32 Development and Activities of Roots of Crop Plants. 



Environmental Conditions of Growth. 



To understand the causes of such remarkable root development, it will 

 be necessary to consider somewhat in detail the environmental conditions 

 under which the crops grew. 



The mellow silt-loam soil, underlaid at a depth of 1 to 1.5 feet with loess 

 of very loose texture, not only absorbs water readily, but has a high water- 

 holding capacity. This ranges from 57 to 64 per cent and is rather uniform 

 to a depth of at least 4 feet, the same type of subsoil extending to depths of 

 many feet. The mechanical analyses of these soils (table 2) shows that they 

 are approximately one-half silt, while the remainder is almost entirely com- 

 posed of very fine sand and clay. 



The chemical analyses are given in table 3. According to the Truog test, 



the soils were slightly acid. The volatile matter and nitrogen are not low, 



while the other critical elements are present in sufficient quantities to assure 



good yields. 



Table 3. — Chemical analyses of soils fr&m Peru.^ 



> Phosphorus determinations were made by digestion with HNO3 and HF; sulphur by fusion 

 with NajOj; potassium by fusion with calcium carbonate; and nitrogen by the modified Gunning 

 method. 



As pointed out elsewhere, (Weaver, 1920 : 100) the chief limiting factor to 

 crop production in the grassland area west of the Missouri is soil-moisture. 

 In general, it may be stated that in eastern Nebraska most of the precipitation 

 falls during the growing-season and less than one-tenth of it during the three 

 winter months. About half of the rainfall of May, June, and July is from 

 rains of an inch or more in 24 hours. Such a seasonal distribution of moisture 

 is very favorable for the growth of crops. Not infrequently, however, storms 

 occur with a rainfall exceeding 2 inches and occasionally 4 or 5 inches in a 

 period of 24 hours. Such storms invariably result in a high run-off, and they 



