LOSS OF WATER BY EVAPORATION 27 



the texture of the soil may be changed by cultivation 

 and by the use of manures. If the soil is of very fine 

 texture, as a heavy clay, percolation is slow, and before 

 the water has time to sink into the soil, evaporation 

 begins ; with good cultivation, the water is able to 

 penetrate to a depth -beyond the immediate influence 

 of evaporation. Compacting an open porous soil by 

 rolling, checks rapid percolation and prevents the 

 water from being carried beyond the reach of plant 

 roots. In order to prevent excessive losses by perco- 

 lation, the management must be varied to suit the re- 

 quirements of different soils. In regions of heavy 

 rainfall and mild winters the losses of both water and 

 plant food by percolation are often large. 



23. Loss of Water by Evaporation. The factors 

 which influence evaporation are temperature, humid- 

 ity, and rate of movement of the air. When the air 

 contains but little moisture and is heated and moving 

 rapidly, the most favorable conditions for evaporation 

 exist. In semiarid regions the losses of water by 

 evaporation are much greater than by percolation. 

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

 atmosphere gives up its water, which has been taken 

 from the soil, and, unless checked by cultivation, the 

 subsoil water is brought to the surface by capil- 

 larity and lost. In porous soils, a greater free- 

 dom of movement of the air is possible, which 

 increases the rate of evaporation. When the sur- 

 face 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 



