E. J. Russell and H. B. Hutchinson 121 



factor is shown to be biological. Large organisms (protozoa) were 

 found in the untrcatcil, but not in the partially sterilised soils, at 

 least two of which are known to destroy bacteria. 



(7) These large competing and destructive organisms are killed by 

 heat and most of them by toluene, and can then serve as food for 

 bacteria. In both these directions the effect of partial sterilisation 

 is beneficial. 



(8) As the effect of partial sterilisation in increasing productiveness 

 is shown on so many soils, and apparently always in the same way, 

 it may be expected that these competing and destructive protozoa 

 are of common occurrence and constitute an important factor in soil 

 fertility. 



(9) In relation to plant growth partially sterilised soils are peculiar 

 in that they supply not nitrate, but other nitrogen compounds such as 

 ammonia, to the plant. The nitrifying organisms will develope if they 

 get into the toluened soil, but they did not work in our heated soils. 

 With this difference in the course of nitrogen nutrition may be cor- 

 related the difference in nitrogen content of the plant and in the 

 character of growth. 



Part 2. 



Experimental. 



§ 13. Crop results. The soil was taken from the outside strip of 

 Barnfield, and had been unmanured for many years. It was brought 

 down in quantity to the plant-house, spread on a clean cement floor, 

 sieved, and carefully picked over to remove worms. The picking over 

 requires great care and is very laborious; unless it is properly dune 

 the crop weights in duplicate experiments are likely to be discordant. 

 The soil was next weighed into pots, tipped out, and mixed with 10 per 

 cent, of sand ; then it was either replaced, heated to 98° C. in a large 

 steam oven, or treated with toluene, according as it was to be an 

 " untreated," a " heated," or a " toluened " soil. In the latter case 2 c.c. 

 of toluene were added for every kilogram of soil, left to act for three 

 days, then allowed to evaporate by spreading the soil out in a thin 

 layer for a sufficient length of time. Finally all the pots were weighed, 

 and water added till 18 per cent, was present, an amount which was 

 kept fairly constant throughout the period of plant growth. 



The results obtained with successive crops are given in Table 1. 



