PHYSICAL PROPERTIES OF SOILS 35 



lation and loss. Good drainage is accomplished by 

 thorough cultivation, and in regions of heavy rainfall, 

 by tile drainage. Well-drained land is warmer in the 

 spring, has a larger reserve store of water, and is in 

 better condition for crop growth. Many swampy lands 

 are highly productive when properly drained. A high 

 state of productiveness cannot be maintained without 

 suitable provision for drainage. When the pores of the 

 soil are filled with water, air is excluded and the neces- 

 sary chemical and bacteriological changes which result 

 in rendering plant food available fail to take place. The 

 drainage of wet and low lands forms an important fea- 

 ture of rural engineering. 



t 



26. Influence of Forest Regions. The deforesting of 

 large areas near the sources of rivers has an injurious 

 influence upon the moisture content of adjoining farm 

 lands. By cutting over and leaving barren large tracts, 

 less water is retained in the soil. Also near forests the 

 air has a higher moisture content, due to the water given 

 off by evaporation. Lands adjacent to deforested dis- 

 tricts lose water more rapidly by evaporation, because 

 the air is so much drier. In Section 24 it is stated that 

 losses of water by transpiration can be indirectly influ- 

 enced. This can be accomplished by retaining the 

 forests. 



Good drainage is necessary not only for individual 

 farms, but also for an entire community. Good storage 

 capacity in the form of forest lands, for the surplus 



