RUSSELL AND HUTCHINSON'S PROTOZOAN THEORY 135 



flora, which does not return so easily to its normal state. It is quite 

 possible that the return to the normal conditions is prevented by 

 the exhaustion for years to come of the more available portions of the 

 plant nutrients." 



Evidence corroborating this theory has been brought forward by 

 Heinze, Stoklasa, Lipman, and Brown, whereas Sirker furnishes 

 evidence in the cultivation of the mulberry which opposes it. He 

 found that the addition of carbon bisulphid to a completely fertilized 

 mulberry plant increases the vegetation 44 per cent., whereas a 

 heavy application of sodium nitrate was of slight value. 



Russell and Hutchinson's Protozoan Theory. They consider that 

 the microscopic flora of the ordinary arable soil includes a wide 

 variety of organisms performing very different functions which 

 may be divided roughly into two classes: (a) saprophytes, tending 

 to increase the fertility of the soil, for example, producing ammonia, 

 fixing nitrogen, and similar changes; and (6) phagocytes and large 

 organisms inimical to bacteria which limit fertility. Between these 

 two classes of organisms there is an equilibrium under natural 

 conditions, but when partial sterilization takes place the phago- 

 cytes are killed but the bacterial spores are not; and subsequently 

 the latter develop with great rapidity, since they are freed from the 

 attacks of their enemies, and there is an increase not only in 

 ammonia but likewise in crop production. 



In support of this theory they point out the following: "In 

 untreated soil there is no accumulation of ammonia, whereas the 

 'toluene evaporated' soil, as well as the soil heated to 98 C., show 

 an increased production of ammonia. That this is mainly the work 

 of microorganisms is proved by the following considerations: (a) 

 The curves belong to the type associated with bacterial, rather than 

 with purely chemical activities. (6) Soil which has been heated to 

 125 C. (at which temperature all organisms are killed) behaves 

 altogether differently; after the first production of ammonia due 

 to heating there is no further change, (c) If the toluene is left in 

 the soil there is only a slow production of ammonia, and never a 

 rapid rate; the curve is more nearly linear. The action of micro- 

 organisms is here excluded, but enzymes may still act. (d) The 

 rapid period sets in only when the soil is sufficiently moist. Thus 

 the two significant changes induced by partial sterilization are, 

 (1) an increase in the amount of ammonia; and (2) cessation of the 

 nitrifying process. 



"It now becomes necessary to determine the part played by 

 bacteria, and why they can increase so much more rapidly in the 

 partially sterilized soil (which accounts for the increased ammonia 

 production) than in the untreated soils. That the comparative 

 inertness of the bacteria in the untreated soil cannot be caused by 

 any bacterial factor is evidenced by the following considerations: 



