26 THE CHEMISTRY OP THE FARM 



tance ; such water is said to be raised by capillary 

 action, which is simply a manifestation of the attrac- 

 tion for water exerted by the surfaces of soil particles. 

 The finer are the particles of the soil, and the closer 

 they are packed, the greater will be the height to 

 which water will be carried by capillary action. The 

 quantity of water reaching the surface diminishes, 

 however, rapidly as the distance it has to travel in- 

 creases. The quantity of water raised also diminishes 

 when the fineness of the particles exceeds a certain 

 point. It is not always the soil with the finest par- 

 ticles which brings most water to the surface. For 

 every distance between the water-level and the surface 

 there is a certain degree of fineness of the soil particles 

 which will act most effectively. Capillary action is 

 seldom able to maintain a sufficient supply of water 

 at the surface. At Wisconsin crops suffer from 

 drought, though a permanent water supply exists five 

 feet below the surface. Capillary action is most 

 effective in the case of silty soils ; such soils, having 

 been deposited from running water, consist of very 

 fine uniform particles, but without any true clay. 



The average annual evaporation from a water surface 

 in the neighbourhood of London amounts to 20'6 

 inches, according to Greaves ; the maximum monthly 

 evaporation of 3'4 inches occurs in July, the minimum 

 of 0'5 inches in December. The evaporation from a 

 saturated soil is greater than from a water surface; 

 as the soil dries the rate of evaporation rapidly 

 diminishes. The average annual evaporation from a 

 bare loam at Rothamsted is about 17 inches. Soils 

 of various character evaporate equal amounts while 

 saturated, but exhibit great differences as drying pro- 



