CHAPTER XIII 



PHOSPHORUS 



PHOSPHORUS is the only element that must be purchased and 

 returned to the most common soils of the United States. Phos- 

 phorus is the key to permanent agriculture on these lands. To main- 

 tain or increase the amount of phosphorus in the soil makes pos- 

 sible the growth of clover (or other legumes) and the consequent 

 addition of nitrogen from the inexhaustible supply in the air; 

 and, with the addition of decaying organic matter in the residues 

 of clover and other crops and in manure made in large part from 

 clover hay and pasture and from the larger crops of corn and other 

 grains which clover helps to produce, comes the possibility of 

 liberating from the immense supplies in the soil sufficient potas- 

 sium, magnesium, and other essential abundant elements, supple- 

 mented by the amounts returned in manure and crop residues, 

 for the production of large crops at least for thousands of years; 

 whereas, if the supply of phosphorus in the soil is steadily de- 

 creased in the future, in accordance with the past and present most 

 common farm practice, then poverty is the only future for the 

 people who till the common agricultural lands of the United States. 



And this does not refer to the far-distant future only, for the 

 turning point is already past on most farms in our older states and 

 on many farms in the corn belt; and lands that have passed their 

 prime with sixty years of cultivation will decrease rapidly in pro- 

 ductive power and value during another sixty years of similar 

 exhaustive farm practice. 



The world's supply of phosphorus exists in three principal 

 sources: First are the supplies in the various soils, concerning 

 which the reader of the preceding pages will have sufficient posi- 

 tive knowledge for intelligent thought. 



Second are the natural beds of calcium phosphate, varying in 

 purity from a few per cent, to as high as 80 per cent, of tricalcium 

 phosphate, Ca 3 (PO 4 ) 2 . 



183 



