Composition of Soil Types

Physical Properties

• Soil is comprised of minerals, organic matter, air and water. The physical characteristics of the minerals and the distribution of the organic portion of the soil affect the availability of water and air to plant roots. Soils are classified into three textures -- sand, silt and clay. Clays are composed of the finest particles and sands the coarsest. Soils are rarely pure sand, silt or clay, but are, instead, mixtures of all three textures.

  The size and aggregation of particles affect movement of water and air through soil. More tightly packed particles and aggregates create narrower pathways, called pore spaces, which force air and water to move more slowly. Clays, due to their particle size, are prone to drainage problems and oxygen starvation of plant roots. Sandy soils that drain readily present a different problem because they may not hold water long enough for roots to absorb.

Chemical Properties

• Plants also require minerals, including nitrogen, phosphorus and potassium, which exist in soil as charged ions. Cation-exchange capacity and soil pH affect availability of these nutrients.

  Water exists in soil as hydrogen and hydroxyl ions. The pH is a measure of hydrogen ion concentration; it also indicates what nutrients are available to plants. Soil with a low pH level is lacking in nitrogen, potassium, calcium and magnesium, while soil with a high pH level is lacking in iron, manganese and zinc. Plants require all these nutrients.

  The pH level also affects the solubility of sodium, calcium and magnesium. At high soil pH, these three elements affect soil structure in such a way as to change the movement of water through the soil.

Biological Properties

• Soil is teaming with life from bacteria to fungi to insects. Some bacteria and fungi have a symbiotic relationship to plants' roots by fixing various mineral elements into forms that are absorbed and usable to plants. Other soil organisms, such as fungi, may associate with plant roots and aid roots in water uptake. Physical and chemical properties can affect fungi and other organisms that aid plants in uptake of nutrients and water.

Managing Garden Soils

• Amending soils can improve water and nutrient absorption by plants and ensure adequate aeration. To determine which amendments to use, you must know the soil's physical and chemical properties. Soil texture is easily determined by adding a little water to a soil sample and trying to roll it into a ribbon. A soil testing kit, or a soil laboratory, can help determine chemical properties.

  Once you know soil type, check with your local cooperative extension agent for specific soil-amending recommendations. They will also know what plants do best in your garden soil.