A good garden soil contains approximately 25 percent water. It takes approximately 300 pounds of water to produce 1 pound of plant dry matter. Because clays and silts have more exposed surface in a given amount of soil, they have a greater water-holding capacity than sandy soils. Because water will rise higher in clays and silts, these soils will support plants better than sandy soils in dry weather.
Water exists in the soil in three forms: capillary, hydroscopic and gravitational.
Capillary water adheres to a soil particle in the same way a film of water adheres to any object. This film of water moves, by "capillary attraction," from one soil particle to another. The smaller particles, such as clay, have greater exposed capillary surfaces. As a result, water will rise higher in a 1-inch tube containing clay than in a 1-inch tube containing sand.
This is a very thin film of moisture that "sticks" to each soil particle. Even in very dry soil some hydroscopic water is present. The only way to remove all of the hydroscopic water from a soil sample is to bake the sample in an oven for a long time. Hydroscopic water is so tightly bound to soil that roots cannot absorb it.
Gravitational water is pulled out of large pores by gravity after rain or irrigation. As the water is pulled out, it pushes out toxic gases and a new oxygen supply moves into the soil.