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OPERATING store's Soil Water Samplers, which are also referred to as T lysimeters" or "lysimeters", have been in general use the world for many years. Soil ater is held largely under a state of tension (negative pres- sure) ithin the soil by capillary forces. The capillary force is the sumo the adhesive and cohesive forces. The adhesive force ischar- acteri ed as the attraction of water for soil solids (soil and organic matt r). Cohesive force is characterized as the attraction of water for it lf. Adhesive force is far greater than the cohesive force. Wate is naturally attracted to soil particles (by its adhesive qual- ity) a d "sticks" to the surface of each particle and in the various sized capillary" spaces or "pores" between the soil particles. When the s it 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. Asa oil dries out, the "excess" water first evaporates as it re- quire less energy to break the cohesive bonds. The remaining water held tightly inside the capillary spaces by adhesive quali- ties, r quires more energy to remove it from the soil. The f lowing illustration (see Figure 1) shows the increasing force requi ed 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 ~norc ener to remove the water from these pores. Even though there may b a considerable volume of water in the soil, the tension that holds he water determines how readily it can be removed. - ~ ~` ~ ~~- ',~- ~a ~~ ~'~ Wet Soil Dry Soil Figure 1. This t nsion that determines how moisture moves in the soil is referr d to as "soil water tension", "negative pore pressure", oz' "soil suctio ". For simplicity's sake we refer to this tension as "soil suc- tion" i these instructions, but keep in mind that negative pres- sure i the most descriptive term. The flowing graph shows the relationship between the percent of mo tore in a soil and the soil suction required to remove the moist re from three types of soil: clay, loam, and sand. The graph SOIIMOISTURE EQUIPMENT CORP. ~ 4 ^° P.O. Box 30025. Sonto Borboro. Colifornio 93105 USA / \ Phone: (8051 964-3525 ~ Fox: (8051 683-2189 SOIlMOISTURE e-moil: soles©soilmoislure.<om ~ Web: hftp://wwwsoilmoi ture.com