18 PRINCIPLES 6 AMERICAN FO&ESTRY. 



ing, and then commence to wash very badly. The washir g 

 away, then, is due to the humus having become used up 

 and there being nothing left to bind the soil particles to- 

 gether. In such cases the application of organic matter 

 will help very materially. For this purpose manure, straw, 

 or other material may be applied, or crops like clover and 

 the grasses, which leave considerable organic matter, may 

 be grown on the land. Crops that leave very little humus 

 in the ground, such as nursery stock, which is dug out by 

 the roots, are most harmful in exhausting the humus in the 

 soil, and land used for this purpose needs heavy manuring 

 with stable manure and an occasional seeding down to 

 grass or clover. 



Alkali Soils. In the prairie portions and occasionally 

 elsewhere we have a kind of soil in which there is a super- 

 abundance of carbonate and sulphate of soda. This kind 

 of soil seldom extends over large areas and generally 

 occurs in places lower than the surrounding land. In some 

 places the alkali occurs in such abundance as to coat the 

 surface of the soil with a white crust. On such land very 

 few agricultural crops or trees grow w r ell. The leaves of 

 the trees growing there generally take on a yellowish color 

 and the wood does not mature well in the autumn. Such 

 land should be drained so that the surface water at least 

 can run off. In this way the alkali can generally be 

 washed out in a few years. It is seldom advisable to plant 

 trees on these places, but if this seems desirable, as is some- 

 times the case on prairies, the best trees to plant are proba- 

 bly the Cottonwood and White Willow. 



EFFECT OF SLOPE AND ASPECT ON TREE-GROWTH. 



The slope of the land affords drainage and so affects the 

 growth of trees, but trees will grow on any slope, even on 

 precipices, if they can find room for their roots and the soil 

 is somewhat moist. The direction of the slope usually 



