408 



BENEFICIAL ACTIVITIES OF BACTERIA 



chapter. When a pod-bearing plant, such as the clover, alfalfa, 

 bean, or pea, is young, its roots are attacked by the nitrogen-fixing 



bacteria found in the soil. They 

 penetrate the roots through the deli- 

 cate roothairs and establish them- 

 selves in the outer layer of the root 

 cells. The root accommodates them 

 by building more cells in that region, 

 forming little nodules or tubercles. 

 In these nodules, the nitrogen-fixing 

 bacteria multiply. They take the 

 free nitrogen from the air which 

 permeates the soil, and build it into 

 nitrites which are later converted 

 to nitrates. When the roots of these 

 plants are plowed under, they decay, 

 and the nitrates are liberated into 

 the soil. Soil may be inoculated with 

 cultures of nodule bacteria. A crop 

 of clover or alfalfa plowed under 

 supplies the soil with about 100 

 pounds of nitrogen to the acre. Such 

 a crop is as valuable as many loads 

 of manure. A good crop of corn or 

 wheat will take from 50 to 75 

 pounds of nitrogen per acre from soil. There are certain free- 

 living soil bacteria which build nitrogen into nitrates when con- 

 ditions are favorable. The result of the activity of these organ- 

 isms is not unlike the nitrates formed by nitrogen-fixing bacteria. 

 But they differ from the nitrogen-fixing bacteria in that they do 

 not need roots of plants for their homes. 



Lightning and similar electric discharges in the air unite some free 

 nitrogen with oxygen to form nitrates. These are washed from the 



The value of nitrogen-fixing bacteria 

 was demonstrated in the above experi- 

 ment. When the roots of the plants 

 were examined, those on the left had 

 numerous well-developed tubercles ; 

 those on the right had no tubercles. 

 They were both watered with a nutrient 

 solution containing all the necessary 

 nutrients but nitrogen. Explain the 

 relation of the tubercles to the growth 

 of the plant. 



