SOURCES OF PLANT-FOOD 



Profit from its use was legitimate, but the necessity 

 of supplying organic matter became even greater 

 than it would have been otherwise. Tens of thou- 

 sands of our most successful farmers use heavy 

 applications of acid phosphate, but they keep 

 their soils in good physical condition by the use 

 of manure or clover, and they apply potash and 

 nitrogen when needed. The clover is assured by 

 using lime wherever it is in too limited supply, 

 and that is the case in most instances, regardless 

 of the use of any kind of commercial fertilizer. 



Basic Slag. When iron ores contain much 

 phosphorus, its extraction by use of lime gives a 

 by-product in the making of steel that has agricul- 

 tural value. The ores of the United States usually 

 do not give a slag sufficiently rich in phosphorus 

 to be valuable. Nearly all the basic slag used as 

 a fertilizer is imported from Germany, and usu- 

 ally contains 17 to 18 per cent of phosphoric acid. 

 The availability of the plant-food in this fertilizer 

 has been the subject of much discussion. The 

 chemist's test which is fair for acid phosphate is 

 admittedly not fair when used for basic slag. 

 Field tests, at experiment stations and on farms, 

 are our best sources of knowledge. When the 

 soil is slightly acid, each 1 per cent of phosphoric 



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