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Colorado State University Master Gardener - Soil Water Holding Capacity and Irrigation Management Page 2 of7 <br />For additional discussion on texture, structure and pore space, refer to the CMG GardenNotes #213, Managing Soil <br />Tilth. <br />The terms, saturation, field capacity, wilting point, and available water describe the amount of water in a soil. <br />[Figure 1] <br />..5 <br />,t <br />_7 V" <br />SafuraliAn <br />Field Copacity witting P*in! <br />Figure 1, At saturation water fills the pore spaces. At f<e/d <br />capacity air occupies the large pore spaces while water fills the <br />smaller pore spaces. At the wilting point, plants cannot extract <br />additional water from the soil. <br />t <br />httr <br />SAftm-Rion refers to the situation when the soil's pore spaces are filled with water. With water replacing air in the <br />large pore spaces, root functions temporarily stop (since roots require oxygen for water and nutrient up-take). <br />Prolonged periods without root oxygen will cause most plants to wilt (due to a lack of water uptake), to show <br />general symptoms of stress, to decline (due to a lack of root function and possible root dieback), and to die. <br />During summer flooding of the Mississippi River In Iowa and Illinois it was observed that healthy trees were <br />somewhat tolerant of a flooding period, while trees under stress or in a state of decline were very intolerant. <br />Fs. !? d capacity refers to the situation when excess water has drained out due to gravitational pull. Air occupies <br />the large pore spaces; and water coats the soil particles and organic matter, and fills the small pore spaces. A <br />handful of soil at or above field capacity will glisten in the sunlight. In clayey and/or compacted soils, the tack of <br />large pore space slows or prohibits water movement down through the soil profile, keeping soils above field <br />capacity and limiting plant growth. <br />Permanen wiit,Ig,point refers to the situation when a plant wilts beyond recovery due to a lack of water In the <br />soil. At this point the soil feels dry to the touch. However, it still holds about half of its water; the plant just does <br />not have the ability to extract it. Plants vary In their ability to extract water from the soil. <br />tAVajfab[e water is the amount of the water held in a soil between fled capacity and the permanent wilting point. <br />This represents the quantity of water "available" or usable by the plant. Note from the illustration below that the <br />amount of available water is low in a sandy soil. Loamy soils have the largest quantity of available water. In <br />clayey soils, the amount of available water decreases slightly as capillary action holds the water so tightly that <br />plants cannot extract it. [Figure 2] <br />30 <br />24 <br />AVAILABLE WATER <br />• i? j1 t S 1t 4 f F' <br />lF PER?MAN£NT' <br />333CCC WILTJIW POINT <br />a c tKS-AvntCA9t.6 WATER <br />//www. ext.colostate.edu/mztfi les/eardennotesi261-Soi lWater.htmt <br />65 <br />Figure 2. <br />Relationship <br />2?g5 between <br />snit texture <br />c7? <br />'g and <br />available <br />water <br />4 <br /> <br />I <br />1/3/2009