MEANS FOR CAUSING THE DIFFUSION OF FOOD. 77 



When the e2diaustiori of a field is not caused by tlie 

 absohite 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 consequently fal- 

 lowmg will again render the crop remunerative, the farmer 

 has means at his disposal to assist the action of the natural 

 agencies, whereby the conversion of the food elements 

 mto the state of physical combination 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 phosphates 

 through tlie 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 conditions, a favour- 

 able action upon the productiveness of a field. 



These salts, even in their most dilute solutions, possess, 

 hke carbonic acid, the remarkable poAver of dissolving 

 phosphate of hme and phosphate of magnesia ; and when 

 such solutions are filtered through arable soil, they behave 

 just hke the solution of these phosphates in carbonic acid 

 water. The earth extracts from these salt solutions the dis- 

 solved 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 dis- 

 solve 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 hme or magnesia enters into solution in 

 combination with the acid. 



