450 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



are also much greater than suspected. The smallest are usually the 

 most numerous: the protozoa, nematodes, and arthropods, roughly 

 in that order. 



They are also the most active, consummg more food and more oxy- 

 gen per unit of bodyweight than the larger ones. The protozoa vary 

 greatly in size, and weight is a better standard of comparison: in 

 Stockli's experiments and at Rothamsted their weight was 3.4 percent 

 of that of the bacteria. The nematodes range from about 4 to 17 

 million per square yard, mostly in the top 4 inches of soil. The 

 majority are free living, but some are parasitic on plants: these have 

 been the most fully studied. 



The chief arthropods are the mites and springtails. The mites 

 range considerably in size: between 12,500 to 100,000 of them could 

 be packed into a drop of water. Salt found about 100,000 per square 

 yard in the top 6 inches of grassland, and about 40,000 in the second 



Protozoa -' 



' • Nematodes 



Nematodes /' 



Nematodes 



> Nematodes 



Alqae 



► Nemofodes 



Figure 1. — The food cycle of free-living nematodes. The inner circle represents the part 

 of the soil organic matter derived from plant residues; the surrounding ring indicates the 

 contribution made by the dead bodies of soil population. Solid line means "enter into"; 

 light broken line, 'turn into when dead"; heavy broken line indicates that several stages 

 are involved. (C. Overgaard Nielsen.) 



