PHYSICAL PROPERTIES OF SOILS 33 



be retained and not carried beyond the reach of crops 

 by percolation, there would be an ample supply for 

 agricultural purposes. The texture of the soil may be 

 changed by cultivation and by the use of manures so 

 as to prevent losses by percolation. 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 takes place. With good cultivation, the 

 water is able to penetrate to a depth beyond the im- 

 mediate influence of evaporation. Compacting an open 

 porous soil by rolling, checks rapid percolation and pre- 

 vents the water from being carried beyond the reach of 

 plant roots. Thus it will be seen that the treatment 

 necessary to prevent excessive losses by percolation, 

 varies with different soils. In regions of heavy rain- 

 fall 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, 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 semi-arid 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 taken from the soil, 

 and, unless checked by cultivation, the subsoil water is 

 brought to the surface by capillarity and lost. In porous 



D 



