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500 SCIENCE PROGRESS 



at the end of this time, the soluble organic matter was still more 

 than double what it was in the unheated soil and this excess of 

 soluble matter, which is no longer toxic or is barely so, must 

 represent the presence of so much extra plant-food ; it is not 

 surprising, therefore, to find that plants flourish much better 

 after a time in soil which has been heated than in ordinary soil. 



The occurrence of such changes in soil which has been 

 heated renders the investigation of its behaviour towards plant- 

 growth very difficult ; it is possible that the action of the 

 toxic substance present (if it be toxic towards plant-growth 

 as well as towards seed-germination) may be masked, by its 

 becoming decomposed before the plant can be grown ; the only 

 results which will follow from its presence will be an increase of 

 growth owing to the excess of soluble organic matter left in the 

 soil by its decomposition. 



These two opposing factors are, as a matter of fact, recognis- 

 able in the results obtained when plants are grown in soil 

 which has been heated ; whether the one or the other pre- 

 dominate depends on the sensitiveness of the plant to the 

 action of the toxin and on the amount of the latter present. 



Fig. 4 shows tomato and tobacco plants grown in soil heated 

 to 30 (so-called unheated soil), 6o°, 8o°, ioo", 125 and 150 . The 

 presence of some toxic substance after heating to the higher 

 temperatures is placed beyond dispute by the dwarfed condition 

 of the plants in these cases, tobacco being evidently more 

 sensitive to this effect than the tomato. Photographs taken 

 at an earlier date show a much more marked effect than those 

 given here, whilst others taken later show less effect ; eventually, 

 before growth was completed, the toxic effect had almost 

 entirely disappeared and the beneficial effects of the products of 

 its oxidation had so far asserted themselves that the plants, 

 even in the most highly heated soils, had outstripped those 

 in the unheated soil. 1 When the soil is heated to temperatures 

 of ioo° or lower, owing to the smaller quantity of toxin present, 

 the effect persists during a still shorter period and even in the 

 early stages we get a stronger growth than in the unheated soil. 



The disappearance of the toxic effect in the most highly 

 heated soils was further illustrated by growing a second crop of 

 these same plants in the samples of soil used for the first crops. 

 The results are shown in fig. 5. As will be seen, it is only in the 



1 See Journal of Agricultural Science, iii. 280. 



