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.