298 TwENTY-SIiVKNTH ANNUAL REPORT 



This observation on the behavior of phosphoric acid has important 

 bearing on the theory of soil fertihty. Probably aside from physical 

 and chemical processes, plant foods are fixed from the soil water and 

 conserved by lower organisms in deposits other than their own tem- 

 porary remains. 



CALICHE 



Large areas of mesa soil in the Southwest are underlaid at various 

 depths by deposits of lime-cemented soil, sand, and gravel commonly 

 called caliche, but recently named desert limestone by geologists. While 

 these areas, because of the caliche, are less valuable for agricultural 

 purposes, the analyses published in the Twenty-fourth Annual Report 

 of this Station, page 567, show caliche itself to be supplied with plant 

 food as richly at least as the soils with which it occurs. Because of its 

 dense, hard character caliche takes water badly and plant roots cannot 

 penetrate it. Nevertheless, where covered with a fair amount of soil, 

 these desert limestone areas may become important in the production of 

 special crops such as cactus, agave, guayule, and desert olives. It has 

 generally been held that caliche resulted from some form of evapora- 

 tion. The usually accepted theories are given in detail in Guild's 

 Mineralogy of Arizona, page 48. Reasoning from observations made 

 in connection with the study of the Salton Sea the writer was led to 

 suspect the organic origin of these limestone deposits. Consequently, 

 field and laboratory studies have been begun. In the field the lower 

 courses of caliche are dense, while toward the top of the deposit 

 calcareous caps with occasional interlying layers of uncemented soil 

 are more frequent. Occasionally, however, caps are found beneath 

 strata of dense caliche a foot or more in thickness. The strongest evi- 

 dence of organic origin of caliche is found in these caps. The to- 

 pography of the capped surface is very irregular and successive over- 

 lying caps often approach within two or three inches at one place, 

 while ten feet away they may be separated by a foot or eighteen inches 

 of cemented gravel. The caps in passing over boulders take the curva- 

 ture of the boulder, and even may be found overhanging. Instances 

 occur of erosion of the massive caliche and a subsequent deposition of a 

 cap over the nearly perpendicular eroded surface. It would be difficult 

 to explain these features by evaporation or simple sedimentation. Gen- 

 erally the caps occur on massive beds of cemented gravel, but frequently 

 seams of gravelly loam an inch and even more in thickness may be 

 found overlying one cap with a second thin cap overlying the loam. 

 The lower face of the cap in contact with the loam is an exact cast of 



