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When it comes to the size and type of septic system you will need, the condition of your soil is the major deciding factor. When your soil is good and untouched, things usually work well.

In its natural state, soil develops passageways through it made by generations of plant roots, worms, and small animals – often over hundreds or thousands of years. This leaves channels for the water to flow through as it makes its way down to the water table. Some of your water is wicked up through live roots and evaporates as the sun pulls it upward.


Human development can significantly disturb the soil. Driving heavy vehicles directly over an absorption area will compact the soil and close off these natural drainage channels, leaving water to surface in ugly black pools in your yard.

When you add soil or move it around, it can take decades for plant life to repair existing channels. When you remove soil, you remove these channels altogether, often leaving lower layers made of clay, which don’t provide much drainage or absorption.

You typically get one shot at using your soil for a septic system. If it fails, the new system either has to go in a new, undisturbed area or the existing area has to have very specific equipment and materials added so that it can be used, which almost always comes with a big price tag.


Soil composition matters, too. The ratio of the mix between silt, sand, clay, and other materials determines its drainage ability and the rate at which water can be absorbed into the earth. Soil that has a high clay content has a lower absorption rate, while soil with a high sand content has a higher absorption rate.

Septic systems require reliable soil drainage in order to function properly, especially during times of high rain fall. If you must place your septic system in high-clay soil, additional measures may need to be taken to ensure proper drainage in the absorption field.


If you’re looking to buy land, have a licensed professional check the soil first to make sure it’s suitable for a septic system. If you already have a system in the ground, make sure you are keeping up with its care and maintenance so it doesn’t become full of solids that will clog the soil. Also make sure you know where your absorption field is and avoid driving heavy machinery directly over it.

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