LIFE IN THE SOIL 225 



for certain black ants which also live in the corn fields. It 

 has been found that the ants will dig furrows down at the 

 side of the corn plants until they can reach the roots, and 

 will then carry the aphides and place them upon younger and 

 more tender roots. When food for the aphides becomes scanty, 

 they are moved to new plants. The aphides thus secure abun- 

 dant food, and the ants secure the honeydew as their own food. 

 Both are parasites upon corn, one directly, the other indirectly. 

 Corn and grass are often killed by these animals, and the soil, 

 rendered porous by the burrows, dries more rapidly than it 

 otherwise would. This is but one of many illustrations of the 

 interrelationship of animals and plants in the soil. 



243. Bacteria of the soil. In connection with the discussion 

 of bacteria those living in the soil were mentioned, but addi- 

 tional statements need to be made in this connection. 



Bacteria are present in most soils in very large numbers. 

 These bacteria are of very many kinds, and they affect the 

 soil in many ways. Some of them live upon dead bodies or 

 parts of bodies of plants and animals, and in so doing they 

 change these things so as to increase the organic content of 

 soils. As instruments of decay in the soil, bacteria and small 

 animal forms are highly important. Different kinds of bacteria 

 carry on different stages in decay. One group of them breaks 

 up ammonia, forming nitrogen compounds that higher plants 

 can use. It will be remembered from an earlier statement that 

 nitrogen is one of the things essential to plant growth. 



But there is another group of" soil bacteria of peculiarly 

 striking interest. They jare often found growing in great 

 numbers in little nodules (or tubercles, as they are called) on 

 the roots of clover, beans, peas, and other plants that are 

 related to these. It has also been found that these tubercle 

 bacteria are able to use nitrogen from the air of the soil and 

 combine it with oxygen in such a way that higher plants, as 

 corn, wheat, and oats, can then use it. One of the greatest 

 problems in maintaining desirable fertility of soils consists in 



