Soil Moisture-Holding Capacity

Texture

• Soil texture is primarily a measure of the fine mineral portion of a soil -- the percentages of sand, silt and clay -- excluding all materials over 1/4 inch. The finer the soil's texture, the more clay and silt it will have and the less sand it will contain. The more fine-grained materials in a soil, the greater the surface area of the soil. With increased surface area the soil will have a greater capacity to hold water. For example, a sandy beach soil does not hold water well, but a soil with lots of clay can soak up a huge amount of water. One problem with clay soils and moisture is that even though they hold more water, they may hold it too tightly for a plant's roots to absorb. In these cases the attractive force of the tiny clay particles is greater than that of the plant roots.

Organic Matter

• The decayed structures of plants and animals as well as their droppings provide the material for organic matter in the soil. Organic matter is another material that increases soil's surface area. Organic matter has a tremendous ability to absorb and hold onto water. The more organic matter in a soil, the greater its ability to retain moisture. Adding organic matter to the soil in the form of compost or mulch can greatly increase its water-holding capacity.

Structure

• A soil's structure is defined by how its small particles form into stable aggregates. These clods influence the surface area of the soil and how easily water will drain through it. A soil with poor structure, such as one found near a sandy beach, will allow water to pass right through it. The fertile topsoil of Iowa, which has excellent soil structure, will absorb a large amount of water while also draining excess moisture. Soil structure depends in large part on the amount of clay and organic matter in the soil. These materials act like glue for holding together large materials. Organic matter exuded through plant roots helps to knit the soil together into tiny clods that create good soil structure.

Porosity

• The amount of space between soil particles is called porosity and is measured by the soil's bulk density. The less dense a soil, the more space it has between particles to hold water. A soil with lots of clay will have more total spaces between particles than will a very sandy soil. This is because the volume of all the tiny spaces between clay particles will exceed the volume of the larger spaces between sand particles. Moisture is held more securely in small spaces between particles due to the capillary force of liquids. In larger pore spaces, gravity can more easily drain moisture from a soil.