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become favourable the survivors multiply with extraordinary rapidity 

 and bring about a more intense production of nitrogenous plant 

 food in the soil. They supposed that a larger amount of at- 

 mospheric nitrogen is " fixed " and the complex substances thus 

 formed in the bacterial cells are slowly broken down to yield plant 

 food. Other investigators have also supposed that increased ni- 

 trogen-fixation is the main cause ot the increased productiveness. 



On the other hand, Koch maintains that nitrogen-fixation is 

 decreased by partial sterilisation. Stormer considers that the larger 

 organisms are killed and decomposed by the surviving bacteria, 

 with production of ammonia. The dark green colour of the plants 

 grown on partially sterilised soils has generally been regarded as 

 an indication that the nitrogenous food stuff in the soil has in some 

 way been increased by the treatment. 



The authors give in this volume of the Agricultural Science 

 Journal a statement of the experiments made by them in this field 

 and the conclusions to which these experiments lead. 



The soil employed in the^experiments was taken from an arable 

 field and contained moderate amounts of nitrogen, organic matter, 

 and calcium carbonate. Partial sterilisation was effected either by 

 heating to 98 C. or by addition of 4 per cent of toluene, which 

 at the end of three days -was allowed to evaporate by spreading 

 out the soil in a thin layer for as long as necessary. In a third 

 series of experiments the toluene was left in the soil during the 

 whole of the experimental period. A fourth series consisted of 

 untreated soils. A few experiments were also made with soils heated 

 to 125 C., at which temperature all organisms are killed. 



After treatment the soils were moistened and kept for definite 

 periods in bottles stopped with cotton-wool at the ordinary labor- 

 atory temperature. In these circumstances various changes soon 

 set in ; in fact bacteria multiply more rapidly and reach far higher 

 numbers in the partially sterilised than in the untreated soils, from 

 6 millions to 40 millions per gram. Considerable evidence was ob- 

 tained that the whole surviving bacterial flora is more active than 

 the original one in effecting the decomposition of nitrogenous or- 

 ganic substances such as peptone etc., and in hydrolising urea ; that 

 the comparative inertness of the bacteria in the untreated soil 

 cannot be caused by any bacterial factor; that by toluening or 

 heating the soil this non-bacterial factor, limiting the development 

 of bacteria, is put out of action. 



The limiting factor must be biological. Large organisms such 

 as infusoria, amoebae and ciliata were not found in the heated soil, 



