostly decaying plant material) and above the parent material below. The organic materials interact with the parent materials in this layer and support what is known as the soil food web.

Generally speaking, an even mix of the three components (a little less clay, though) is optimum for root growth.

What horticulturists and soil scientists refer to as the soil food web is an intricate network of life forms, mostly microbial, that work together to create the support needed for the foliage above.

The two most important conditions for this network of life forms are oxygen and water. Beneficial fungi and bacteria that assist plant roots with the absorption of water require oxygen to live. Earthworms, which create channels for root growth as well as nutrients, must respire underground.

Aeration devices placed in the garden will broaden and deepen the profile of available oxygen and water for all of these organisms to live.

A more robust soil food web means a more robust garden!

Perhaps more important than the composition of soil is the structure of the soil. The soil composition will determine the amount of micropores that are present in the soil. Soil absorbs water into its mircropores and holds it. Because of the small size of the micropores, water does not drain from them. Drainage occurs through larger spaces in soil known as macropores. It is the drainage of water through these larger spaces that allows air to be drawn into the soil. Without air (Oxygen) roots cannot live.

So soil structure essentially refers to the amount of macropores in the soil.

Soil structure can be compromised by breaking it down during construction, over tilling, compacting it, or killing off the organisms that provide the structure. Organisms that create structure can be complex organisms such as earthworms that create rather large spaces in soil by tunneling through it. There are also microorganisms that contribute to soil structure by creating a glue known as Glomalin that holds pieces of soil together and preserves the macropores in soil.

All of the organisms that contribute to soil structure require oxygen to live. So if oxygen is not constantly available in the soil there will be no organisms in the soil food web to preserve the structure of the soil.