Laserfiche WebLink
(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 capiIlary spaces within the soil particles <br />under a higher soil suction, whereas the sandy sail holds water in <br />large capillary spaces under a lower sail suction. <br />M <br />.~ <br />T <br />L <br />Q_ <br />Y <br />N <br />O <br />0 <br />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, "Com- <br />SOILMOISTURE EQUIPMENT CORP. <br />P.O. Box 30025. Sonto Borboro, Colifornio 93105 USA <br />Phone: (8051 964-3525 ~ Fox: 1805) 683-2189 <br />e-moil: soiesQsoilmoisture.com ~ Web: http: //wwwsoitmoisture.com <br />/ \ <br />SOILMOISTURE <br /> <br />