Water Retention in Soil

Plants depend on the moisture retention properties of soil.

Whether you're growing trees and shrubs or flowers and vegetables, plants need water and soil to survive. Soil absorbs and retains moisture from rainfall and plants tap into this natural reservoir when they need water. Soil continues to take in moisture until it reaches saturation point when the excess water is released into streams and other waterways in a process known as "run off."

Function

Soil is a porous material with many spaces between soil particles. Water accumulates in the spaces between soil particles, allowing the soil to retain moisture. The "field capacity" of soil is the maximum volume of water that a soil type can retain at saturation point.

Types

The capability of soil to retain water depends on the type of soil. Soil with fine particles, such as clay and peat, retains moisture well because the water molecules stick well to smaller particles. Sandy soils contain much larger particles and so usually retain less moisture. This makes sandy soil ideal for improving drainage and clay-based soil perfect for retaining moisture.

Effects

Plants and, ultimately, animal life depend on the water retention properties of soil. The soil provides a constant supply of moisture and water to plants in the periods between rainfall. Without water retention, rainfall would immediately run off into streams, rivers and other waterways, depriving plants of the regular supply of water required. In drier areas, soils can retain enough moisture to sustain plants through periods of drought.

Considerations

Water retention in soil is an important consideration for gardeners. For example, vegetables usually require plenty of water to thrive and so do well in clay-type soil that retains moisture well. New plants and shrubs need less moisture and adequate drainage and often grow well in sandier soils. Gardeners can improve drainage and alter moisture retention levels by mixing sand into the existing top soil.

References

• University of Florida: Retention of Water, Basics of Soil-Water Relationships; J. Bouma, et al; September 2003
• Your Healthy Garden: A Guide to Soil Preparation

Resources

• Colorado State University: Choosing a Soil Amendment; J.G. Davis, et al; May 12, 2010
• Ohio State University Extension: Physical Characteristics of Growing Mixes; Dr. Claudio C. Pasian