THE ACTION OF ONE CROP ON ANOTHER. 
379 
If the growth of plants in soil produces a toxic substance, we 
should expect that some toxin must exist in all soils under normal 
conditions. That such is the case has been established by our examina- 
tion of heated soils. When a soil is heated to various temperatures 
short of that at which destruction of organic matter begins, the pro- 
portion of soluble matter, both organic and inorganic, in it increases 
rapidly, and the soil becomes toxic to plant growth, and also to the 
germination of seeds ; and, from a comparison of the amount of soluble 
matter present after heating to different temperatures, it is clear that 
such changes must extend down to ordinary atmospheric tempera- 
tures, that is, that a certain amount of such toxic substances must be 
present in soils which have not been heated by artificial means. This is 
also shown by the fact that seeds germinate more easily in a wet 
inert medium than in soil. As to the toxicity of heated soils, there 
can be no question : when freshly heated soil is examined by its effect 
on the germination of seeds, it is found that it retards this germination, 
so that the period of incubation is increased by nearly 100 per cent, 
when the temperature of heating has been 150 0 C. (curve 0 in fig. 58). 
But this toxic effect disappears after a time ; forty-four days after 
the heating its retarding effect is greatly reduced (curve marked 44) , 
and after 106 days it has nearly entirely disappeared. When 
the toxic properties of the heated soil are examined by growing- 
plants in it, the results are, in consequence of this reduction of toxicity 
with time, as well as of certain bacterial changes consequent on the 
heating, somewhat obscured, for the growth of the plants occupies a 
much longer period than the germination of seeds, and during this 
period the toxicity is rapidly disappearing. When the temperature 
to which the soil has been heated is not high — these temperatures are 
marked on the pots in fig. 59 — the proportion of toxin is not large, 
and it becomes more or less completely oxidized into food-material 
before growth progresses very far, so that we get an increased vigour 
in such cases ; but where the temperature has been higher, and more 
toxin is present, its effect is more persistent, and becomes apparent 
from the stunting of the plants. This was still more evident at an 
earlier date in the growth of the plants, though, unfortunately, no 
photographs of the results were taken then, but a photograph taken 
at a later date is enough to show that, as time goes on, the toxic effect 
diminishes, and the plants in the more highly heated soil, though still 
somewhat stunted, have outstripped in weight those in the less heated 
soil, giving clear proof that much more food-material must have been 
available in their case, and that the only reason why it did not operate 
at an earlier date must have been the existence of some adverse cir- 
cumstance preventing the plant from benefiting by the food which 
was there : that is, that some toxic influence was operating. Similar 
results with other plants were obtained, telling the same tale. 
That the toxicity of heated soils is due to the decomposition of the 
organic matter in the soil cannot be doubted, and a like decomposition 
must occur, though less readily, at ordinary atmospheric temperatures 
(indeed, the germination results prove that so-called unheated soil 
