The life sustaining ability of soil is best understood by appreciating the complex cycles of decay and erosion. Its natural formation occurs in a series of layers starting at the surface but gradating down to the deepest bedrock. The surface layer is where active decomposition begins. Exposure to atmospheric elements, surface warmth and moisture helps to break organic matter into loose mulch like material. At the microscopic level, this layer is teeming with a diversity of bacterial, fungal and algal life forms. In combination with larger organisms like beetles and worms they provide the additional recycling activity to enable minerals and nutrients to be retrieved from the decaying organic matter and returned to the soil. Another family of soil based micro-organisms are involved in relationships that enable plants to absorb nitrogen from their roots.
Ideally the layer directly beneath the surface will be humus rich topsoil. The quality of this topsoil will depend on the amount of organic material available near the surface and the activity of the recycling organisms.
A coastal rainforest provides almost ideal conditions for the creation of richly fertile topsoil. With increased temperatures and humidity an abundance of organic material reaching the ground begins to decompose almost immediately. It is then broken down by organisms which thrive under the conditions. The entire process is accelerated resulting in a generous layer of finely blended topsoil.
A descent through deeper soil layers will reveal gradually decreasing quantities of humus before reaching the substratum of bedrock. Deep layers contribute to the surface quality of soil by providing mineral particles and compounds through erosion. The deep layers also support the structure of the soil by providing its foundation and drainage characteristics.
A technical analysis of structure can isolate the important layers of soil, their relationship to each other, aeration and drainage characteristics along with the mineral components characteristic to a particular location. It can also indicate the comparative rates and efficiency for recycling organic material. Information about structure will assist the serious gardener to predict how soils behave under varying seasonal conditions.
Soil type is a classification based on the major particle constituent along with the average pH reading. The most typical examples of soil type are sand, clay, and silt based. In some respects this information has limited value because soils tend to vary significantly across regions even when described to be of similar type. This is where an understanding of structure will provide a clearer picture.
From the perspective of the organic grower, good soil structures need to be protected. This can be achieved by minimising digging, replacing disrupted layers in their correct order when necessary and renewing surface layers by providing a supply of organic material such as compost and manure. The addition of organic material will improve the water and nutrient holding ability of the soil.
