310 AMMONIA AND NITEIC ACID. 



power in exhausted corn-fields, it is absolutely necessary 

 that the arable soil should contain a surplus of all nutri- 

 tive substances for cereal plants, nitrogenous among 

 others, but no one in greater proportion than the rest. 

 It is assumed that the farmer by a right succession of 

 crops, that is, by a proper proportion between his corn 

 and fodder fields, is always in a position, by carefully 

 husbanding the ammonia in his farm-yard manure and 

 avoiding all unnecessary waste, to provide the arable 

 soil with such a surplus of nitrogenous food as will cor- 

 respond to the proportion of the other nutritive sub- 

 stances therein stored ; and that the atmosphere annu- 

 ally makes up what he removes in his crops. 



The nitrogenous food conveyed by the atmosphere 

 and rain, is upon the whole sufficient for his cultivated 

 plants, but not enough for many of them in point of 

 time. In order to give a maximum crop, many plants 

 require, during the period of vegetation, much more 

 than the air and rain afford in that time ; and therefore 

 the farmer makes use of fodder plants in order to in- 

 crease the crops of his corn-fields. The fodder plants, 

 which thrive without rich nitrogenous manure, collect 

 from the ground and condense from the atmosphere, in 

 the form of blood and flesh constituents, the ammonia 

 which is supplied from these sources ; and the farmer, 

 in feeding his horses, sheep, and cattle with the turnips, 

 clover, &c., receives, in their solid and fluid excrements, 

 the nitrogen of the fodder in the form of ammonia and 

 products rich in nitrogen ; and thus he obtains a supply 

 of nitrogenous manures or nitrogen, which he gives to 

 his corn-fields. 



The rule is, that for certain plants, weak in devel- 

 opement of leaf and root, and which have but a short 

 period of vegetation, the farmer must compensate by 

 the quantity of manure for the time which is wanting 

 for the absorption of the requisite amount of nitrogen 

 from natural sources. 



It is easy to see that the accumulation of nitro- 



Eenous food by farm-yard manure in the uppermost 

 tyers of the ground, so very important for the perfect 



