MANURES. 223 



As a general rule, it may be said that we have only to 

 occupy ourselves with these three all-important substances 

 (with the possible exception of lime), and it is to supply 

 any deficiency in them which may exist that manures 

 are employed. 



To determine whether such deficiency does exist, chem- 

 ical analyses are often resorted to, though from one point 

 of view these are of little direct value as evidences of 

 fertility. 



As has already been mentioned, it is only that portion 

 which is soluble that can be taken up by the roots of 

 plants ; but what an ordinary chemical analysis indicates 

 is the total amount present. It indicates potential rather 

 than actual fertility. To overcome this difficulty, attempts 

 are now made to estimate the amount actually available 

 for the plant's use, thaj: is the "available" part, which is 

 continually being augmented by the breaking down of 

 the insoluble or "dormant" ingredients by the numerous 

 complicated processes which are ever taking place in the 

 soil. 



The soil water which acts as the natural solvent is 

 cliarged with -weak acids in solution, while again, the 

 rcotlets of the plant themselves have a certain dissolving 

 power, so that any solvent used in chemical determinations 

 must be acid in its nature. Approximate estimates, it is 

 considered, are obtained by the employment of a 1 % solu- 

 tion of citric acid, this being the nearest approach to 

 natural soil conditions obtainable. 



By comparing the results of such determinations with 

 i'< innl fiplrj experiments, a similarity is observable which 



