76 THE SOIL. 



than in a clay soil ; tlie presence of the alkahne earth, 

 under these circumstances, servino; to oxidise the car- 

 bonaceous matter, and to convert the ammonia present in 

 the soil into nitric acid. 



All kinds of lime, when Hxiviated, give up nitrates to the 

 water. Nitric acid is not retained by the porous earth, 

 as is ammonia ; but it is carried down combined with 

 lime or magnesia by the rain-water into the deeper layers 

 of the soil. While the formation of nitric acid taking 

 place in the ground is useful for plants which, like clover 

 and peas, draw their food (here including nitrogen) from 

 a greater depth, yet for this very reason fallowing has a 

 less beneficial effect, with a view to the culture of 

 cereal plants, upon a hme soil rich in animal remains ; 

 for by the conversion of ammonia into nitric acid, and its 

 removal, the ground becomes poorer in one of the most 

 important elements of the food of plants. The case is 

 conceivable that a field of the kind, if not cultivated for 

 a number of years, may ultimately have its productive 

 powers impaired by a deficiency of nitrogenous food in 

 the soil. 



The cause of the exhaustion of a field by the culture 

 of any plant is always, and under all circumstances, 

 dependent upon a deficiency of one or more nutritive 

 substances in those portions of the soil which are in con- 

 tact with the roots. A field in which these portions are 

 deficient in phosphoric acid in the state of physical com- 

 bination, will be found unsuited for the production of a 

 proper crop, though it should contain abundance of avail- 

 able potash and sihcic acid. The same results will follow 

 from a want of potash, even though phosphoric and silicic 

 acids be plentiful ; and equally so from a want of sihcic 

 acid, lime, magnesia, or iron, even where potash and 

 phosphoric acid are in abundance. 



