238 HOW CROPS FEED. 
A few experiments might easily be devised which would 
completely settle this point beyond all controversy. 
Organic Matters 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 carbonic acid found in various soils have 
already been given (p. 219). The beneficial effects of such 
a source of carbonic acid in the soil are sufficiently obvious 
(p. 128). 
Organic Matters not Essential to the Growth of 
Crops.—Although, on the farm, crops are rarely raised 
without the concurrence of humus or at least without its 
presence in the soil, it is by no means 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). 
g 4, 
THE AMMONIA OF THE SOIL. 
In the chapter on the Atmosphere as the food of plants 
we have been led to conclude that the element nitrogen, 
so indispensable to vegetation as an ingredient of albumin, 
etc., is supplied to plants exclusively by its compounds, 
and mainly by ammonia 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 the atmosphere, become of appreciable use 
