Index age and tree species must be stated when referring to site index because the site index of one species will be different from the site index of another species growing on the same area. There is a close relationship between site index and timber yield. The depth of the uppermost soil layer is a critical factor affecting tree growth. Topsoil is highest in organic matter and nutrients, is usually well aerated and drained, and allows maximum root growth and root penetration. The proportion of sand, silt and clay in the topsoil and subsoil layers is called texture.
Sandy soils are normally very well drained and often lack nutrients due to constant leaching loss.
At the other end of the spectrum are the pure clay soils comprised of very small, fine soil particles. Subsoil Consistence Class. Consistency of the subsoil layer is another important factor in forest soil productivity. The combination of soil-sized particles and the physical and chemical properties of each individual particle type in a given soil determine the soil's consistence class. A layer which restricts the downward penetration of a tree's root system will reduce tree growth in direct relation to the depth of layer.