Water moves through the soil because of the forces of gravity (pulling downward) and capillary action (the force with which water molecules cling to each other and to soil particles). Gravity is relatively easy to understand, but capillary action is less familiar to us.
Capillary action in soil
You can observe capillary action by rolling up a paper towel and inserting the bottom of it in a glass of water. Water will climb through the towel because of the capillary action of water molecules clinging to towel fibers and to each other.
Capillary action is caused by the surface tension of the water, the force that makes water climb the side of a glass slightly. The thinner the film of water on a surface, the stronger the capillary action.
When water enters the soil it moves downward in the pore spaces between the soil particles. As water fills the pores, the soil becomes saturated and the water moves deeper, replacing the air in pores. The rate at which this wetting front moves depends on the soil texture: it moves faster in sand, with its large pore spaces, and slower in clay, which has small pore spaces. Eventually, when the film of water on the soil particles is thin enough, the force of gravity pulling downward is balanced by the capillary action of the water molecules clinging to soil particles, and the water stops draining downward.
The Wetting Zone
When water is applied to dry soil, it is absorbed and tightly held by the capillary action of the soil. As more water is applied, the pores in the soil fill with water and the soil becomes saturated. Water flows easily through saturated soil. If water is continuously applied, as from a sprinkler, the saturated part of the soil moves deeper.
The front of this saturated soil is called the wetting zone. As long as water is applied, the wetting zone moves deeper into the soil. When watering stops, the wetting zone continues to move downward for a while, as the pores above it empty of water and air enters the soil. Eventually, the wetting zone stops moving downward. The capillary forces have become strong enough to balance the force of gravity, and the water stops moving. This level of wetness is called field capacity.
To summarize, when you water, you are saturating the soil to a certain depth, depending on how much water you put on. The saturated soil will drain until it reaches field capacity, then stop draining. The soil below the wetting zone remains unaffected by the water you applied. If you apply a small amount of water, the surface becomes muddy, but the soil below remains dry. If you apply more water, the saturated region is deeper, but the soil below it is still dry.
It is impossible to keep soil “slightly moist.” Whenever water is applied, some of the soil is saturated, then drains to field capacity and stays there until plants and surface evaporation dry it again. Drip irrigation, which applies water very slowly, comes closest to keeping soil “slightly moist.” Sprinklers, flood irrigation, and other irrigation methods always saturate the surface soil.
The rate at which water enters the soil depends on the size of the soil pores. The fine pores of clay cause more friction, and water enters the soil slowly. Sand, with its large pores, accepts water rapidly.
If water is applied faster than the wetting front can move downward, puddles will form on the surface and water will run off the soil. To avoid puddles and runoff, apply water slower. If this is difficult because of your watering equipment, apply water until runoff begins, then stop and give the wetting front time to move downward before resuming. This is called “cycling” irrigation. Many sprinkler timers have settings to allow three or more short cycles of watering.