Many factors affect the ability of soils to hold water. Temperature, humidity and a host of other subtle variables add nuance to a dynamic, natural process. Soil scientists characterize and classify soils into different groups or categories, and they often refer to “a soil” in the sense of it being a particular kind of soil. Different kinds of soils have differing abilities to hold water.
Sand, silt and clay are the three basic mineral particle sizes that make up soil. Although the types of minerals can vary within the three categories, it’s mostly the particle’s sizes that are key. Sand is the largest or coarsest particle, silt is medium in texture -- about the consistency of talcum powder -- and clay is the smallest or finest particle. Sandy soils tend to allow water to percolate down through and drain rapidly. So, sandy soils have relatively poor ability to hold water. At the opposite extreme are clayey soils. Soils that are high in clay content can hold relatively large amounts of water for extended periods of time. Silt is intermediate in its water holding properties. Soils, overall, vary widely in their proportions of sand, silt and clay and their water holding capacities.
Given proper moisture conditions, sand, silt, clay and organic matter -- the basic soil components -- tend to adhere to themselves and to each other, forming what soil scientists call “soil structure.” This structure can be granular, platelike, blocky or in some other form. Just as soil texture affects soil’s ability to hold water, it’s structure has an influence as well. Organic matter, including decomposing plant material, is a component of soil, and its presence influences soil structure and, in turn, the soil’s ability to hold water.
The reason that most people, whether soil scientists, farmers or weekend gardeners, are interested in soil’s ability to hold water is for the purpose of growing plants. And, as it turns out, plants themselves influence a soil’s ability to hold water. That’s because plants put a significant demand on a soil’s water supply. Through the process of absorption, through the roots, up through the stems and leaves, and then out into the atmosphere, plants “drain” water from the soil, but upward. This has practical implications since a weed in the garden, in addition to stealing nutrients and possibly shading the desired crop plant, consumes valuable water.
Soils, as they form over time, naturally develop what soil scientists call layers or horizons. In addition to layering of mineral soil components, a soil that has formed with vegetation growing in it typically has an organic layer at the surface and mineral layers that may increase in coarseness as depth increases. Cultivating a soil, as for gardening, alters this natural condition but some layering generally remains. Although a garden soil may be fairly uniform near the surface, layers deeper down can differ in texture and in other characteristics. This vertical profile has an effect on the ability of a soil to hold water.
- University of Minnesota: Soils Lecture
- The Nature and Properties of Soils, Eighth Edition; Nyle C. Brady
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