CHAPTER VIII 
PHOSPHORUS AND LIMESTONE 
The three most important elements in the soil are nitrogen, 
phosphorus and potassium. Of these three soil elements, 
nitrogen is required in the largest amount. While nitrogen 
is no more essential to the growth of corn than the other 
two, it is the element most easily lost from the soil. As we 
have stated under ‘‘Leguminous Crops,’’ nitrogen can be 
gathered from the air and stored in the soil by the growing 
of such crops as clover, cowpeas and soy beans. 
For this reason the growing of clover for the first time 
on over-cropped corn land often makes the soil very produc- 
tive for years to come because the supply of phosphorus and 
potassium has not yet become lowered. But the supply of 
phosphorus, like that of nitrogen, can become so low that 
farm crops (especially clover) will not do well until enough 
phosphorus has been replaced to bring the supply back to 
normal. 
On many soils in the Corn Belt the crop yield is limited 
by the lack of phosphorus rather than by the lack of nitro- 
gen. Now, the only way by which phosphorus can be added 
to the soil is to buy it in some form or other and apply it to 
the land. If the phosphorus content of the soil is actually 
lower than the nitrogen content, there is no doubt but that 
the application of phosphorus would be a profitable invest- 
ment; but to apply phosphorus to soil that is already very 
low in humus and nitrogen is nothing less than throwing 
money away. If clover crops grow large and luxuriant on 
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