34 SOILS AND FERTILIZERS 



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soils a greater freedom of movement of the air is possi- 

 ble, which increases the rate of evaporation. When the 

 surface of the soil is covered with a layer of finely pul- 

 verized earth, or with a mulch, excessive losses by 

 evaporation cannot take place, because a material 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 ob- 

 served 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 in this way. The loss of water by evapora- 

 tion 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 excess 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 prevent surface accumu- 



