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• I (see Figure 2) illustrates that it is easier to remove water from a <br /> sandy soil with 10% moisture, than it is to remove water from a <br /> clay soil with 30% moisture.This is because the water in the clay <br /> soil is held in very small capillary spaces within the soil particles <br /> under a higher soil suction,whereas the sandy soil holds water in <br /> large capillary spaces under a lower soil suction. <br /> so <br /> Y <br /> RIM <br /> RI <br /> m <br /> 3 <br /> Y0 CHINO SILTY CLAY <br /> r <br /> d <br /> 3 20 <br /> N <br /> O <br /> 10- PACHAPPA FINE SANDY LOAM <br /> o HANFORD SAND <br /> 0 <br /> 2 5 15 <br /> SOIL SUCTION BARS <br /> Figure 2. <br /> Soilmoisture's Soil Water Samplers allow water to be removed from <br /> the soil by creating a vacuum(negative pressure or suction)inside <br /> the sampler greater than the soil suction holding the water in the <br /> capillary spaces.This establishes a hydraulic gradient for the wa- <br /> ter to flow through the porous ceramic cup and into the sampler. <br /> Note: when evaluating soil suction ratings of a ceramic plate or <br /> cup, a positive pressure rating is used.Water can be held at ten- <br /> sions far greater than 1 atm (the limit for vacuum-type measure- <br /> ments). Positive pressure can force water out of capillary pores <br /> equivalently as negative pressures, and is the practical method <br /> for evaluation of soil suction. <br /> In practice, a vacuum is drawn in the Soil Water Sampler that <br /> exceeds the soil water tension.Then liquid water will flow to the <br /> ceramic cup due to the potential gradient(i.e.water will move from <br /> less negative potential to more negative potential).The practical <br /> limit for water flow in soils is about 65 cb (centibar) (although in <br /> some soils,the value can approach 85 cb).When soil moisture ten- <br /> sions exceed 2 bars,the wetted meniscus in the ceramic pores will <br /> break and the Soil Water Sampler will appear to be unable to hold <br /> vacuum.The ceramic cup will have to be rewetted to hold a vacuum <br /> and soil moisture tensions will have to decrease to less than 85 cb <br /> before water can again be moved toward the ceramic cup. <br /> Additional information on the advantages and disadvantages of <br /> Soil Water Samplers in general can be found in Chapter 19,"C^om- <br /> SOILMOISTURE EQUIPMENT CORP. 5 \® <br /> P.O. Box 30025, Santo Barbaro, California 93105 USA / \ <br /> Phone:(805)964-3525•Fox:(805)683-2189 sowosnNtE <br /> e-mail:soles®soilmoisture.com•Web:Nip://wwwsoilmolsture.com �/ <br />