40 FIB8T FBINCIPLE8 OF AOBICULTUBE. 



loss to soils, and is greater or less according to the char- 

 acter of the soil, and the treatment it receives. On the 

 other hand, there is a gain in the fertility of soils due 

 to natural causes. 



Rain carries to the soil appreciable amounts of nitric 

 acid and ammonia, as well as certain solid substances, 

 existing in the atmosphere. The gain from this source 

 is greatest in the vicinity of cities, and least in the open 

 country. The gain due to the action of water, heat, cold, 

 and decaying vegetable matter has already been referred 

 to in previous sections, though the changes taking place 

 in vegetable matter require further notice. 



Nitrification. — Vegetable matter is the source of 

 humus of soils, and the active principle of humus is 

 nitrogen. The nitrogen in humus is combined with car- 

 bon, and in this form it is not available to plants. In 

 order to become most useful to them it must be changed 

 into a nitrate, since plants take up their nitrogen chiefly 

 in this form. This process is called "nitrification," and 

 is caused by minute "organisms" or "ferments," which 

 are present in all fertile soils. 



These ferments are most active in warm, moist, well- 

 drained soils, when nitrification proceeds rapidly; they 

 are not active when the temperature is lower than 41° F. 

 or higher than 131° F. In winter, in temperate climates, 

 nitrification practically ceases altogether, while in sum- 

 mer it proceeds most rapidly. As soon as nitric acid is 

 formed by this process, it immediately combines with 

 some base, preferably lime ; hence, if drainage is allowed, 

 the loss of nitrates is always accompanied by a loss of 

 lime. 



