88 THE SOIL OF THE FARM. 



Tlie soluble phosphoric acid in superphosphate on com- 

 ing in contact with lime in the soil is rapidly converted 

 into an insoluble form, and consequently it does not rap- 

 idly penetrate the soil. This change takes place with 

 certainty in a soil which contains much calcareous matter. 

 A few hours thus often suffices to modify the easy solu- 

 bility of the manure ; and the more rapid the change is 

 brought about the more necessary it becomes to have the 

 superphosphate in as fine a state of division as possible, 

 and well mixed with the soil. The extreme state of di- 

 vision in which, however, this process leaves it, makes 

 it far more soluble^n the carbonic acid rain-water than 

 the most finely divided bones or coprolites which had been 

 reduced by mechanical means ; and the efficiency of the 

 manure is thus not materially impaired. 



It is especially suited as a manure for the turnij) crop. 

 If used alone, as much as four or five hundred pounds 

 per acre may be applied with good effect ; but on most 

 soils it is advisable to use a smaller quantity ; and for 

 mangel wurzels and potatoes it is best applied mixed with, 

 one hundred pounds of sulphate of ammonia and two 

 hundred pounds of potash salts. For the 2:»otato crop, 

 from four to six hundred pounds of superphosphate may 

 be used, in addition to farm-yard manure, or ammonia 

 and potash salts. As a mangel manure, suiocrphosphate 

 is considerably less effective than guano. On barley it 

 has been largely used as a top-dressing of late years at the 

 rate of about three hundred pounds per acre. The oat 

 crop, too, as grown in the fen districts, receives a dressing 

 of superphosphate with great advantage during the spring. 

 A greater bulk of both grain and straw has thereby been 

 produced, and an earlier harvest is obtained. 



Ground Phosphate— Its value as a fertilizer is derived 

 from the phosphate of lime of which the mineral is partly 

 composed. Coprolites, as to seventy to eighty per cent. 



