134 



SOIL CONDITIONS AND PLANT GROWTH 



of bacteria from the untreated soil to the partially sterilised soil led to 

 still further increases both in bacterial numbers and in the rate of de- 

 composition, whilst addition of bacteria from partially sterilised to un- 

 treated soils had little or no effect. 



The experiments, therefore, indicate that the increased production 

 of ammonia in the partially sterilised soil is due to the increased num- 

 bers of the bacteria rather than to any other cause ; the problem reduced 

 itself to finding out why the bacteria can increase so much more 

 rapidly in the partially sterilised than in the untreated soils. No 

 evidence could be obtained that partial sterilisation produced any 

 sufficient increase in bacterial food to account for the results, it appears 

 rather that the untreated soil contained some factor detrimental to bac- 

 teria and put out of action by heat or antiseptics. All attempts to 

 find a soluble toxin in the untreated soil failed. The factor appeared 

 to be biological and could be reintroduced into the partially sterilised soil 

 by inoculation. Addition of small quantities (O'5 per cent.) 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 LVI. 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, amcebae 

 and other protozoa known to destroy bacteria. None were found in 

 the heated soil, and only small flagellated infusoria in the soil treated 



