234 PLANT SOCIOLOGY 



Shelford (1913) and Buckle (1923) have written about the ecology 

 of soil insects. The original assumption that special communities of 

 soil insects correspond to certain plant communities has been shown to 

 be greatly overdrawn. The great majority of the species inhabit soils 

 of the most diverse compositions. 



According to the food habits of soil animals, Buckle (1923) distin- 

 guishes earth eaters, meat eaters (carnivores), and plant eaters 

 (herbivores). The meadow and grazing soils of central England which 

 he examined contained at least 50 per cent of herbivores, while carni- 

 vores were in the majority in tilled soils. This fact is probably related 

 to the circumstance that, unlike meadow and grazing lands, the tilled 

 vegetation does not cover the soil throughout the year. 



In the mangrove swamps of the tropics small crabs grind up the 

 organic remains. 



An abundant microfauna, composed of infusoria, rhizopods, rotifers, 

 and nematodes, inhabits the soil and especially the root layers down to 

 a depth of 15 cm., and participates in the processes of soil formation, 

 even at the extreme outposts of life.' The biology of the microorgan- 

 isms of the soil has been well discussed by Waksman (1931). Heinis 

 (1920) has investigated the development of the microfauna in the 

 highest plant cushions of the Alps. He found at 3,500 m. and higher 

 numerous rhizopods, rotifers, and nematodes inside the Androsace and 

 Saxifraga cushions. He considers them of extreme importance in the 

 formation of humus. 



Examination of soil samples from Spitzbergen have shown that in 

 the humus as well as in the mineral soils of the far north a rich protozoan 

 fauna occurs, which is strikingly similar to that of the temperate zone. 

 A high development of the protozoan fauna corresponds, as a rule, to an 

 impoverished bacterial flora. The development of bacteria and their 

 activity are affected unfavorably by the presence of large numbers of 

 Protozoa. 



Soil Flora. — The microflora of the soil plays a much more significant 

 role than the animals in soil economy. This flora is composed of 

 countless bacteria, fungi, and algae, which live preferably in the root 

 layer of the soil and are exceedingly active there. Their significance is 

 indicated by their function of nitrogen fixation, as well as by the fact 



1 Useful papers may be cited: Cutler, D. W., and L. M. Crump, Daily periodicity 

 in the numbers of active soil-flagellates, Ann. Appl. Biol. 7: 11-24, 1920; France, 

 R. H., "Das Edaphon," Stuttgart, 1921; Russell, E. J., Soil Protozoa and soil 

 bacteria, Proc. Roy. Soc. Lond. 89, 1915; Sandon, H., Protozoa from the soil and 

 mosses of Spitzbergen, Jour. Linnean Soc. 34, 1923; Shelford, V; E., ^'Laboratory 

 and Field Ecology," 125-127, 1929. 



