WATER IN THE SOIL • The water content and the rate of water movement in soils depend to a large extent on soil type and soil structure. • In sandy soils, the spaces between particles are so large that water tends to drain from them and remain only on the particle surfaces and at interstices between particles. • In clay soils, the channels are small enough that water does not freely drain from them; it is held more tightly Field Capacity • The moisture-holding capacity of soils • Field capacity is the water content of a soil after it has been saturated with water and excess water has been allowed to drain away. • Clay soils or soils with a high humus content have a large field capacity. • As a soil dries out, water is first removed from the center of the largest spaces between particles. • Because of adhesive forces, water tends to cling to the surfaces of soil particles, so a large surface area between soil water and soil air Water has a high surface tension that develops tends to minimize air–water interfaces. • As the water content of the soil decreases, the water recedes into the interstices between soil particles, and the air–water surface develops curved air–water interfaces. • The value of p in soil water can become quite negative because the radius of curvature of air– water surfaces may become very small in drying soils.