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Mr. E. J. Russell. 



soil soon contains higher bacterial numbers per gramme and accumulates 

 ammonia at a faster rate than partially sterilised soil alone. 



4. The improvement in the soil brought about by partial sterilisation is 

 permanent, the high bacterial numbers being kept up even for 200 days or 

 more. The improvement, therefore, did not consist in the removal of the 

 products of bacterial activity, because there is much more activity in partially 

 sterilised soil than in untreated soil. Further evidence is afforded by the 

 fact that a second treatment of the soil some months after the first produces 

 little or no effect. 



It is evident from (3) and (4) that the factor limiting bacterial numbers 

 in ordinary soils is not bacterial, nor is it any product of bacterial activity, 

 nor does it arise spontaneously in soils. 



5. But if some of the untreated soil is introduced into partially sterilised 

 soil, the bacterial numbers, after the initial rise (see (3) ), begin to fall. The 

 effect is rather variable, but is usually most marked in moist soils that have 

 been well supplied with organic manures ; e.g., in dunged soils, greenhouse 

 soils, sewage farm soils, etc. Thus the limiting factor can be reintroduced 

 from untreated soils. 



6. Evidence of the action of the limiting factor in untreated soils is 

 obtained by studying tbe effect of temperature on bacterial numbers. 

 Untreated soils were maintained at 10°, 20°, 30° C, etc., in a well moistened 

 aerated condition, and periodical counts were made of the numbers of 

 bacteria per gramme. Eise in temperature rarely caused any increase in 

 bacterial numbers ; sometimes it had no action, often it caused a fall. But 

 after the soil was partially sterilised the bacterial numbers showed the 

 normal increase with increasing temperatures. Similar results were 

 obtained by varying the amount of moisture but keeping the temperature 

 constant (20° C). The bacterial numbers in untreated soil behave erratically 

 and tended rather to fall than to rise when the conditions were made more 

 favourable to trophic life ; on the other hand, in partially sterilised soil, the 

 bacterial numbers steadily increased with increasing moisture content. 

 Again, when untreated soils are stored in the laboratory or glasshouse under 

 varying conditions of temperature and of moisture content the bacterial 

 numbers fluctuate erratically ; when partially sterilised soils are thus stored 

 the fluctuations are regular. 



7. When the curves obtained in (6) are examined it becomes evident that 

 the limiting factor in the untreated soils is not the lack of anything* but 

 the presence of something active. 



* The soils included fertile loams well supplied with organic matter, calcium 

 carbonate, phosphates, etc. 



