Soil permeability refers to the ease with which water and other liquids flow through soil. Between 20 and 30 per cent of soil consists of air spaces through which water can flow, according to John Howell of the University of Massachusetts Extension. Different soil types contain air spaces of different sizes, allowing water to permeate some soils faster than others. Soil permeability affects several aspects of plant health and care. Highly permeable soils, such as sand, quickly dry out and leach nutrients. Impermeable soils, such as clay, retain moisture and nutrients but can also hold too much water around plant roots, restricting access to oxygen. Drainage also becomes an issue when constructing structures or sewage systems.
Soil Type Test
You can get a rough idea of soil permeability by determining the soil type, according to the New York State Department of Environmental Conservation. Moisten a sample of soil roughly the size of a small egg and work it in your hand until the soil is moist throughout. Squeeze the soil into a ball. The easier it is to form and the harder the soil feels in your hand, the smaller the particles and the less permeable the soil. If the soil won't form a ball or breaks apart easily, the soil contains large particles that will allow it to drain quickly. Clay soil--the most impermeable type--feels sticky to the touch, while highly permeable sandy soils feel gritty.
Constant and Falling Head Permeability Tests
In a laboratory setting, engineers use the constant and falling head tests to determine the permeability of soil samples. The constant head test evaluates coarse-grained soils, while the falling head test measures the permeability of fine-grained soils. These tests measure how quickly and easily water flows out of a soil sample. Although the testing process is more complex than what you'd likely want to use to measure permeability for your garden, these tests become relevant during large construction projects, particularly when evaluating the stability of earth-containing structures, according to the California State University at Fullerton.
The percolation, or perc, test is a field test that measures how quickly water drains into soil. Because it is conducted in the field rather than the lab, it gives an accurate picture of how soil performs and is cheap and easy to undertake. When performing a perc test, you dig out of column of soil, fill it with water and measure how the water drops in timed intervals. Use the perc test when planning to construct an underground sewage treatment system.
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- New York State Department of Environmental Conservation: Permeability Testing Methods for Secondary Containment Systems
- University of Massachusetts Extension: Soil Basics--Part I
- NASA Soil Science Education: Soil Texture
- California State University, Fullerton: Constant Head and Falling Head Permeability Test
- University of Minnesota Extension: How to Run a Percolation Test
- University of Connecticut: Soil Mechanics and Foundations--Permeability III