Drainage is a problem in many soils, especially in those that contain a lot of clay. Clay is comprised of extremely small particles surrounded by very small channels called micro-pores. That is why these soils drain slowly and hold only small amounts of oxygen needed for plant roots.

Larger soil particles such as sand and gravel have plenty of large spaces called macro-pores. Therefore, it would seem logical that drainage could be improved by mixing quantities of these materials into clay or placing a layer of them beneath the clay. When the water moved through the fine clay and hit the coarse textured sand or gravel, it should rapidly drain away.

Sounds logical. Unfortunately, it does not work that way.

Water moves through the soil from large particles to small, not the other way around. At the conference, we saw a film of water moving through a fine textured soil toward a layer of gravel beneath. When the water reached the gravel, instead of moving more rapidly, it stopped and backed up. The water did not move into the gravel until the entire layer above it was totally saturated. Then it moved into the gravel.

French drains which are basically trenches filled with stones or rocks only work if the stones extend all the way to the surface. If a layer of fine soil is placed on top of them, drainage is slowed considerably since this layer must become totally saturated before the water moves into the stones.


Determine how well your soil drains.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

 

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