330 AMMONIA AND NITEIC ACID. 



It is easily perceived from this table that the quanti- 

 ties of nitrogen which could be obtained from the field 

 and restored in the form of farm-yard manure, bear a 

 proportion not exact but sufficiently well marked, to the 

 crops of clover produced by the field ; and there can be 

 no doubt that the farmer who takes the right way to 

 make his fodder plants thrive, obtains at the same time 

 the means of em^iching his arable soil with a surplus of 

 nitrogenous food for his corn-plants. 



We do not mean to imply that in every possible case 

 the farmer must renounce the idea of supplying to his 

 land ammonia from other quarters ; for soils vary so very 

 much in their natm-e, that even though we can assert 

 that by far the greater proportion of them may not 

 require a restoration of nitrogenous food, yet this will 

 not hold good for all without exception. In a soil rich 

 in hme and humous materials, in consequence of the 

 process of decay going on, a certain quantity of the 

 ammonia fixed in the earth is converted into nitric acid, 

 which is not retained by the soil, but is conveyed into 

 the lower layers in the form of salts of Hme or magnesia. 

 Under certain circumstances, this loss may amount to 

 much more than is compensated by the atmosphere, and 

 for such fields a supply of ammonia mil always be useftd. 

 The same holds good for certain soils which have not 

 been tiUed for many years, and in which, by the opera- 

 tion of the causes above-mentioned, the necessary sur- 

 plus of nitrogenous food, formerly present, is gradually 

 expended. On recommencing the cultivation of such 

 soils, the employment of nitrogenous manures will at 

 first produce a remarkably beneficial effect. Afterwards, 

 these too require no further supply. 



There is one reason which excites in the farmer's mind 

 a prejudice in favour of nitrogenous manures, and that 



