Waterscout SMEC 300 Soil Product Manual
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20Volumetric WaterContentThe SMEC 300 measures volumetric water content. Thevolumetric water content (VWC) is the ratio of thevolume of water in a given volume of soil to the total soilvolume. At saturation, the volumetric water content(expressed as a percentage) will equal the percent porespace of the soil.In-field soil moisture content will range from air-dry tosaturation. However, plants cannot extract all the water ina saturated soil and can extract none of the water in an air-dry soil. Instead, two other moisture content levels, fieldcapacity and permanent wilting point are often used toindicate the upper and lower limit of plant available wa-ter. Field capacity is defined as the condition that existsafter a saturated soil is allowed to drain to the point wherethe pull of gravity is no longer sufficient to remove anyadditional water. Water draining from a soil profile can-not, in general, be taken up by plant roots. On the oppo-site end of the spectrum, permanent wilting point is thehighest moisture level at which an indicator plant cannotrecover turgor after being placed in a humid environment.Irrigation should be scheduled somewhere between thesetwo extremes. One rule of thumb is to apply water whenhalf the plant available water has been depleted. Howev-er, individual circumstances may dictate a more conserva-tive or liberal approach. Figure 1 illustrates the plantavailable water range for the 12 USDA-defined soil tex-tures. Keep in mind that these numbers are merely guide-lines and will vary for individual soils. |
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