124 SOILS AND MANURES 



phosphate of lime. Calcium phosphate, again, is probably 

 more readily soluble than phosphates of iron and alumina, 

 especially if the latter have been dried or partially dried. 

 All that can be said with certainty is that some substances 

 which are not soluble in water can be assimilated by 

 plants, and that much of what is dissolved by concentrated 

 acids cannot. In any case, the available state cannot be 

 defined in terms of solubility, because plants differ in their 

 assimilative capacities; what is available to one is not 

 available to another. There is, therefore, no absolute 

 available state at all. Attempts to define it in general 

 terms are apt to be misleading, and methods for estimating 

 the amount of " .available plant food," in the strict sense, 

 are impossible. 



This, however, does not preclude the possibility of deter- 

 mining by chemical analysis whether or not a soil stands 

 in need of potash or phosphatic manures. It has been 

 established that .this cannot be predicted with certainty 

 from the amounts of potash and phosphoric acid extracted 

 from the soil either by strong acids or by pure water, and 

 the fact suggests that dilute acid might prove a more suit- 

 able solvent. Acids of different kinds and various strengths 

 have been tried by several authorities, but that which has 

 been most generally approved in recent years is a cold, 

 1 per cent, solution of citric acid. This solvent was 

 proposed by Dyer after a careful examination of the acidity 

 of the root sap of a number of different species of plants. 

 It was thought that it would nearly imitate the dissolving 

 action of the roots, and the phosphoric acid and potash 

 extracted by it have been called " available plant foods." 

 The term is, perhaps, an unfortunate one, but, as it is 

 in common use, it must be accepted. It must be clearly 

 understood, however, that no sharp line of distinction can 

 be drawn between what is available and what is not. Dyer 

 applied the reagent to soils of known character and 



