FERTILITY AND HUMUS IN DRY AREAS 415 



Sources of humus. The sources of humus in dry 

 areas includes: (1) organic matter that decays on the 

 surface of the soil ; (2) the roots and stubbles of plants ; 

 (3) green crops grown and buried, and (4) farmyard 

 manures. 



The supply of organic matter from the decay of 

 plants on the surface of the ground is inconsiderable, 

 for the reason, first, that the growth of these is rela- 

 tively small, and, second, that what is produced is in 

 a great measure consumed by animals when grazing. 

 This was true to a very considerable degree, even before 

 the introduction of domesticated animals. The com- 

 paratively small extent to which vegetable soils occur 

 in dry areas in thus accounted for. 



The roots and stubbles of plants are one of the 

 most important sources of humus in dry areas. The 

 plants grown on these are possessed of a relatively 

 long and deep root system. The roots are encouraged 

 to go down because they may secure moisture thereby 

 and also additional food. The opposite is true of plants 

 grown under irrigation. The proportional weight of 

 the' roots of plants differs greatly, and the same also 

 holds true of the weight that the roots bear to the stems 

 and leaves. The weight of the roots of clover is about 

 the sani as that of the portion grown above ground. 

 The weight of the roots of alfalfa is probably much 

 greater than the weight of any single crop of the hay. 

 The roots of brome grass are about the same weight 

 as that of the stems and leaves. The roots of oats 

 are about 40 per cent, of the stems and leaves, whereas 

 the roots of peas are only about 9 per cent, of the por- 

 tion above ground. It would seem correct to say that 

 no plant grown in dry areas will equal alfalfa in the 

 quantity of the organic matter which it brings to the 

 soil in its roots, or in the deep distribution of the same. 



