SOILS 387 



clay, lime, organic matter, and salts are represented, and 

 this is true of colour as well of the other physical conditions. 



The soil is constantly changing, it loses water by direct 

 evaporation, evaporation from plants, also by drainage. 

 The water passing through a soil carries the soluble salts 

 with it, some of which like the nitrates are of the utmost 

 importance, as being th« form in which the plant assimilates 

 nitrogen. The soil gains material from the atmosphere, 

 both nitrogen, carbonic acid, and water, and is enriched by 

 the residue of crops and manures. 



Besides these there are gradual changes taking place in 

 soil ; it is a well-known fact that the most carefully selected 

 grass seeds may fail to produce a luxuriant growth on 

 certain soils, until with the lapse of years these slow 

 changes have become accomplished. 



In the production of changes the part played by the 

 earthworm must not be overlooked, it gradually reduces 

 the soil to a finer condition through the earth which it 

 ejects as castings ; it enriches the soil with nitrogen, and 

 by its physical effect keeps it open for the penetration of 

 both rain and air. It was calculated by Darwin that on an 

 ordinary pasture, the whole of the fine surface soil to a 

 depth of ten inches was passed through worms and turned 

 up in the course of fifty years. 



Ground' Water. — The water in the soil is known as the 

 ground water ; its level varies with the rainfall and with 

 different soils ; it may be at the surface as in the case of 

 marshes, or many feet below as in the case of deep wells. 

 A ground water 5 feet from the surface is too high for 

 health ; 15 feet is a common and safe depth. The ground 

 water is the residue of that which has fallen on the earth, 

 it is constantly seeking its natural outlet into streams 

 and rivers, so that it is always in motion. Its height is 

 determined by the height of the water in neighbouring 

 wells, and fluctuations in its height have been connected 

 with the onset of disease, especially by Pettenkofer, who 

 was the first to direct attention to the question. 



In addition to the ground water there is always moisture 



25—2 

 Digitized by Microsoft® 



