76 AGRICULTURAL BACTERIOLOGY 



known of the effect they produce in the soil. They are, 

 however, to be considered as agents in the decomposition of 

 organic matter and in the elaboration of plant food. 



Protozoa, unicellular animals, are present in the soil in 

 numbers reaching several thousand per gram. It is to be 

 remembered that every animal is a factor in the decompo- 

 sition of organic matter. The relatively large size of the 

 protozoa makes them an appreciable factor in the elabora- 

 tion of plant food. Many protozoa live on bacteria, and it 

 has been claimed by some students of the soil that they are 

 the cause of the reduced fertility of many soils, since they 

 cause the destruction of certain classes of bacteria that are 

 essential in the cycle of nitrogen. It is a well known fact 

 that the bacterial content -of surface waters is kept at a low 

 level by the protozoa. 



The macroscopic lower animal forms in and on the soil 

 are likewise abundant and also function in the decompo- 

 sition of organic matter. It has been shown that from ten 

 to fifteen million insects of macroscopic size may be found 

 on a single acre of meadow-land. Within the soil the com- 

 mon earthworm occurs in varying numbers. Determina- 

 tions indicate that as high as 350 pounds per acre of these 

 organisms may be found in the more fertile soils. All of 

 these forms are living on complex, organic compounds, and 

 are giving off simple waste products. They also function 

 in the pulverization of organic matter, and thus make it 

 more easily attacked by microorganisms. The earthworm 

 is also of importance in the aeration of the soil through its 

 burrows. The soil is passed through the alimentary tract 

 of the earthworm, and is brought by them to the surface 

 in their "castings." This reworking of the soil particles 

 materially improves the texture of the soil. When land is 

 plowed that has been under grass for a number of years, it 

 will be found to possess a granular structure, due to the 



