18 TRUCK-FARMIHG AT THE SOUTH. 



and comminuted to support lichens and other of the 

 lowest forms of vegetation, which, by their decay in- 

 creased the film of soil. These were followed by plants 

 of a higher organization, successive generations pre- 

 paring for those which followed them. Thus organic 

 constituents accumulated, until, in time, every arable 

 soil contained in varying proportions every element 

 of plant food. The variations are such, however, that a 

 soil in its natural or original condition may be more or 

 less deficient in one or other constituent, and therefore 

 be better adapted for one crop than another. Finding 

 from experience or analysis, which of the principal con- 

 stituents is absent in a soil, we may supply it by the ap- 

 plication of a special fertilizer for the production of a 

 particular crop. 



Humus, or vegetable matter undergoing decay, which 

 gives the dark color to fertile soils, disintegrates miner- 

 al substances, supplies large quantities of carbonic acid, 

 ammonia, and nitrates, and is indispensable to fertility; 

 and yet the direct absorption of humic matters into 

 plants is by no means so well established, as not to be a 

 mooted question between the best chemical authorities 

 and vegetable physiologists. 



Ammonia and nitric acid have been shown to be pres- 

 ent in the air; but it is generally held that the soil must 

 absorb these, or they must be conveyed to it in rain water, 

 before they can be generally appropriated by plants. 

 Yet Peters and Sachs proved, experimentally, that a bean 

 plant grew and flourished by direct absorption of am- 

 monia through its foliage alone; but they failed with 

 every plant but the bean, which, like our cow pea, is a 

 leguminous plant. Chemical analysis shows that the le- 

 guminosae are richer in nitrogenous matter than any other 

 family of plants, and yet we know that our cow pea will 

 grow on the poorest soil, deriving its nourishment in 

 part from the atmosphere. Ozone, or active oxygen, is 



