The rate at which soil absorbs water is called permeability. Taken from the Latin "permeare" meaning "to pass through," soil permeability determines how readily water passes through the individual particles of soil. The porosity of different types of soil affects how much water the soils can absorb and hold. Soil that does not absorb water most likely has a very high clay content.
For optimum permeability, soil particles must be large and far enough apart from each other to allow water to move easily between them. The larger the particles, the more space is between them, giving clay the lowest rating when it comes to permeability. Clay particles are very small and packed closely together, leaving little room for water or anything else to get through. It can take as long as 200 years for water to travel through three feet of clay, and just two minutes for water to move through gravel.
It is difficult for water moving through dense soil such as clay to get through smaller openings, which slows the process down considerably. Soil that doesn't absorb water also may have been packed down, which leaves water sitting on the surface of soil with a high clay content.
The shape of soil particles also contributes to how quickly water can pass between them. Clay particles are tiny and smooth, while sand and gravel particles are larger and more unevenly shaped. Water is able to move much more quickly between the sand and gravel particles, because they are not so tightly packed together as clay particles. Clay also attracts water molecules as they pass through. These trapped molecules essentially fill in the spaces between the tiny clay particles, making any further water absorption impossible.
Sand, loam and clay are the three basic soil types; from a gardening perspective, loam is the best choice. A perfect blend of clay, silt and sand, loam absorbs water readily but drains well, so plants aren't left standing in puddles after a rainstorm. It absorbs and holds nutrients well, and has enough coarse material to protect plants and keep them anchored.
Pure clay is hard, dry and clumpy, while sand is loose and granular. Plants will grow in sand, but they will need frequent watering and feeding . Conversely, clay holds on to water and nutrients for a long time, but its density makes it nearly impossible for plant roots to develop and grow.
Chances are, if your soil is high in sand, permeability won't be a problem, but there are things you can do to improve your clay soil. Perform a simple test by squeezing a handful of it. Sandy soil compacts but doesn't hold its shape and crumbles easily. Loam holds its shape but isn't sticky and crumbles easily. Clay compacts tightly, feels wet and sticky, holds its shape indefinitely and does not crumble.
Once you've determined the type of soil you have, experiment by adding components that will enhance its permeability. Add lots of loose rich organic material, such as aged manure or compost, to your clay soil, and work it in well. If it still feels dense, work in some sand until you have a soil that meets the loam requirements.
- Ask A Scientist: Water and Soil Types; Andy Johnson, et al.
- Michigan Environmental Education Curriculum: Permeability
- Clean-Water-For-Laymen.com; Soil Permeability; 2010
- Colorado State University Extension; Choosing a Soil Amendment; J.G. Davis, et al.; May 2005
- University of California; Managing Clay Soils in the Home Garden; Falih K. Aljibury
- Basics of Gardening.com: Different Types of Soil