The word soil has such a diverse meaning, it could refer to volcanic black soil or iron rich red soil or grainy and golden desert sand or even the wet and nutrient rich topsoil of the rainforest. These differences in soil are dependent on the climate, the weathering process, and whether soil is transported or sedimentary. Another important characteristic is water retention. Various characteristics of soil including soil composition, grain size and the presence of organic matter determine the percolation and water holding capacity of the soil. This is important because the primary source of entry for water into plants is through the roots in the soil. If the soil cannot retain water, then it cannot foster plant growth.