154 SOIL CONDITIONS AND PLANT GROWTH 



form an almost insoluble double ammonium silicate and a 

 soluble calcium salt, and also, like clay, they lost this property 

 after ignition. Although he did not establish the existence 

 of such double silicates in soil, their resemblance to the 

 reactive constituent in the soil was so close that he considered 

 himself justified in assuming their presence. 



This chemical view was generally accepted in England, 

 but it was controverted by Liebig, who held that the whole 

 phenomenon was physical and comparable with absorption 

 by charcoal. Plant food constituents occur in the soil in two 

 states : chemically combined and physically retained : the 

 latter being the looser is the more suited for the purposes of 

 plant nutrition. " The power of the soil to nourish cultivated 

 plants," he writes, " is therefore in exact proportion to the 

 quantity of nutritive substances which it contains in a state of 

 physical saturation. The quantity of other elements in a state 

 of chemical combination distributed through the ground is 

 also highly important, as serving to restore the state of 

 saturation, when the nutritive substances in physical combina- 

 tion have been withdrawn from the soil by a series of crops 

 reaped from it" (174^, pp. 67-69). 



Knop (150) combined both chemical and physical hy- 

 potheses. The absorption of acid radicles he attributed to 

 precipitation by the iron and aluminium hydroxides supposed 

 to be present in soil : phosphoric acid, however, reacted 

 first with the calcium compounds to form calcium phosphate 

 and then with the iron compounds. With bases the action 

 was rather more complex : the absorption in the first instance 

 was due to a surface attraction, which was followed by a 

 combination with silica or aluminium silicates : there was, 

 however, invariably an equilibrium, the whole of the base 

 never being removed, no matter how dilute the solution. 



Liebig's proposition which we have quoted above was 

 expressed more tersely by Knop as follows : Soils of great 

 fertility have a high content of easily replaceable bases ; and 

 he measured this by determining the ammonia absorbed from 

 ,a 0"5 per cent, solution of ammonium chloride, assuming that, 



