ii8 SOIL CONDITIONS AND PLANT GROWTH 



by inoculation. Addition of small quantities (0*5 percent.) of untreated 

 soil, or of a filtered aqueous extract of the soil containing bacteria, 

 considerably increased the total number of organisms, while addition of 

 large quantities (5 per cent.) of untreated soil led to a considerable 

 reduction. The depressing effect was not shown at once, as the effect 

 of a toxin should have been, but only after the lapse of some time. 



TABLE L. EFFECT OF REINFECTING UNTREATED SOIL INTO PARTIALLY STERILISED SOIL. 



The factor does not operate unless sufficient moisture is present, and 

 it reaches its maximum development in moist, warm soils well supplied 

 with organic matter and bacteria such as sewage farm soils and green- 

 house soils. It does not appear to be bacterial, since its effects do not 

 show in the aqueous extract of the soil ; and it does not come into evi- 

 dence in the partially-sterilised soils as the bacteria develop. Search 

 was, therefore, made for larger organisms, such as infusoria, amoebae 

 and other protozoa. None were found in the heated soil, and only 

 small ciliated infusoria in the soil treated with toluene. But the un- 

 treated soil contained a variety of them, and the evidence at present 

 available goes to show that such large organisms constitute the factor, 

 or one of the factors for there may be others limiting bacterial 

 activity, and, therefore, fertility in ordinary untreated soils. 



The micro-organic flora of an ordinary soil thus appears to be very 

 mixed and includes a wide variety of organisms performing very 

 different functions. Saprophytes, parasites attacking plants and appar- 

 ently also living on and decomposing organic matter, and large organisms 

 inimical in various ways to bacteria all seem to be present. The action 

 of the saprophytes tends to increase the fertility of the soil, e.g., they 

 produce ammonia, fix nitrogen, and so on. On the other hand, the 

 parasites are obviously harmful, while the phagocytes and similar 

 organisms are detrimental to fertility because they limit the number 

 of bacteria, and, therefore, the rate at which ammonia is produced. 



Between these classes of organisms there is an equilibrium under 

 natural conditions. When, however, toluene is added or when the soil 



