174 SOILS AND FERTILIZERS 



227. Relative availability of phosphoric acid fertilizers. 



— The availability of these fertilizers has been casually men- 

 tioned as each was discussed, but a brief resume will serve 

 to make the matter more definite. Acid phosphate, including 

 dissolved bone, is the most readily available of the phos- 

 phoric acid fertilizers. The reverted portion is more or less 

 available, depending on the character of the original rock, 

 and on the kind of soil to which it is applied. It is not as 

 valuable as the soluble phosphoric acid. The insoluble por- 

 tion has no greater availability than the rock from which the 

 acid phosphate was made. 



Next to acid phosphate in availability comes basic slag, 

 then steamed bone and finally the rock phosphates. 



Acid phosphate and basic slag may be used for top dressing 

 grass or winter grains, but the other fertilizers must be in- 

 corporated in the soil in order to become available. It is 

 necessary that they shall be acted on by the soil water having 

 carbon dioxide in solution and possibly by other acids formed 

 by the decomposition of organic matter. 



228. Rock phosphate versus acid phosphate. — The ques- 

 tion has frequently been raised in the last few years regarding 

 the use of ground rock phosphate or floats as a substitute for 

 acid phosphate. Which of these practices is the better must 

 be largely determined by practical experiment, and by a study 

 of the conditions under which floats become available. 



It is urged in favor of floats that the price of phosphoric acid 

 is much less in this form than in the form of acid phosphate, 

 which is made by a more or less expensive process. It is 

 further argued that even if much more material must be used 

 in order to get a pound of available phosphoric acid the re- 

 mainder stays in the soil to increase the total supply, and that 

 gradually it will become available, finally perhaps reaching 

 a point where no more need be applied. 



On the other side is the well-established practice of using 



