The moisture content of the soil fluctuates between the 

 lower limit represented by PWP and the upper or wet end of 

 the available moisture range — field capacity (FC). FC, 

 which is defined as the amount of moisture retained by a 

 given soil after a period of free drainage, is approximately 

 twice the PWP. Tests have shown that the SP over a wide 

 textural range is equal to four times the PWP. Therefore, 

 the salt concentration in a saturated extract is roughly 

 one-half that of the soil solution at FC and one-fourth the 

 concentration of the soil solution at the PWP. 



Soil -Moisture Relationships . In addition to the difficulty 

 evident in the selection of samples representative of 

 average soil conditions from a given field and the bio- 

 chemical disturbances created during soil preparation in 

 the laboratory, certain field conditions exist which cannot 

 be duplicated. For example, tiled fields seldom occupy 

 areas having the homogeneity that is found in a small sample 

 of soil. Stratigraphic and structural variability is 

 prevalent in the soils on the west side of the Valley. 

 Efficient tile operation is dependent upon the presence of 

 coarse-textured material within the soil profile; therefore, 

 certain portions of fields are not only drained at different 

 times depending upon the portion of the field being irriga- 

 ted, but also at different rates depending upon differences 

 in soil permeability and lateral hydraulic conductivity. 

 Vast areas of soils along the west side have been classified 

 as having greater lateral than vertical hydraulic conduc- 

 tivity (29). These soil conditions were somewhat substantia- 

 ted by the quantities of effluent discharged from several 

 tile systems investigated in the study area. 



Whenever tile systems are installed in soils such as those 

 Just described, the coarse-textured material (aquifers) is 

 leached rapidly and the finer textured portions of a field 

 sire leached more slowly. The intensity of irrigation and 

 its duration may determine to some extent the effective 

 leaching of these fine-textured regions of a field. (These 

 isolated conditions are seldom shown on a soils map.) These 

 dense areas approach saturation more slowly than the coarse - 

 textured areas during normal irrigation. If nitrate ions or 

 other ions are present, they may not have the opportunity 

 to solvate and drain into the shallow ground water. There- 

 fore, soil moisture extracted directly from the field by 

 means of porous cups may be more indicative of the actual 

 field salinity and soil nitrate levels than saturated 

 extracts. The number of samples collected and timing of 

 sampling (for different periods of agricultural activity), 

 the depths sampled and the specific textural areas sampled 

 all play an important part in obtaining a soil sample repre- 

 sentative of moisture and salt conditions within an entire 



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