PHOSPHORUS, ITS SOURCES AND USE IN PLANT FEEDING Go 



phate of lime as it exists in the phosphatic rocks, and it is in a similarly 

 insoluble state, while the potash is. in the form of a mineral known as glau- 

 conite, and is also insoluble at once. Mr. Ruffin, of Virginia, whose book 

 on calcareous manures was for many years the standard authority on the sub- 

 ject in this country, attributed the effects of marl, which he used largely on 

 his lands in Southeastern Virginia, to the lime it contained. But it has been 

 shown that in every instance the most valuable marls are those which contain 

 the largest percentages of phosphoric acid, though shell marls are also valu- 

 able as a source of lime for certain soils. 



PHOSPHATIC GUANO. 



After the exhaustion of the rich deposits of guano in the Peruvian 

 islands, large quantities of phosphatic guano were brought from islands in the 

 Carribean Sea, where the soluble nitrogen had been washed out by rains, leav- 

 ing only the insoluble phosphate of lime. These guanos are used to a con- 

 siderable extent in a pulverized state, and answer about the same purpose as 

 the Florida soft phosphates of recent days. Most of these guanos are not 

 adapted to the making of acid phosphate, and they are now little used since 

 the discovery of the great deposits of phosphatic rock in North and South 

 Carolina, Florida and Tennessee. 



THE GREAT PHOSPHATE ROCK DEPOSITS 



The phosphatic rock that has entered more largely into the manufacture 

 of acid phosphate is the South Carolina rock. This is pulverized and treated 

 with sulphuric acid, and the result is the article called acid phosphate, which 

 contains usually about 12 to 13 per cent, of soluble phosphoric acid, a smaller 

 percentage of what is called "reverted" phosphoric acid, or acid not soluble 

 at once in water, but soluble in citrates. The soluble and the reverted phos- 

 phoric acid are added to make what is called the "available" phosphoric acid. 

 Then there will always be a small portion of the phosphoric acid which is in- 

 soluble. Some Station chemists assign no value to this in their valuations of 

 fertilizers, while others value it at 2 cents per pound when the available is 

 rated at 4 cents. This is the fairer valuation, since there is evidence that it 

 does finally become available in the soil, just as the pulverized rock does. 



Large deposits of phosphatic rock are found also in Florida, much of 

 which is known as "soft" phosphate, and is not available for dissolving with 

 sulphuric acid, and strenuous efforts have been made to get it into use under 

 the name "Natural Plant Food." Tt has value, if one has time to wait on it. 

 Another important deposit of phosphate rock has more recently been dis- 



