SOIL RELATIONS 235 



same direction in which the proportion of humus and 

 mineral bases increases, which is to the east and northeast. 

 In the Great Plains such increase occurs to the north and 

 west. It is true that protein production is largely 

 dependent upon climate (317), but its source, to begin 

 with, must be in the soil. 



247. Humus. Experiments show that the loss of 

 food materials from the soil after continuous cropping is 

 not simply a decrease in the amount of these present. 

 There is also a gradual loss of humus, and the soil gets out 

 of the proper condition for using the valuable salts that 

 may be present. An important influence of the humus 

 in this connection is its ability to conserve moisture. 

 Humus, therefore, appears to play a double role, of fur- 

 nishing nitrogen and conserving moisture (Snyder, 1896). 



Such facts may at least partially explain why grain soils in places 

 in the middle West and Northwest have failed to give such returns 

 in recent years as formerly, apparently from some other cause than 

 unfavorable seasons. By crop rotation and other means, the 

 humus content should be restored. 



In the direction of diminishing rainfall and as the native grass 

 covering becomes more scant, the depth of humus diminishes, until 

 finally, in the strictly arid regions, there is little or none. As an 

 offset to this condition, it is also a fact, as shown by Hilgard and 

 Jaffa, that the humus of arid regions is much richer in nitrogen 

 than that of humid regions. The actual percentage of humic 

 nitrogen in the soil, therefore, is apparently about the same in arid 

 regions as in semi-arid regions. 



248. Importance of lime. One must always keep in 

 mind the great importance of lime as a grain soil constit- 

 uent. It is indirectly an indispensable substance. The 

 necessity for lime applications in humid areas for correct- 

 ing soil acidity is well known. There are several other 



