PHYSICAL CONDITION OF AVAILABILITY 285 



beneficial as the soluble phosphate, but in the greater part of soils, 

 if it produces an action, this action is less than that of superphos- 

 phates and the inferiority of this action appears to be caused, at 

 least in part, because no portion of it can enter immediately into 

 the plant in a condition of aqueous solution. 



"To resume, the whole of my experiment seems to make clear 

 that the favorable action of superphosphate is not only caused by a 

 greater dissemination of the combinations of phosphoric acid in 

 the arable earth, but that it is also necessary to take into account 

 the absorption in the form of combinations, soluble in water, of a 

 portion of soluble phosphoric acid of superphosphates. If we 

 desire to obtain a maximum result it is necessary to distinguish two 

 sorts of soil ; first, the soil analogous to those, of which numerous 

 examples are found in Bretagne, in which insoluble phosphates 

 succeed as well as superphosphates and where it is natural to em- 

 ploy phosphate simply ground. Second, the other soils which are 

 far more numerous and in which the phosphoric acid fertilizers in 

 combinations soluble in water are absolutely indispensable to ob- 

 tain the maximum effect." 



246. Physical Condition of Availability. In general it may 

 be said that the physical state of subdivision of raw phosphates 

 and basic slags is one of the most important factors in respect 

 of their availability. This degree of fineness, however, depends 

 for its efficiency largely on other conditions, especially where arti- 

 ficial means are employed for determining availability. For 

 this reason it is advisable, when determining the availability of 

 these materials by chemical means, to employ extremely dilute 

 solutions. The method of Dyer depends on the use of an organic 

 acid and of Moore upon the use of a mineral acid ; both require 

 great degrees of attenuation. 43 Only by the use of some such re- 

 agent can the conditions which occur in nature be simulated, 

 yet it must not be considered that the same degree of attenuation 

 must be present in the artificial means as in the natural. Otherwise 

 the time of experimental determinations of the artificial means 

 would have to continue over several months instead of several 

 hours. It is possible that a revision of the common idea respect- 



** Principles and Practice of Agricultural Analysis, and Edition, 1906, 

 1 : 394, 459- 



