THE SOIL 



B 



T 5 ^ 







3EE 



Diagram illustrating relation 

 of soil to sub-soil. 



A. Soil. B. Sub-soil. 

 C. Underlying rocks. 



remain for a while in suspension. Some small pieces of dark 



material will probably be noticed floating on the surface of 



the water. The larger particles are 



sand; the fine particles, silt; the 



very fine particles, clay; the float- 

 ing bits of dark material, organic 



matter, or humus. Sand, silt, and 



clay are rock particles, differing 



mainly in size. Organic matter, 



or humus, is the remains of decayed 



plants or animals. 



Where soil comes from. A 



long time ago the upper surface of 

 the earth 

 was solid 



rock, and in some places this condition 

 still exists. By action of wind and rain, 

 heat and cold, plants and animals, and 

 other agencies, many of the rocks have 

 been broken into particles small 

 enough to be washed away by water 

 or blown about by winds. Through 

 a long period of thousands of years 

 these processes have been going on. 

 The rocks on higher places like moun- 

 B. Granule composed tains and hills have been broken up, and 



of very fine particles. ^ ^^ fragments have been carried 

 O* /\ir sjpcicc. 



to lower levels by water and wind, 



where they were left to form the soil as we see it. In the 

 meantime, plants have grown and died and their remains have 

 become mixed with the rock particles. This fact accounts 

 for the presence of organic matter, or humus, in the soil. 



Diagram of soil particles 

 enlarged. 



A. Sand. 



