LIFE IN THE SOIL 313 



315. Interrelationship of living things in the soil. The 

 different things living in the soil may prey upon one another 

 quite as do things that live aboveground. The soil is often 

 filled with a dense population of living things, some living 

 upon mineral matter in the soil, some upon dead organic 

 matter, some upon other living things. 



An instructive illustration of the extent to which these 

 interrelations may go is seen in the case of the roots of the 

 corn plant and certain animals that may often be found upon 

 or near these roots. Extremely small insects, known as plant 

 lice, or aphides, bite into the tender roots of the corn and 

 suck out the nourishing juices. The aphides are almost 

 stationary animals and cannot ordinarily make their way 

 from one plant to another without assistance. When well 

 nourished they excrete small drops of a glistening, sweetish 

 solution sometimes called honeydew. This honeydew is an 

 article of food 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 abundant food, and the ants secure the honey- 

 dew 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 bur- 

 rows, dries more rapidly than it otherwise would. This is 

 but one of many illustrations of the interrelationship of 

 animals and plants in the soil. 



316. Bacteria of the soil. In connection with the discus- 

 sion of bacteria those living in the soil were mentioned, but 

 there are additional statements to be made. 



Bacteria are present in most soils in very large numbers. 

 They are of very many kinds, and they affect the soil in a 

 number of ways. Some of them live upon dead bodies or 



