218 HOW CROPS FEED. 
little to add here to what has been remarked in previous 
paragraphs. 
Free Oxygen, as De Saussure and Traube have shown, 
is indispensable to growth, and must therefore be access- 
ible to the roots of ‘plants. ‘ 
The soil, being eminently porous, condenses oxygen. 
Blumtritt and Reichardt indeed found no considerable 
amount of condensed oxygen in most of the soils and sub- 
stances they examined (p. 167); but the experiments of 
Stenhouse (p. 169) and the well-known deodorizing effects 
of the soil upon fecal matters, leave no doubt as to the 
fact. The condensed oxygen must usually spend itself in 
chemical action. Its proportion would appear not to be 
large; but, being replaced as rapidly as it enters into com- 
bination, the total quantity absorbed may be considera- 
ble. Organic matters and lower oxides are thereby ox- 
idized. Carbon is converted into carbonic acid, hydrogen 
into water, protoxide of iron into peroxide. The upper 
portions of the soil are constantly suffering change by the 
action of free oxygen, so long as any oxidable matters 
exist in them, These oxidations act to solve the soil and 
yender its elements available to vegetation. (See p. 131.) 
Free Nitrogen in the air of the soil is doubtless indiffer- 
ent to vegetation. The question of its conversion into 
nitric acid or ammonia will be noticed presently. (See p. 
259.) 
Carbonic Acid.—The air of the soil- is usually richer in 
carbonic acid, and poorer in oxygen, than the normal at- * 
mosphere, while the proportion (by volume) of nitrogen 
is the same or very nearly so. The proportions of car- 
bonic acid by weight in the air included in a variety of 
soils have already been stated. Here follow the total 
quantities of this gas and of air, as well as the composi- 
tion of the latter in 100 parts by volume, as determined by 
