24 



MAINE STATE COLLEGE 



has been chiefly caused by the fact that phosphoric acid has cost 

 much less in the crude condition than after treatment with sulphu- 

 ric acid. It has been generally conceded that the less soluble and 

 cheaper form of plant food is more likely to prove useful in grass 

 and grain growing than with hoed crops, because in the first case 

 permanency of effect is desired and in the latter, immediate availa- 

 bility is the essential thing. 



It is true, however, that in later years we have come to more 

 fully appreciate the ability of plants to make their own solutions 

 of food from the minerals of the soil that are highly inert so far as 

 it is a question of solution in water, and we have known for a long 

 time that the water-soluble phosphates of fertilizers revert very 

 rapidly after contact with the soil to these forms that are easily 

 dissolved only by acids, the chief advantage secured by the manu- 

 facture of a superphosphate being that its compounds become dis- 

 tributed in the soil in a more finely divided condition than is pos- 

 sible in any other way and so become more fully accessible to root 

 action. The results of these experiments are in accord with the 

 tendency to accord to insoluble phosphates a larger place in prac- 

 tice. (See Table 4. ) 



TABLE IV. 



Yield per acre of air-dry 

 erops. 



^ ?, - -^ 



z ■'■ " - 



Plots receiving no fertilizer (average of 6) 



Complete fertilizer, pbosphoric acid largely dissolved. 



Complete fertilizer, phosphoric acid from groimd iDone, 



Complete fertilizer, phosphoric acid from crude ground 

 South Carolina rock 



Fertilizer containing nitrogen and potash, but no phos- 

 uhorlc acid 



lbs. 



lbs. 



14,050 





21,381 



7,331 



20,610 



6,560 



18.983 



4,933 



16,S20 



2,770 



lbs. 



916 

 8-20 



617 



.346 



"We have in the above figures conclusive evidence that the phos- 

 phoric acid of the bone and ground South Carolina rock was quite 

 freely appropriated, though not to the same extent as from the dis- 

 solved phosphate. 



