62 The Sugar-Beet in America 
and thawing, (4) alternate wetting and drying, (5) or- 
ganic matter, (6) soluble salts, (7) animal life, and (8) 
storms. The tilth of the soil is the result of a combined 
action of a number of these factors, all of which improve 
it, except certain kinds of storms and certain soluble salts 
like sodium carbonate. 
AIR IN THE SOIL 
Oxygen is as necessary for the growth of plants as it is 
for that of animals. It is, therefore, impossible to have a 
fertile soil unless there are spaces through which air can 
circulate. Seeds in germinating, and roots in growing, 
require oxygen which is absorbed while carbon dioxid is 
given off. The decay of organic matter uses oxygen 
and forms carbon dioxid which accumulates in the soil 
air. If conditions in the soil do not favor a free move- 
ment of air, the oxygen supply soon becomes reduced to 
a point at which plant growth is retarded. The aération 
of the soil is dependent on texture, structure, drainage, 
and a number of other factors. In a coarse sand, air 
moves readily, but in a clay, especially if it is compact, 
the movement is slow. Puddling greatly reduces aération, 
whereas flocculating the soil particles into groups pro- 
motes the ready movement of air. 
A water-logged soil, on account of the lack of oxygen, 
usually has a low crop-producing power. A free circu- 
lation of air, resulting from placing drains under such a 
soil, is in part responsible for the increased yields that 
follow drainage. The beneficial nitrifying and nitrogen- 
fixing bacteria require an abundant supply of oxygen for 
