22 SOILS 
Most sandy, clay or heavy soils contain sufficient pot- 
ash and phosphorus, but are wholly lacking in humus, 
organic matter and nitrogen. 
One writer says that “ corn, oats and wheat are made 
of plant food; that they consist of ten certain elementary 
substances; that a ton of corn contains a ton of these 
plant food elements, of which only three are secured by 
the corn plant from air and water. The seven are taken 
from the soil. The three elements make up more than 
ninety per cent. of the corn, but the other seven are no 
less essential to plant growth.” 
The seven elements mentioned and which make up but 
ten per cent. of the corn crop, are generally found in 
sufficient quantities in all soils to last from 500 to 17,- 
600 years. 
The chief lack of worn-out soils is humus, organic 
matter and nitrogen. 
Humus is the residue of decayed organic matter. 
Organic matter is vegetable or animal matter, like 
leaves, roots, sticks, grasses, manure, straw, etc. 
Therefore that which is left of organic matter after it 
has passed through its process of decay is humus. It 
appears in the soil as a dark-colored substance, and where 
it exists in abundance renders the soil black. 
Soils originally procured their entire supply of humus 
and a large portion of nitrogen from decayed vegetation 
or organic matter, secured in their progress of formation. 
Virgin soils procured their largest supply of nitrogen 
from the air through the work of those soil bacteria who 
make their homes in the root nodules of those plants 
known as the nitrogen-gathering plants or the legumes, 
