AGRICULTURAL EXPERIMENT STATION. 29 



Reverted phosphoric acid is a term originally signifying phos- 

 phoric acid that has once been "soluble," but which from some 

 cause has "reverted" or "gone back" to forms insoluble in water. 

 Now it is used to designate that which is dissolved b^^ a solution of 

 ammonium citrate, and includes not only the truly reverted, but also 

 more or less of phosphoric acid as combined in the original, undis- 

 solved phosphatic material. Reverted phosphoric acid, in so far as 

 it comes within the strict meaning of the term, most probably has a 

 value for crop production equal to that of the soluble form, but it is 

 not clear that this holds true of that which would be dissolved by 

 ammonium citrate from iinely ground South Carolina rock, for 

 instance, at a temperature of 65*^ C. 



Insoluble phosphoric acid is that which is readily soluble neither 

 in water nor in a solution of ammonium citrate, but which can be 

 dissolved in strong acids. In superphosphate it comes from some 

 of the original phosphatic material that has not been acted upon by 

 sulphuric acid, and depends somewhat for its value upon the kind 

 of material used, whether bone or rock phosphate. In au}^ case it 

 has less value than the soluble or reverted forms. 



It should be remembered that the terms "soluble," "reverted," 

 and "insoluble" are merely relative in their significance. There is 

 no compound of phosphoric acid that is not dissolved to a slight 

 extent, at least, by pure water, and to a still greater degree by 

 ammonium citrate, and the extent of the solubility of raw phos- 

 phates in these liquids, and in weak acids such as are found in the 

 roots of plants, depends very largely upon their mechanical con- 

 dition, or the degree of fineness to which they are ground. 



The potash used in this country for agricultural purposes comes 

 mostly from German}^ in the so-called "German potash salts," 

 which include potassium sulphate, potassium chloride (the muriate) 

 and kainite. Except for a few special purposes, potash is equally 

 valuable in all these forms, but costs least in the muriatic and in 

 kainite. 



All fertilizers contain more or less water. The presence of this 

 ingredient does not affect the actual value of a fertilizer unless it is 

 in sufficient quantity to cause stickiness and thus render it difficult 

 to distribute the fertilizer uniformly by hand, or by a corn planter 

 or other machine. That is to say, two fertilizers containing the 

 same quantities of the same kinds of plant-food will be equally 

 valuable, no matter whether the amount of water present is the 



