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oul~22 VIII.6 The neutron meter does have several drawbacks. It is a somewhat complex electronic device, of which the earlier portable models were subject to breakdown' or malfunction. Later models have proven reasonably dependable. However, procedures for checking and standardizing instru- ments must be followed faithfully to assure dependable results. Certain measurement problems are inherent in the nuclear techniq\l.e. At depths less than 1 foot beneath the soil surface the meter's sphere of influence intElrsects the air-soil interface; because some neutrons pass into .the air and are not moderated, the meter reads too little a water content. The surface-moisture probe provides a means for observing moisture content at and near the soil surface. The large sphere of influence also "dullS" the vertical resolution of the instrument--lines of demarcation between zones of different water content are not defined sharply. The meter readings are integrated values that are influenced somewhat by wet or dry zones above or below the neutron source. The neutron meter provides a very convenient means for periodic observation of changes in total water content and in depth of water penetration in a soil profile. The depth-density meter provides data that may improve interpretations of soil-water content at depth. Tensiometers Tensiometers are porous cups filled with water and embedded in the soil. Capillary potential, or the tension of the soil water, is measured by a vacuum gage or manometer connected to the porous cup by means of a tube. The tension between the soil water and the soil particles, as . measured by the tensiometer, has a vital relationship to the well being of plants; in this regard, the tension is perhaps more meaningful than volumetric moisture content. However, for many hydrOlogiC purposes, volumetric soil-water content is desired as a basis for determining the equivalent depth of water. Volumetric water content can be related directly to soil-water tension in the particular soil; however, this relationship may vary with changes in soil texture, density, or temperature. The relationship between soil-water content and tension as deter- mined in the laboratory may not be fully applicable to field conditions. A tensiometer may be calibrated in the field by gravimetric sampling close at hand. Such calibration, however, generally is only approximate as the relationship of tension to water content varies according to a wetting-and-drying hysteresis--that is, a soil that is drying after having been wet contains more water at the same tension than when that soil is being wetted.