70 The Sugar-Beet in America 
tion. The carbon dioxid given off by roots assists in 
dissolving the minerals of the soil. 
The making available of reserve plant-foods as fast as 
needed by crops is one of the chief problems of soil man- 
agement. This is done: (1) by tillage, which aids the 
weathering agencies in their action on soil particles; (2) 
by drainage, which allows air to circulate more freely 
through the soil; (3) by plowing under organic matter, 
which in decaying helps to make the minerals soluble ; 
and (4) by numerous other less important means. The 
nitrogen present in the soil is made available by nitrifica- 
tion, which is favored by tillage and by a desirable mois- 
ture-content. Plant-foods that are likely to be scarce are 
discussed in Chapter VI. 
SOIL BACTERIA 
The soil is not a mass of dead matter, but is filled with 
myriads of living organisms, which are constantly trans- 
forming its compounds and renewing its productiveness. 
These organisms work on the bodies of plants and dead 
animals and make the material composing them useful to 
growing plants. All life on the earth is dependent for its 
continuance on these unseen organisms, but for whose 
renewing action the available plant-food would in time 
be consumed, all plant life would then cease, and animals 
would soon follow. 
The most important of these organisms of the soil are 
the bacteria, the existence of which was discovered in 
1695. They are so small that it would take about 25,000 
of them placed side by side to reach an inch. They in- 
