Souls 69 
PLANT-FOOD IN THE SOIL 
The method by which plants secure their food from 
the soil has been known less than a century. From the 
time of the ancient Greeks and Romans down to the 
beginning of the nineteenth century, investigators sought 
to find some one substance in the soil that was the real 
food of plants. At different times it was thought to be 
fire, water, niter, oil, and many other materials. During 
this period all plant-food was supposed to come from the 
soil; it was not known that the greater part of it comes 
from the air. 
Of the ten elements required by plants, seven, in ad- 
dition to those obtained from water, come from the soil. 
These are potassium, phosphorus, calcium, magnesium, 
iron, sulfur, and nitrogen. A number of non-essential 
elements, including sodium, chlorine, and silicon, are also 
taken up by most plants. All crops require the same 
elements for their growth, although they do not use them 
in the same proportion. Sugar-beets and potatoes use 
relatively large quantities of -potassium, the grain crops 
require considerable phosphorus, while alfalfa and clover 
use more calcium. 
Soils are made up largely of insoluble material of no 
food value to plants. The amount of actual plant-food, 
in the soil is comparatively small, but since plants do not 
use large quantities of this food, the supply of most of the 
elements is sufficient for crop production. Only a small 
part of the total plant-food of the soil is available during 
any one year. Roots penetrate every part of the surface 
soil, but they can absorb only the material that is in solu- 
