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OPERATING PRINCIPLES <br />Soilmoisture's Soii Water Samplers, which are also referred to as <br />"suction lysimeters" or "lysimeters", have been in general use <br />around the world for many years. <br />Soil water is held largely under a state of tension (negative pres- <br />sure) within the soil by capillary forces. The capillary force is the <br />sum of the adhesive and cohesive forces. The adhesive force is char- <br />acterized as the attraction of water for soil solids (soil and organic <br />matter). Cohesive force is characterized as the attraction of water <br />for itself. Adhesive force is far greater than the cohesive force. <br />Water is naturally attracted to soil particles (by its adhesive qual- <br />ity) and "sticks" to the surface of each particle and in the various <br />sized "capillary" spaces or "pores" between the soil particles. When <br />the soil is very wet, the large pores fill with water. This "excess" <br />water has no direct surface contact with the soil and is held cohe- <br />sively, one water molecule to another, and can move quite freely. <br />As a soil dries out, the "excess" water first evaporates as it re- <br />quires less energy to break the cohesive bonds. The remaining <br />water, held tightly inside the capillary spaces by adhesive quali- <br />ties, requires more energy to remove it from the soil. <br />The fallowing illustration (see Figure 1) shows the increasing force <br />required to remove water from the small-sized capillary pores com- <br />pared to the large pores as the soil dries out. When the remaining <br />water is held only in extremely small pore spaces, it requires more <br />energy to remove the water from these pores. Even though there <br />may be a considerable volume of water in the soil, the tension that <br />holds the water determines how readily it can be removed. <br />{ <br />~ s -~~ ~~ <br />' ~l <br />Wet Soil Dry Soil <br />Figure 1. <br />This tension that determines how moisture moves in the soil is <br />referred to as "soil water tension", "negative pore pressure", or "soil <br />suction°. For simplicity's sake we refer to this tension as "soil suc- <br />tion" in these instructions, but keep in mind that negative pres- <br />sure is the most descriptive term. <br />The following graph shows the relationship between the percent <br />of moisture in a soil and the soil suction required to remove the <br />moisture from three types of soil: clay, loam, and sand. The graph <br />SOILMOISTURE EQUIPMENT fORP. ¢ ^° <br />P.O. Box 30025. Santo Barbaro, California 93105 USA / \ <br />Phone: 1805) 96q-3525 ~ Fox: (8051 683-2189 SOILMOISTURE <br />e-moil: soles©zoilmoisture.<om ~ Web: http://wwwsoilmoisture.com <br />