THE SOIL’ 
147 
Sources of Soil Material.—Soil material comes from 
inorganic and organic matter. By inorganic matter 
we mean material that is made from the decomposi¬ 
tion of rocks, by such agents as water, air, and heat. 
The organic matter or humus is the remains of de¬ 
cayed plants and animals. The greater part of cul¬ 
tivated soil is made up of inorganic matter and does 
not require as much of our time and attention as 
the humus supply in the soil. (Fig. 112, Soil Materials). 
'IG 112.—Soil materials, air, water, plants, and heat working to breaK up xne 
rocks into finer particles. 
Soil Building Forces.—There are many forces such 
as water, wind, ice, heat, cold, frost, plants and animals, 
and elements in the atmosphere that are working hard 
day and night, breaking, grinding, transporting and de¬ 
caying the solid rock of the earth’s surface. Let us 
summarize briefly the special work done by our princi- 
pal weathering agencies. 
Water pounds, grinds, and carries material from 
place to place. It also penetrates large pieces of rock, 
