THE SOIL AND ITS MANAGEMENT 



certain of the minerals are decomposed and new substances are 

 formed. For instance, a mineral called feldspar, a common 

 part of rocks, is decomposed under the influence of water con- 

 taining carbonic acid, and forms clay and other products. Most 

 of our clay soils are made in this way. As rocks break down, 

 the soluble material is dissolved by water and carried by streams 

 to the sea, where it con- 

 stantly helps to supply 

 the salts of sea water. 



The fact that little 

 actual breaking down 

 of the rocks can be 

 observed during a life- 

 time goes to show the 

 extreme slowness with 

 which these weathering 

 processes go on. A 

 stone building stands in 

 the weather for a hun- 

 dred years with only 

 a slight roughening of 

 the rock surface. 



52. The mineral mat- 

 ter of soils. The min- 

 eral matter of a Soil In the upper picture, a ledge of partly disintegrated 



makes nn the larcrpr limestone is shown - Roots are growing in the crev- 

 ices of the rock, thus assisting weathering agencies 

 in reducing the rock to soil. In the lower picture, 

 clay and loam have been formed. The roots penetrate 

 through these to a depth of several feet 



FIG. 28. How soils are formed 



part of its weight, the 

 organic matter being 

 usually less than 5 per 

 cent of the total weight, 

 plant foods are derived. 



From the mineral matter most of the 

 This mineral matter varies in texture 

 from coarse gravel to fine clay. The finest of these clay par- 

 ticles are so small as to be visible only with the highest-power 

 microscopes. It would require 25,000 of these minute clay par- 

 ticles, laid side by side, to measure I inch in length. For sandy 

 soils or loess it would ordinarily require from 500 to 1000 soil 



