86 THE SOIL. 



even where potash and phosphoric acid are in abund- 

 ance. 



When the exhaustion of a field is not caused by the 

 absolute deficiency of food elements, when even a 

 more than adequate supply of all the needful nutriment 

 is there, but not in the proper form, and where conse- 

 quently fallowing will again render the crop remuner- 

 ative, the farmer has means at his disposal to assist the 

 action of the natural agencies, whereby the conversion 

 of the food elements into the state of physical combina- 

 ation is effected, and thus to shorten the fallowing 

 season, or even in many instances to make it altogether 

 superfluous. 



We have seen that the diffusion of earthy phos- 

 phates through the soil is effected exclusively by water, 

 which, if containing a certain amount of carbonic acid, 

 dissolves these earthy salts." 



Now, there are certain salts, such as chloride of 

 sodium, nitrate of soda, and salts of ammonia, which 

 experience has proved to exercise, under certain condi- 

 tions, a favourable action upon the productiveness of a 

 field. 



These salts, even in their most dilute solutions, pos- 

 sess, like carbonic acid, the remarkable power of dis- 

 solving phosphate of lime and phosphate of magnesia ; 

 and when such solutions are filtered through arable 

 soil, they behave just like the solution of these phos- 

 phates in carbonic acid water. The earth extracts from 

 these salt solutions the dissolved earthy phosphates, and 

 combines with the latter. 



Upon arable soil mixed with earthy phosphates in 

 excess, these salt solutions act in the same way as upon 

 earthy phosphates in the unmixed state, that is, they 

 dissolve a certain proportion of the phosphates. 



Nitrate of soda and chloride of sodium suffer, by 

 the action of arable soil, a similar decomposition to 

 that of the salts of potash. Soda is absorbed by the 

 soil,, and in its stead lime or magnesia .enters into solu- 

 tion in combination with the acid. 



If we compare the action of arable soil upon salts 



