1 88 SYSTEMS OF PERMANENT AGRICULTURE 



soft rock average 50 to 60 per cent pure; while the Florida hard 

 rock and the Tennessee brown rock contain from 60 to 75 per cent 

 of calcium phosphate. The Tennessee blue rock varies from less 

 than 50 to more than 70 per cent, or from 200 to 300 pounds of 

 phosphorus per ton of rock. The Florida soft rock contains chiefly 

 phosphates of iron and aluminum, while in the other rocks the 

 phosphorus is largely in the form of tricalcium phosphate. 



Aside from the deposits of high-grade phosphate, containing 

 45 or 50 to 75 or 80 per cent of calcium phosphate, there are known 

 to exist very much more extensive deposits of lower grade phos- 

 phates and phosphatic limestones containing from less than 10 

 per cent to 40 per cent or more of calcium phosphate, correspond- 

 ing to from 2 to 8 per cent of phosphorus, or from 40 to 160 pounds 

 of phosphorus per ton of rock. At present, these deposits have no 

 market value, because, if the phosphate costs $4.00 per ton fine- 

 ground and on board cars at the mine, and if the freight charges 

 are $3.00 per ton, the freight on two tons of low-grade rock would 

 amount to $6.00; while the delivered cost of one ton of high-grade 

 rock supplying the same amount of phosphorus would be only 

 $7.00, leaving but 50 cents a ton for the low-grade rock, which would 

 barely pay for the expense of easy quarrying and grinding. 



As the supplies of high-grade phosphate become exhausted and 

 prices advance, the lower grades will no doubt be utilized in this 

 country as they are in Europe, where 35 to 40 per cent Belgian 

 phosphate is now one of the chief commercial grades. 



About 62^ per cent calcium phosphate, or 12^ per cent of phos- 

 phorus, is the average grade of the fine-ground natural rock phos- 

 phate now used in Illinois, and to some extent in other states, for 

 direct application to the soil in intimate connection with abundance 

 of decaying organic matter, as farm manure, clover, or other green 

 manures. In this form the element phosphorus costs the farmer 

 about 3 cents a pound. 



The information thus far secured amply justifies the adoption 

 of a system of farming in which fine-ground natural phosphate 

 rock should be applied at the rate of 1000 to 2000 pounds per acre 

 every three to six years, for three or four successive crop rotations, 

 after which the application may be reduced one half, or to 200 

 pounds per acre for each year in the rotation, which would still 



