The Best Garden Soil Constituents

Mineral Matter

• Mineral matter accounts for almost half of soil volume. Mineral matter is mostly composed of rocks and rock particles that have been broken down over time by wind, water, erosion, natural decay and other natural forces. Common mineral matter in soils include quartz, mica, limestone and the like. Some mineral matter particles exist in the layer just below the top soil, but most mineral matter exists in the "parent material" layer, since the erosion and degradation of the rock in this layer is what gives way to the soil in the layers above.

Air

• Air generally makes up around a quarter of total soil volume in a typical soil sample. If you've ever purchased a plant or plant seeds from a nursery to plant at home, you have probably seen recommendations for establishing the plant in "well-drained" or "less well-drained" soil. Soil drainage is mostly a result of the amount of moisture in the soil, but the amount of air in the soil is what decides how well-drained that soil is. The importance of air in soil is highlighted by common gardening recommendations that tell the gardener to "aerate" the soil beneath a lawn or garden by tillage, for example.

Water

• Water also makes up around a quarter of soil volume in a quality soil sample. Water is of course critical for plant growth. A plant's root structures absorb water in the soil and transmit the water to all parts of the plant's vasculature to support new growth and keep existing growth healthy. If a soil is too water-logged, it can leave the plant susceptible to fungal and bacterial diseases, while if a soil is too dry the plant will be unable to supply sufficient moisture to the parts of the plant aboveground.

Organic Matter

• Organic matter usually only makes up around 5 percent of soil volume, but its importance cannot be overstated. Decaying plants and animals, as well as the almost limitless amount of fungi and bacteria that naturally exist in the soil, all make up the soil's organic matter. So critical is organic matter to plant growth that growers frequently need to add organic matter in the form of compost, peat moss, animal manure, fish meal or many other materials in order to improve the soil's fertility.