THE VALUE OF MINERAL MANURES. 223 



the phosphatic guanos are almost devoid of nitrogen in any 

 form and contain from 20 to 50 per cent, of phosphate of 

 lime; equivalent to about half as much phosphoric acid. 

 The guanos now in the market are practically phosphatic 

 manures, and are reduced to superphosphate by means of 

 sulphuric acid, as will be explained in the next chapter un- 

 der the head of superphosphate of lime. 



All these mineral manures are of exceedingly great value 

 for the culture of farm crops ; so much so that no farmer 

 can afford to neglect them. They furnish plant food in the 

 most available form and when used with skill and judgment 

 return a large profit on their cost. The example given of 

 the production of 15 tons of hay by the use of 600 Ibs. each 

 of salt and plaster costing less than $10 while the hay was 

 worth at that time $300 is perhaps an unusually favorable 

 one ; but thousands of cases are on record in which the use 

 of this class of manures has returned in profit several times 

 the money expended, while extra labor has been only re- 

 quired to take care of the increased harvest. When an 

 acre of land is made to produce double its former yield by 

 the use of manures liberally applied, the cost of the manure 

 is all the extra charge; the land, the labor in preparing it, 

 and in the culture of the crop, are all the same whether the 

 yield be 10 bushels of wheat or 40; or 25 bushels of corn or 

 80. The enhanced crop then, less the cost of the manure, 

 is the measure of the profit. 



