48 THE ELEMENTS OF PLANT FOOD 



In the form of a superphosphate the phosphorus 

 is available, and the treated rock becomes a valuable 

 fertilizer. 



The untreated jock, called floats, may be acted on 

 by the carbonic acid or the humic acids in the soil and 

 rendered slowly available. If an immediate return 

 of fertility is desired, a superphosphate should be 

 used. Permanent improvement in a soil may be made 

 by use of the mineral phosphates or floats, which be- 

 come slowly available as plant food, spreading their 

 effect over a number of years. 



Reverted phosphoric acid represents a condition of 

 turning back of the superphosphate, or the available 

 phosphoric acid, to the unavailable form. It is still 

 counted as available, though its degree of availability 

 is lessened. 



Note. The conservation of the fertility of the soil is a most 

 serious material problem, and the vital factor in this problem is 

 the supply of phosphorus. The solution of the question of where 

 the supply of phosphorus for future generations is coming from is one 

 which can be delayed but a few years longer, if the fertility of the soil 

 is to be maintained. It has been estimated that there is not enough 

 native phosphorus in the upper seven inches of the average soil of the 

 corn belt to last fifty years, if maximum crops are taken off each year. 

 Long before the expiration of the half century, maximum crops will 

 probably be impossible because of the gradual exhaustion of phosphorus. 

 The visible supply of rock phosphate, at the present rate of use, will 

 not last fifty years. The other commercial source is bone meal, a prod- 

 uct made by fertilizing companies and packers by grinding the bones 

 of animals slaughtered for food. Although the supply from this source 

 is slowly increasing, the total output is not adequate to make good the 

 depletion of the soil by continuous harvesting of crops. 



