SOURCES OF PLANT-FOOD 



now chiefly used in this country is found in South 

 Carolina, Tennessee, and Florida. It varies greatly 

 in content of phosphoric acid. When pulverized 

 for direct use on land, without treatment with 

 sulphuric acid to make the plant-food available, 

 a grade running 28 per cent phosphoric acid, or 

 less, usually is selected, the higher grades being 

 reserved for treatment with acid or for export. 

 This untreated rock, pulverized exceedingly fine, 

 often is known as floats. 



The value of a pound of phosphoric acid in 

 floats, as compared with that of a pound in the 

 treated rock, known as acid phosphate, is a matter 

 upon which scientists differ widely. Only a small 

 percentage of the plant-food is immediately avail- 

 able, and the question of wise use hinges upon the 

 degree of availability gained later, and the time 

 required. The large amount of experimental 

 work that has been done affords data that cause 

 the following opinion to be stated here: Rock- 

 phosphate, known as floats, is not a profitable 

 source of plant-food for soils deficient in organic 

 matter, when compared with acid phosphate. It 

 is more nearly profitable in an acid soil than in 

 one that has no lime deficiency. It gives more 

 satisfactory results when mixed intimately with 



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