Scientific Agriculture. 



128 



[February, 1910. 



of changes go on in the soil during the 

 process and have arrived at some under- 

 standing of the reasons for contradictory 

 results mentioned above- 



The experimental investigation of the 

 subject began with the discovery, in 

 which several men shared, that soil 

 which has been heated to the temper- 

 ature of boiling water will grow larger 

 crops than the same soil which has not 

 been treated. In this country Russell 

 and Darbishire carried out a long series 

 of such experiments, and showed that 

 the heated soil will produce double the 

 yield of the untreated soil, and that the 

 beneficial effect persists as far as four 

 crops after the original heating. All the 

 plants they tried were benefited, except 

 the leguminous species, and all kinds of 

 soils behaved in the same way. More- 

 over, not only did the gross Aveight of 

 the crop increase, but on analysis it 

 proved to be richer in such essentials of 

 plant-food materials as nitrogen and 

 phosphoric acid, so that the crop grown 

 on the soil after heating actually con- 

 tained about four times as much nitrogen 

 as that grown upon the soil which had 

 not been heated. Various explanations 

 of the action were put forward, mostly 

 depending upon changes which were 

 supposed to have been set up in the 

 bacterial flora of the soil ; but Pickering, 

 as the result of his experiments, sug- 

 gested that in the main the action was 

 due to the splitting up of the organic 

 matter (humus) of the soil by heating. 

 He showed that the germination of seeds 

 is retarded in soil that has been heated, 

 and that the retardation is greater the 

 higher the temperature to which the 

 heating is pushed ; he also showed that 

 the soil after heating actually contained 

 more nitrogen compounds in a soluble 

 state. Hence he concluded that the 

 heating had split off from the humus 

 soluble nitrogen compounds which are 

 injurious to germination, but which 

 later will serve as food for the growing 

 plant. Pickering's results are undoubt- 

 edly correct, in that ammonia and 

 other soluble nitrogen compounds are 

 split off from the humus by the heating ; 

 but some work which has just been 

 published by Russell and Hutchinson, of 

 the Rothamsted Laboratory, shows chat 

 this is only nart of the story, the in- 

 crease in fertility of the heated soil 

 being chiefly due to a rearrangement of 

 the living organisms inhabiting the soil. 

 In the first place, it can be shown that 

 heating to the temperature of boiling 

 water for ten hours or so does not ster- 

 ilise the soil ; certain groups of organisms 

 are killed off entirely, but others which 

 exist in the form of spores resist the 



heat, and as soon as the soil cools down 

 again begin to develop and multiply 

 with great rapidity. For example, the 

 bacteria bringing about nitrification are 

 wiped out entirely, but most of the 

 other groups retain some represent- 

 atives, especially that class which take 

 the complex organic matter of the soil 

 and break it down into ammonia and 

 kindred compounds. In one of the 

 Rothamsted arable soils used in the 

 experiments the normal number of 

 bacteria in the soil before treatment 

 was about 5,000,000 per gramme ; immedi- 

 ately after heating the number had 

 fallen to 60 per gramme, but then fol- 

 lowed a very rapid rise ; in a fortnight 

 the original 5,000,000 had been reached, 

 and a month or five weeks afterwards 

 the number had risen to 26 000,000 per 

 gramme. Step by step with this in- 

 crease in the number of bacteria in the 

 soil came a similar increa&e in the rate 

 of production ammonia, i.e., of a soluble 

 nitrogen compound on which the plant 

 could feed. It was thus demonstrated 

 that, in the soil that had been heated, 

 the increased crop is due to the greater 

 amount of ammonia which becomes 

 available for the plant, and that this 

 increase in the ammonia is brought 

 about by the larger number of bacteria, 

 chiefly splitters-off of ammonia, which 

 get a footing in the soil. Various ex- 

 periments, which need not here be 

 detailed, also demonstrated that the 

 increase in numbers of the bacteria is 

 not due to any stimulus derived from 

 the heating, but to the removal of some 

 factor which is at work in ordinary soil 

 keeping down the numbers of bacteria. 

 This new and unknown factor turns out 

 to be the presence in ordinary soil 

 of large non-bacterial organisms like 

 amoebse and infusoria, which habitu- 

 ally feed upon the bacteria, and thus, 

 by keeping their numbers down, estab- 

 lish a certain numerical equilibrium 

 between themselves and the bacteria. 

 These higher organisms are wholly des- 

 troyed by the heating or other steril- 

 isation methods, whereas the bacteria 

 are only partially exterminated and 

 afterwards develop to a much greater 

 extent than before, because they have 

 the field to themselves. With this in- 

 crease in the number of bacteria goes 

 an increased production of soluble plant 

 food from the insoluble reserves in the 

 soil and a corresponding increase in 

 crop. With certain differences these 

 results are repeated when othe*" methods 

 of sterilising the soil are adopted ; if, for 

 example, the soil in a dry state is ex- 

 posed for some hours to the vapour of 

 chloroform, carbon bisulphide, toluene 

 or other volatile antiseptic, there is a 



