THE SOIL AS A HABITAT FOR LIFE — RUSSELL 455 



ous and unexpected hcarm. The soil bacteria can decompose many 

 stable types including benzene and naphthalene compounds ; the for- 

 mula should be designed to insure destruction when their work is done. 



FOOD CYCLES IN THE SOIL 



The ultimate source of food for the soil population is the dead vege- 

 tation falling on the soil surface and dragged in by the larger ani- 

 mals. Some organisms feed on it directly, some at second hand, 

 some even more indirectly. The process has been studied by the 

 foresters, and considerable work is now being done on grassland and 

 on arable soils. W. Kiilmelt, in the book already mentioned, de- 

 scribes the process as seen in a Swiss forest. In the first stage the 

 forest litter is attacked by numerous members of the population. Bac- 

 teria and many of the animals take the carbohydrates and proteins 

 which they can digest, and the fungi feed on the celluloses in the wood, 

 which other organisms cannot take. In the second stage, organisms 

 of the first stage are devoured by the predators. In neither stage do 

 the organisms assimilate all the material they ingest : the rest is ex- 

 creted. But the excretions still have some food and energy value 

 and in the third stage they are consumed by the coprophagous or 

 dung-eating organisms; this process is repeated while any utilizable 

 material remains. Kiilmelt states that among the tipulid larvae some 

 of the smaller individuals consume the excreta of the larger ones — a 

 monstrous piece of economy. The feces of many soil animals are rich 

 centers for germination and growth of microorganisms. 



The process appears to be of the same pattern on grassland and on 

 arable land. Earthworms and other large animals begin operations 

 by dragging the plant material into the soil and ingesting it. The 

 decomposition is intensified by the vast numbers of bacteria in their 

 guts which seem to find conditions there more favorable than in the 

 soil outside: oxygen absorption and carbon dioxide evolution are 

 much increased by the presence of these larger animals in the soil. 

 Other actions are stimulated also: in the Arnheim experiments the 

 introduction of a millipede into a soil culture increased fungus ger- 

 mination 400-fold. These are examples of what A. Macfadyen has 

 called the catalytic action of the soil fauna. The small animals take 

 over as in forest soils. 



CONCLUSION 



The most remarkable part of the story is the conclusion. The final 

 unconsumed and indigestible material and excretions constitute the 

 soil humus (most important for plant growth), and the simple salts to 

 which much of the digested material has been reduced are nutrients 

 essential to plant growth. They were taken up by the plant wliile 

 it lived, they helped to build up the plant tissues, and now at the end 



672-174—63 34 



