HEATED AND TREATED SOILS 257 



increase, however, did not manifest itself till after the grasses 

 had been growing for some time. 



That the free access of air is necessary for the toxic substance 

 in heated soils to become converted into plant-food, was estab- 

 lished by a series of experiments in which young apple trees 

 were grown in large bottles containing soil heated to different 

 temperatures, the necks of the bottles being closed so as to admit 

 of only a very limited supply of air, such as would pass through 

 wide tubes packed with cotton wool. The growth measurements 

 of the trees showed a fairly steady decrease according as the 

 temperature of the heating had been higher. 



Temperature . . 30 60 80 100 125 150 C. 

 Growth . . . 100 97 96 79 49 36 



The original object of packing the air-tubes with cotton wool 

 was to exclude bacteria ; also, half of the trees were sterilised 

 before planting ; but no indications were obtained that the results 

 were influenced by the degree of sterility of the conditions. At 

 the same time other experiments showed that the changes 

 occurring in heated soil on its exposure to air took place equally 

 under septic and aseptic conditions, so that in the above experi- 

 ments it is the limitation of the air supply, and not the exclusion 

 of bacteria, which must explain the results obtained. 



The evidence thus derived from a study of these heated and 

 treated soils shows that the organic matter existing in soil decom- 

 poses very readily, forming, first, a substance which is toxic 

 towards germination and plant-growth, and, ultimately, one 

 which acts as a plant-food; also, that such changes must be 

 occurring in soils which are not subjected to temperatures higher 

 than those naturally occurring. That such changes, and analo- 

 gous ones in the case of soils treated with antiseptics, are 

 sufficient to explain all the phenomena observed, may be an open 

 question; indeed, there can be little doubt but that bacterial 

 changes co-operate in producing some of them, such as the in- 

 creased growth in moderately heated soils: the role played by 

 bacteria and protozoa in the soil has been placed beyond the 

 region of uncertainty. But there is now no question of accepting 

 one explanation to the exclusion of the other; the bacterial 

 and chemical explanations are equally true, and the only point 

 remaining to be settled is the relative importance of the two 

 in determining the results. The evidence as it stands at present 

 certainly seems to point to the chemical factor as being the 

 more important of the two ; the trophic protozoa are killed at 

 s 



