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- 



