28 



SOIL -MOISTURE STUDIES 



It has been shown that the limitations of soil tanks make 

 them inadequate for some types of consumptive use investigations. 

 Tanks are suited to areas of high ground water where studies are 

 to be made with definite water levels but studies in other areas 

 where the water table is beyond reach of root systems may best be 

 carried on through soil sampling. The Division of Irrigation and 

 various agricultural experiment stations have employed soil sam- 

 pling for agricultural crops and to some extent for natural vege- 

 tation. 



Soil-moisture studies require systematic collection of 

 many soil samples taken to depth beyond the reach of plant roots. 

 This is done through use of soil tubes of different lengths driven 

 into the soil to known depths. The samples obtained are dried in 

 an electric oven at a temperature of 110 C. Standard laboratory 

 practices are followed. 



Collection of soil samples is a laborious process, as the 

 manual effort of driving soil tubes by hand, especially for depths 

 beyond a few feet, is extremely arduous. To lessen the labor and 

 expedite the work a compressed air unit developed by the Division 

 of Irrigation (3) drives the soil tube mechanically. The entire 

 equipment, shown in Plate I-B, consists of an air compressor, a 

 soil tube, and a soil tube jack. The air unit includes a com- 

 pressor mounted on a truck, a light air hammer, and an air hose. 

 It provides a pressure of 100 pounds per square inch, delivering 

 2,250 blows per minute to the soil tube. 



The soil tubes are of l6-gage seamless steel tubing, from 

 5 to 2$ feet in length, fitted with a suitable driving head and a 

 cutting point. The point is of case-hardened nickel steel with a 

 choke bore to overcome friction within the tube. 



A very efficient, light-weight jack shown in the foreground 

 in Plate I^B has been perfected by the Division of Irrigation (31) 

 to draw the soil tube from the ground under difficult conditions. 



