S38 Ho^^■ cuors feed. 



A few experiments might easily be devised which would 

 completely SL'ttlo this point beyond all controversy. 



Orj?anic blatters as Indirect Sources of Carbon to 

 Plants. — The decay of organic matters in the soil supplies 

 to vegetation considerably more carbonic acid in a given 

 time than would be otherwise at the command of crops. 

 The quantities of cai'bonic acid found in various soils have 

 already been given (p. 219). The beneficial eifects of such 

 a source of carbonic acid in the soil arc sufficiently obvious 

 (p. 1:>8). 



Orjfanic Matters not Essential to the Growth of 

 Crops. — Although, on the ianii, crops are rarely raised 

 without the concurrence of humus or at least without its 

 presence in the soil, it is by no meims indispensable to 

 their life or full development. Carbonic acid gas is of it- 

 self able to supply the rankest vegetation with carbon, as 

 has been demonstrated by numerous experiments, in which 

 all other compounds of this element have been excluded 

 (p. 48). 



THE AMMONIA OV THE SOIL. 



In the chapter on the Atmosphere as the food of plants 

 we have been led t) conclude that the element nitrogen., 

 so indispensable to vegetation as an ingredient of albumin, 

 etc., is supplied to p'ants exclusively by its compounds, 

 and mainly by am,monia and nitric acid, or by substances 

 which yield these bodies readily on oxidation or decay. 



We have seen further that both ammonia and nitric acid 

 exist in very minute quantities in the atmosphere, are dis- 

 solved in the atmospheric waters, and by them brought 

 into the soil. 



It is pretty fairly demonstrated, too, that these bodies, 

 as occurring in tlie atmosphere, become of appreciable use 



