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A.J. illust Floy This pr introduc grass ecc ecosystere the prov ment. are used consisten turfgrass GRASS A past pub' sive sumrr develope4 is inform processes • Divide MANAG cesses, a thorough chapters growth ai emphasize on the tax turfgrasses and cultur of grasses componen the atmos ences that gation. and susta quality ar guide to A chapter methods tions whil chapter on yield meth of turfgras 108 THE TURFGRASS ENVIRONMENT soils to 25 to 30 percent for fine - textured (clayey) soils.' Thus, a measure of soil water content cannot serve as a guide to a plant's soil water requirement, but a good relationship exists be- tween a plant's water needs and the work required to remove a unit of water from the soil. Water potential (i ,) is an expres- sion of the energy status of soil water relative to pure, free water. Soil - adsorbed water is not capable of doing as much work as pure, free water; thus, its energy, or water potential, is lower. Differences in water potential between two locations in a soil, called the water potential gradient, provide a force that causes water to flow from locations of higher to lower water potential. Soil water conductivity is an expression of the ease with which the soil conducts water. In compacted soils, conductivity is low because of high resistance to flow. Well - structured soils conduct water more rapidly. Where a plant is pulling water from the soil immediately surrounding its roots, replenishment of this root zone soil moisture is dependent upon: (a) the water poten- tial gradient between the root zone soil and a location in the soil where water is available and (b) the conductivity of the soil. In a saturated soil, water potential near the soil surface approaches zero. As the soil drains, water potential becomes progressively lower (more negative). Water movement in satu- rated and unsaturated soils should be considered as two distinct processes. Saturated flow occurs when all or most of the pores are filled with water. It takes place through large pores and, thus, is most rapid in coarse - textured soils. The principal force acting upon the water is gravity, and the direction of flow is primarily downward. If the number of large pores decreases suddenly at a given soil depth, downward movement is re- stricted, and water may accumulate above the interface where the two media meet, resulting in a temporary water table. This occurs in sand or thatch overlying loam soil, or with a compact subsoil. Unsaturated water flow occurs in soils in which the large pores are not filled with water. The rate of unsaturated flow depends upon the thickness of water films surrounding soil particles; thicker water films allow faster flow rates than thinner films, due to the differences in water potential. Thus, water moves faster in moist soils than in dry soils. Unsaturated flow proceeds in any direction, irrespective of gravitational force. 'Taylor, S.A. and G.L. Ashcroft, Physical Edaphology, (San Francisco, Cali- fornia: W.H. Greeman and Company, 1972), p. 8. EDAPHIC ENVIRONMENT 109 The so- called wick action or capillary flow of water from lower to upper soil locations is actually unsaturated flow. Where the continuity of water films is disrupted, as at the interface between a fine - textured soil and an underlying coarse - textured soil, unsaturated flow is slowed, or may stop alto- gether. This can result in an accumulation of water, called a perched water table, above the interface. Water will not move across the interface until the water potential in the above soil builds to a level sufficient to overcome the attraction between the water and the fine - textured soil. When sufficient water potential has built up from continued downward flow toward the interface, water will enter the coarse- textured soil and be conducted rapidly away. This principle can be applied to turf soils in which layers exist either by design or by error. Consider a fine - textured soil that has been modified by incorporation of sand or other coarse amendments. If the incorporation is not uniform, but results in subsurface layers of coarse material, water flow within the pro- file can be disrupted. On the other hand, where a layer of coarse sand has been intentionally positioned beneath finer - textured sand with some loam soil and organic amendments, a perched water table will develop following irrigation or rainfall. In this instance, the perched water table is desired to compen- sate for the low water retention of the fine sand. A USGA green is constructed in this fashion to combine the advantages of com- paction resistance and moisture retention in the root zone. Results are only satisfactory, however, where the design in- cludes a critical depth of the surface medium; a too - shallow sur- face layer will not drain properly, while a layer that is too deep will be too dry at the surface. This can be illustrated using a rectangular household sponge measuring 5 by 3 by 1 inches. When the sponge is saturated and positioned with its 5 -inch and 3 -inch sides in the horizontal plane, very little water drains out. Turning the sponge 90 degrees to position the 3 -inch side verti- cally results in more drainage. Rotating the sponge so that its 5 -inch side is vertical results in still further drainage. This demonstration shows the relationship between height of the water column and water retention within the pore volume of the sponge. Cutting the sponge horizontally into three equal sections while it is still in the 5 -inch vertical orientation and squeezing each section to remove internal moisture would reveal that the uppermost section was driest and the lowermost section was wettest. Thus, the distribution of moisture within (continued