28 SOILS AND FERTILIZERS 



of different texture is interposed between the soil and 

 the air. 



24. Loss of Water by Transpiration. Losses of 

 water may also occur from the leaves of plants by the 

 process known as transpiration. Helriegel observed 

 that during some years 100 pounds more water were 

 required to produce a pound of dry matter than in 

 other years, because of the difference in the amount of 

 water lost by transpiration. The loss of water by 

 evaporation can be controlled by cultivation, but the 

 loss by transpiration can be only indirectly influenced. 

 Hot, dry winds may cause crops to wilt because the 

 water lost by transpiration exceeds the amount which 

 the plant takes from the soil. 



25. Drainage. Good drainage is essential in order 

 to properly regulate the water supply. An excessive 

 amount of water in the soil is equally as injurious as 

 a scant amount. If the water which falls on the land 

 is allowed to flow over the surface and is not retained 

 in the soil, there is not sufficient reserve water for 

 crop growth. The object of good drainage is to store 

 as much water as possible in the subsoil and to pre- 

 vent surface accumulation and loss. Good drainage 

 is accomplished by thorough cultivation, and in re- 

 gions 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. The drainage of wet and low lands forms an 

 important feature of rural engineering. Many swampy 

 lands are highly productive when properly drained. 



