|"Dry" Soils||Infiltration and Runoff||Wetting and Saturation|
|Water Holding Capacity||Percolation and Drainage||Engineering and Bearing Capacity|
If infiltration continues for long periods, it is possible that the soil will reach field capacity. At that point, the soil can no longer hold water against the pull of gravity. The result is water begins to move through the soil profile and out of the plant root zone (called percolation or drainage). Any water that does this is lost from the perspective of the plant. In humid regions where groundwater tables are common, little can be done to decrease percolation without increasing runoff. In these regions, the concern becomes water quality in the runoff or percolated waters. Water is an almost universal solvent, so as it moves across the soil surface, or through the soil surface, it picks up hitchhikers. On the surface, this includes sediments in suspension and salts in solution. In the soil profile, most of the sediments are physically filtered out of the water, as are many of the positively charged ions (cations) from the solution. Soil clay minerals and organic matter both have dominantly negative charges, and so attract cations from the soil solution. However, anions (negatively charged ions) in the soil solution are able to move through the soil in percolating waters almost unimpeded. The movement of salts (especially nutrients or agricultural chemicals) through the soil profile with percolating waters is called leaching.
The images below show a sponge that just as it begins to drain, and a closeup of the water draining from the bottom of the sponge. In order for the drainage to begin, the bottom of the sponge must be approaching saturation.