FERTILITY AND HUMUS IN DRY AREAS 413 



upper soil, that is, the soil which may be regarded as 

 the domain of root growth. 



The supply of humus in dry areas. The supply of 

 humus in dry areas is much less than in humid areas. 

 The difference in the relative proportions cannot be 

 stated, as the humus supply does not increase in all 

 instances as humidity increases, nor is the supply lim- 

 ited in proportion to the dryness of the climate. The 

 character of the soil exercises a potent influence. 



The supply of humus in dry areas is low because: 

 (1) The proportion of the sandy soils in these is greater 

 than in humid areas. Especially is this true of the 

 Inter-mountain areas of the west. These do not pro- 

 duce a strong vegetation in the absence of irrigating 

 waters. (2) The plants are bunched, as it were, in their 

 habit of growth, hence the number of the plants on a 

 given area is relatively small. In dry areas a tough, 

 dense sod is rarely found, though of common occur- 

 rence in humid areas. The proportion of organic matter 

 thus produced is much less than in the latter. (3) The 

 growth in dry areas is less luxuriant than in humid 

 areas. It is in a sense dwarfish in its nature. The 

 hay or straw taken from a piece of land in dry areas 

 is relatively small. The trees show the same character- 

 istics. (4) Much of the growth in dry areas has been 

 burned through long centuries, and this has greatly 

 reduced the humus supply in the soil. (5) The extent 

 to which alkali prevails on western soils has interfered 

 adversely with the growth of vegetation, in many in- 

 stances to its entire exclusion. 



From what has been said it will be apparent that 

 it is more difficult to increase humus in dry than in 

 humid areas. In the latter, when soils are left untilled 

 for a time, they will produce, as it were, spontaneously, 

 a luxuriant growth of weeds. Later these will be suc- 

 ceeded by native grasses which will dislodge the weeds, 



