380 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 
contains some toxin), and, as most of the organic matter in soil is the 
product of plant-growth, it follows that more toxin will be produced 
where plants are growing than where they are not, hence the toxic 
effect of one plant on another. The ultimate decomposition of the 
toxin into food-material, demonstrated by the results with heated soils, 
also explains the increased fertility observed in a soil which has grown 
a surface crop, as soon as that surface crop is removed, and the pro- 
duction of toxin ceases. 
I What the toxic substance is, has not yet been ascertained, but an 
Temperature of Heating. 
Fig. 58, — Incubation Periods of Seeds in Heated Soils, 
examination of it is now in progress from the chemical point of view. 
Those experiments, of which some account has been given here, all 
go to prove that the toxin is a substance which can be oxidized, or, in 
chemical parlance, that it is a reducing body, and it has been found that 
there is such a body present to a certain extent in extracts from all 
soils, and that the proportions of it are increased by growing a crop in 
the soil, or still more so, by heating it. But the question is evidently 
one of great complexity which cannot be dealt with in the present 
communication ; it may be said, however, with confidence that the 
toxin is not dihydroxystearic acid, which, according to the views 
expressed by workers in the Soils Bureau of the United States, is the 
main substance accountable for the infertility of certain soils. 
