AGRICULTURE. 143 



of surfaces is presented, on which the atmosphere, the 

 rains, and the dews percolating and permeating freely 

 through it, act chemically, rendering soluble, and diffusing 

 through it these mineral food constituents. 



The change of position in the mass of the soil and its 

 individual atoms, effected by repeated tillage operations, 

 throws out fresh surfaces to the action of the atmosphere 

 and the sunlight, thereby rendering available a larger 

 quantity and a larger surface of soU saturated with avail- 

 able food. 



From contact with the atmosphere, in circulation 

 through its substance, the soil derives ammonia, carbonic 

 acid, and oxygen — the solvents of mineral and the decom- 

 posers of organic substances. From rain and dew it 

 derives also ammonia and nitric acid ; and the evapora- 

 tion of moisture (water) from any substance, generates 

 ammonia from the nitrogen of the atmosphere ; and in 

 the case of this evaporation from the soil, the latter has 

 the property of fixing or appropriating the ammonia 

 thus generated. Ammonia and carbonic acid are essential 

 to the formation, hfe, and growth of vegetation. 



The absorption of heat by the soil during the day is 

 counterbalanced by its radiation, or cooling, at night, 

 and this 'respiration of caloric' is essential. In hard 

 compact soil the radiation is very slow ; in subdivided or 

 tilled soil, it is much more rapid, and dew, the result of 

 radiation, is deposited on the soil and the vegetation, 

 and is absorbed by them. 



When land is deeply tUled all these influences are ex- 

 tended in proportion to the depth tilled ; the rainfall 

 penetrates, and forms a store of moisture in the substrata, 

 rises, by capillary attraction, to supply the wants of 

 surface vegetation, without stagnating in the soil or 

 obstructing its air passages. 



