|"Dry" Soils||Infiltration and Runoff||Wetting and Saturation|
|Water Holding Capacity||Percolation and Drainage||Engineering and Bearing Capacity|
When a dry sponge is first submerged in water, it wets rapidly. This sponge is not submerged, only the bottom is in contact with the water. So the water is moving from the bottom of the sponge to the top of the sponge in response to differences in potential energy. The factor controlling water movement in nature (and thereby in soil), is that it moves from areas that have higher potential energy to areas that have lower potential energy. This is the reason water flows downhill: It discharges its energy as it moves downhill. This is also the reason water moves upward in the sponge.
Soaking sponge - water is added to the sponge to get it essentially saturated, that is, so that all the pores are filled with water. In this case, the sponge is submerged beneath the water surface.
In truth, it is very difficult to remove all the air from the sponge. Just as water can be held in the pores of the sponge or the soil, air can get trapped in tiny pores, too.
Though the sponge appears saturated, a small amount of applied pressure drives air out of the sponge. (Notice the bubbles at the water surface.)
Additional pressure drives even more air out of the sponge. If the pressure had not been applied, the air would have remained in the sponge.
The same is true for soil. Even soil that has been saturated for long time periods usually contains air in up to 10% of the pores (voids) between soil particles. So, in field conditions, it is difficult to achieve conditions of greater than 85 to 90% of saturation.