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OPERATING PRINCIPLES Soilmoisture's Soil Water Samplers, which are also referred to as "suction lysimeters" or "lysimeters", have been in general use around the world for many years. Soil water is held largely under a state of tension (negative pres- sure) within the soil by capillary forces. The capillary force is the sum of the adhesive and cohesive forces. The adhesive force is char- acterized as the attraction of water for soil solids (soil and organic matter). Cohesive force is characterized as the attraction of water for itself. Adhesive force is far greater than the cohesive force. Water is naturally attracted to soil particles (by its adhesive qual- ity) and "sticks" to the surface of each particle and in the various sized "capillary" spaces or "pores" between the soil particles. When the soil is very wet, the large pores fill with water. This "excess" water has no direct surface contact with the soil and is held cohe- sively, one water molecule to another, and can move quite freely. As a soil dries out, the "excess" water first evaporates as it re- quires less energy to break the cohesive bonds. The remaining water, held tightly inside the capillary spaces by adhesive quali- ties, requires more energy to remove it from the soil. The following illustration (see Figure 1) shows the increasing force required to remove water from the small-sized capillary pores com- pared to the large pores as the soil dries out. When the remaining water is held only in extremely small pore spaces, it requires more energy to remove the water from these pores. Even though there may be a considerable volume of water in the soil, the tension that holds the water determines how readily it can be removed. ~ ,~ Wet Soil Dry Soil Figure 1. This tension that determines how moisture moves in the soil is referred to as "soil water tension", "negative pore pressure", or "soil suction". For simplicity's sake we refer to this tension as "soil suc- tion" in these instructions, but keep in mind that negative pres- sure is the most descriptive term. The following graph shows the relationship between the percent of moisture in a soil and the soil suction required to remove the moisture From three types of soil: clay, loam, and sand. The graph SOILMOISTURE EQUIPMENT CORP. 4 / \° P.O. Boa 30025, Sonto Borboro, ColiFornio 93105 USA / \ Phone: (805) 964-3525 ~ Fax: 1805) 683-2189 soltM\ oisroxe e-moil: solesBsoilmoisture.com • Web: http://wwwsoilmoisture.com