CROP INDICATORS. 261 



"The distribution of Upper Sonoran crops is at present local; and so de- 

 pendent are many of the crops upon natural protection, adequate water supply, 

 and suitable soils, entirely aside from temperature, that they can not be grown 

 over the whole of a region so varied as the Upper Sonoran of Colorado. " 



Whatever may be the shortcomings of the life-zone concept, they are more 

 or less inevitable in a pioneer work covering such a vast field. With Hilgard 

 and Chamberlin, Merriam must be given great credit for having recognized 

 the value of natural indicators so early, and for pointing out many of the 

 major correlations. His method has formed the basis for the surveys of 

 Western States made by the Bureau of Biological Survey during the past 15 

 years. The first of these was that of Texas, made by Bailey (1905), in which 

 little attention was given to crop correlations. In a similar study of New 

 Mexico (1913; cf. Wooton, 1912: 10) he has discussed the crops of the Lower 

 and Upper Sonoran zones in some detail, especially as to the fruits (23, 38). 

 Cockerell (1897) was the first to give a general discussion of the life zones of 

 New Mexico, as well as the first to make use of insects as zone indicators. 

 Cary (1911: 29, 40) has dealt with the agricultural importance of the Upper 

 Sonoran and Transition zones in Colorado. He has also characterized briefly 

 the agriculture of the same zones in Wyoming (1917: 30, 39) and has pointed 

 out the economic importance of the boreal zones (52). Robbins (1917) has 

 described briefly the native plant communities in Colorado with especial 

 reference to altitude and has discussed the general agricultural relations of 

 the grassland, sagebrush, chaparral, woodland, and montane forest. 



Edaphic indicators of crops and methods. — Variation in crop possibilities 

 within a climate, due to edaphic or soil conditions, may be regional or local. 

 Regional and local variations are both caused chiefly by variations in water- 

 content arising from differences in soil, solutes, or topography, and the only 

 important difference between them is that the one determines the general 

 agricultural practice of a region, and the other that of a neighborhood or of a 

 single farm. The responses of plants to local differences in water-content are 

 readily seen, and the corresponding edaphic indicators are of much value 

 in suggesting desirable or necessary local variations in crops or methods. 

 Since practically all such local differences have to do with water-content or 

 temperature, their indicators have the same general significance as in the case 

 of the more general climatic differences. Such local variations in conditions 

 may often be quite as great as those between adjacent climatic regions and 

 edaphic indicators may consequently denote differences in crops and methods 

 quite as great as climatic ones do. Since the number of such indicators is 

 legion, and every small difference of soil or topography has a corresponding 

 indicator, the adjustment of crop and method to any particular variation in 

 conditions is largely a matter of practicability. Locally as well as generally, 

 the chief differences in soil are represented by saline soil, hard land or gumbo, 

 and sand. All of these have their proper indicators, as is well known, and it 

 is only necessary to recognize that their local occurrence has much the same 

 significance assigned to them by Hilgard, Shantz, Kearney and others for 

 more extensive regions. This is particularly well illustrated in the case of 

 dune-sands, which are found in sandhill areas through the prairies and plains. 

 It is best seen in the great sandhill region of Nebraska, where soil and topo- 

 graphy have combined to present an unusual set of conditions. The loose 



