29 

 Withdrawal from depths as great as 25 feet is practicable with 

 this eauipment in soil which is neither too wet nor too coarse. 

 In wet clay the soil sticks to the tube and is difficult to dis- 

 lodge, while saturated soil slips from the tube and is lost be- 

 fore it can be drawn to the surface. Samples of coarse material 

 greater than the diameter of the tube cannot be obtained with 

 this equipment. Most of the valley lands may be sampled with the 

 soil tube, but alluvial fans, gravel areas, and other coarse and 

 rocky places require pits, shafts, or tiinnels. 



Samples of soil obtained through use of the soil tube 

 weigh 150 to 200 grams, but in rocky soil large samples of the 

 material are more representative. Accordingly, from pits or 

 shafts, 4,000-gram samples are obtained without reference to size 

 of particles. After they have dried, the rocky portions are 

 screened out and classified as rock. 



The equivalent depth of water in soil samples may be found 



PVd 

 from the equation D = Yoo'' ^^ which D is the equivalent depth in 



inches; P, percentage of moisture in the sample; V, apparent spe- 

 cific gravity of the soil in place; and d, depth of soil sample 

 in inches. 



The depth to which soil samples are taken depends upon the 

 depth to which roots go in search of moisture. As previously 

 shown, some vegetation is deep-rooted while other species have 

 roots relatively close to the surface. Moisture may percolate to 

 depths beyond the root zone, but root extraction determines the 

 depth to which it is necessary to take soil samples. Beyond this 

 depth percolating water contributes to the underground -water sup- 

 ply. Thus, by sampling, to determine the use of water by deep- 

 rooted shrubs it might sometimes be necessary to drive soil tubes 

 to depths of 25 feet or more, whereas for shallow-rooted grasses 

 4 to 6 feet would be sufficient. 



