Soil Drainage terms you should know: (from Colorado State University Extension- GardenNotes #261 Soil Water Holding Capacity and Irrigation Management)
- Water Holding Capacity: Soil texture and structure (actually the pore space created by soil texture and structure) primarily determine a soil’s ability to hold water. Water coats the soil particles and organic matter and is held in the small pore spaces by cohesion (the chemical forces by which water molecules stick together). Air fills the large pore spaces. Water readily moves downward by gravitational pull through the large pore spaces. In small pore spaces, water moves slowly in all directions by capillary action.
- Saturation refers to the situation when the soil’s pore spaces are filled with water. With water replacing air in the large pore spaces, root functions temporarily stop (since roots require oxygen for water and nutrient up-take). Prolonged periods without root oxygen will cause most plants to wilt (due to a lack of water uptake), to show general symptoms of stress, to decline (due to a lack of root function and possible root dieback), and to die.
- Field capacity refers to the situation when excess water has drained out due to gravitational pull. Air occupies the large pore spaces; and water coats the soil particles and organic matter, and fills the small pore spaces. A handful of soil at or above field capacity will glisten in the sunlight. In clayey and/or compacted soils, the lack of large pore space slows or prohibits water movement down through the soil profile, keeping soils above field capacity and limiting plant growth.
- Permanent wilting point refers to the situation when a plant wilts beyond recovery due to a lack of water in the soil. At this point the soil feels dry to the touch. However, it still holds about half of its water; the plant just does not have the ability to extract it. Plants vary in their ability to extract water from the soil.
- Available water is the amount of the water held in a soil between field capacity and the permanent wilting point. This represents the quantity of water “available” or usable by the plant. Note from the illustration below that the amount of available water is low in a sandy soil. Loamy soils have the largest quantity of available water. In clayey soils, the amount of available water decreases slightly as capillary action holds the water so tightly that plants cannot extract it.
Soil Compaction, compression of soil by heavy machinery or animal traffic, can decrease the soil pore space leading to decreased soil air and increase water and bulk density (harder soil). This can decrease the quality of the soil and make it a more stressful environment for plants–whether crops or forage–to live in.
For more information on this topic, go to the Ohio State University Extension bulletin “Soil Compaction and Drainage” at http://ohioline.osu.edu/b301/index.html.