144 PRINCIPLES OP AGRICULTURE. 



the agriculturist desires, the soil should afford a supply of carbonic 

 acid to the roots, either by the decomposition of vegetable mould, 

 or by absorbing the gas from the atmosphere. Its fertility may 

 be increased by the addition of vegetable substances disposed to 

 decay; or by mixing it with charcoal, gypsum, or some other 

 substance, which possesses in a high degree the property of 

 absorbing carbonic acid from the atmosphere (. 179 193). 



III. In order to effect the same object, the soil should be 

 capable of affording a supply of ammonia to the roots, either by 

 the decomposition of animal matter contained in it, or by attract- 

 ing that gas from the atmosphere. The proper supply of this 

 important article of food may be secured, either by the employ- 

 ment of some animal manure which liberates it freely, or by the 

 admixture of some substance (such as gypsum or charcoal) which 

 absorbs it rapidly from the atmosphere. The first method is 

 most desirable, when the vegetable substances, which it is re- 

 quired to obtain in the greatest quantity, contain much ammonia ; 

 as do the seeds of corn (. 1948). 



IV. In order to promote the luxuriant growth of any tribe 

 of plants, the soil should be supplied with those mineral ingre- 

 dients, which its tissues naturally contain. If these are origin- 

 ally deficient, they must be added ; if they are contained in the 

 soil, but have been for a time exhausted, the land should be 

 allowed to lie fallow, until the action of the weather has further 

 separated the mineral particles (. 199 206). 



V. The soil may be improved nearly as much by a crop 

 of a different kind, as by lying fallow ; provided thai crop do 

 not exhaust it of the same mineral ingredient with the one 

 it replaces, and furnish excretions which are beneficial to it 

 (. 207-211). 



