30 SOILS AND FERTILIZERS 



23. Loss of Water by Evaporation. — The factors 

 which influence evaporation are temperature, humidity, 

 and rate of movement of the air. When the air con- 

 tains but little moisture and is heated and moving 

 rapidly, the most favorable conditions for evaporation 

 exist. In semiarid regions the losses of water bv 

 evaporation are much greater than by percolation. 

 The dry air comes in contact with the soil, the soil 

 atmosphere gives up its water, and, unless checked by 

 cultivation, the subsoil water is brought to the surface 

 by capillarity and lost. In porous soils, a greater 

 freedom of movement of the air is possible, which in- 

 creases the rate of evaporation. When the surface of 

 the soil is covered with a layer of finely pulverized 

 earth, or with a mulch, excessive losses by evaporation 

 cannot take place, because a material of different tex- 

 ture 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 influ- 

 enced. Hot dry winds may cause crops to wilt be- 

 cause the water lost by transpiration exceeds the amount 

 which the plant takes from the soil. 



