THE SOILS OF FLORIDA. 
47 
atmosphere as carbon dioxide. The perennial plants have a longer 
cycle, which in case of trees, aside from the foliage, may last for 
many years, the carbon being temporarily locked up in the structure 
of the tree. In the form of coal, lignite, peat, muck, and limestones 
and other carbonates carbon may remain locked up in the earth for 
an indefinite period. 
Moreover, animals feed upon plants and plant products, and the 
organic compounds are carried into the blood and there meet the 
oxygen taken into the lungs. The action in the lungs results in the 
formation of carbon dioxide which is given off in breathing. In addi¬ 
tion to these usual sources the return of carbon dioxide to the atmos 
phere is being facilitated at the present time by the activities of 
man. This gas is a product of combustion as well as of decay, the 
two processes involving the same reactions. In the extensive use of 
coal, oil, gas, and wood as fuels the return of carbon dioxide to the 
atmosphere is being hastened. So also the calcining of limestone for 
lime and other purposes results in the return of carbon dioxide, the 
gas being given off when carbonates are heated. 
The preceding pages contain estimates of the amount of the sev¬ 
eral essential elements in the crust of the earth to a depth of ten 
miles. These estimates are of value in a general way as bearing on 
the relative abundance of the elements, but it must be borne in mind 
that the amount in soils is not governed strictly by the relative 
amounts in the earth’s crust. Some of the elements are relatively 
more abundant in the superficial than in the deeper deposits. Some 
are more readily soluble than others and hence are quickly removed 
from the soils by surface waters. Thus although the total amount 
of nitrogen in the earth’s crust amounts to merely a trace too small 
to estimate, yet the small amount which does occur is largely in the 
soils, where it exists as a constituent of organic matter, ammonia 
or ammonium salts and nitric acid. The amount present in soils 
varies to such an extent that any attempt to express an average is 
practically useless. Nitrogen in soils to the amount of .01 to .03 
per cent, is not uncommon, while soils rich in organic matter may 
contain 3 to 4 per cent. 'Certain soils of the Orinoco Valley in 
South America are said to contain as much as 30 per cent, of nitro¬ 
gen. This is in the form of nitrates, and is due to the oxidation of 
organic matter through the agency of bacteria. The relative solu¬ 
bility of the ingredients materially affects their accumulation in 
soil. Potassium which occurs in soils in a readily soluble form, is 
often deficient in soils subject to leaching, although abundant in 
arid soils. 
