Water-holding capacity is controlled primarily by soil texture and organic matter. Soils with smaller
particles (silt and clay) have a larger surface area than those with larger sand particles, and a
large surface area allows a soil to hold more water. In other words, a soil with a high percentage of
silt and clay particles, which describes fine soil, has a higher water-holding capacity. The table
illustrates water-holding-capacity differences as influenced by texture. Organic matter percentage
also influences water-holding capacity. As the percentage increases, the water-holding capacity
increases because of the affinity organic matter has for water.
Water availability is illustrated in the figure by water levels in three different soil types. Excess or
gravitational water drains quickly from the soil after a heavy rain because of gravitational forces
(saturation point to field capacity). Plants may use small amounts of this water before it moves out
of the root zone. Available water is retained in the soil after the excess has drained (field capacity
to wilting point). This water is the most important for crop or forage production. Plants can use
approximately 50 percent of it without exhibiting stress, but if less than 50 percent is available,
drought stress can result. Unavailable water is soil moisture that is held so tightly by the soil that it
cannot be extracted by the plant. Water remains in the soil even below plants' wilting point.
One can see from the table that soil texture greatly influences water availability. The sandy soil can
quickly be recharged with soil moisture but is unable to hold as much water as the soils with
heavier textures. As texture becomes heavier, the wilting point increases because fine soils with
narrow pore spacing hold water more tightly than soils with wide pore spacing.
Soil is a valuable resource that supports plant life, and water is an essential component of this
system. Management decisions concerning types of crops to plant, plant populations, irrigation
scheduling, and the amount of nitrogen fertilizer to apply depend on the amount of moisture that is
available to the crop throughout the growing season. By understanding some physical
characteristics of the soil, you can better define the strengths and weaknesses of different soil
types.
The table and figures were originally published by the Institute of Agriculture and Natural
Resources at the University of Nebraska - Lincoln.