Thk Ylm.\ Mesa 



241 



shows the silt in the river water itself and not the residue deliv- 

 ered to the land ; this amount is extremely variable thru the year 

 and the average found might vary widely from that actually de- 

 livered. The phosphorus content of the sediment would probably 

 be very slowly available. Based on these considerations, fertilizer 

 needs on the Mesa would probably be found covered best by light 

 applications of acid phosphate, stable manure and leguminous cover 

 crops — a relatively inexpensive practice when compared with that 

 in use in other citrus districts. See Figure 6 and 7 for cover crop. 



ALKALI 



Small areas of alkali occur in the Yuma Mesa, but are neither 

 so extensive nor will they be so difificult to handle as in the valley. 

 These areas are not readily detected due to the shifting surface 

 sands, but it is said they may be traced immediately after a rain. 

 Where alkali does occur it is probably related to the heavier soil 

 phases, such as the clay bands. Collingwood found the following 

 amounts of alkali in the clay seams exposed in the railroad cut at 

 Yuma : 



TABLE VII — ALKALI IX CLAY SEAMS UNDER YUMA MESA 



The analyses given in Table VIII show the nature and amount 

 of alkali in a few spots that show surface indications of alkali. 



TABLE VIII — ALKALI IN ALKALI SPOTS ON YUMA MESA 



Sample 



No. 1 



No. 2 



No. 3 



6915 



6926 



6927 



Sample 6915 was the alkaline calcareous clay described else- 

 where ; 6926 and 6927 were taken from strong alkali spots that had 

 developed after irrigation on the same tract from which Nos. 1, 2 

 and 3 were taken before irrigation. Further analysis showed 6927 

 to contain much calcium chloride. 



