142 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



The roots of plants, however, may sometimes extend to con- 

 siderable depths, and the presence of the water-table at a mod- 

 erate distance from the surface, is thus an advantage. 



If moist air comes in contact with a cold surface it will deposit 

 water. A soil may gain water from the air when the air is moist 

 enough and the surface of the soil cool enough. It has been 

 thought that water is sometimes distilled, as it were, from the 

 lower layers of the soil into the upper. 



The monthly drainage from Mr. Greaves drain gauge 1 in Eng- 

 land, filled with gravel and free of vegetation, has in fourteen 

 years exceeded the monthly rainfall nineteen times; twice in 

 December, seven times in January, seven times in February, and 

 three times in March. The amount of water condensed from the 

 air must, therefore, have been in each case more than that lost by 

 evaporation from the soil, and may therefore increase the 

 moisture of the soil. In Texas very heavy dews have been 

 observed at various times. Drain gauges in England sometimes 

 run more water than falls as rain, especially in January, 

 February, and March. 



Losses of Water from the Soil. The water which comes to the 

 soil is lost in several ways. Part flows away without penetrating 

 the soil, as surface water. Part percolates through the soil to 

 the water-table and reappears in drains, wells or springs, or 

 passes through subterranean channels to the sea; this may be 

 called percolation water. Another portion of the water is evapo- 

 rated from the soil into the atmosphere. Finally, the water taken 

 up by plants is passed off through their leaves into the air 

 (transpiration). 



The proportion of the rainfall which passes off as surface water 

 depends upon (a) the character and condition of the soil; (b) 

 the slope of the land; (c) the amount and duration of the rain- 

 fall. If the soil is loose and porous, either naturally, or rendered 

 so by cultivation, a larger amount of water will penetrate it. The 

 slope of the land determines the rate at which the water runs off ; 

 the shorter the time of contact between soil and water, other 

 1 Warington, Physical Properties of the Soil. 



