498 SCIENCE PROGRESS 



plants from benefiting from the increased richness of the soil ; 

 but as soon as the grass were removed, the production of toxin 

 would cease and the previously grassed soil would be found to 

 be more fertile than soil which had never had grass growing in 

 it. This is in accordance with the behaviour of trees in soil from 

 grassed and tilled land, as mentioned above; the accumulation 

 of nitrogen in grassed land is a fact which has been known now 

 for many years. It is probable, however, that no soil would 

 ever be quite free from the toxic substance, if such exist, which 

 is produced by the growth of grass. 



The difficulty of examining the action of an easily oxidisable 

 substance by means of growing plants in soil containing it is 

 very great, because even the quickest growing plant takes a 

 considerable time to develop ; the question has been attacked, 

 therefore, by using the germination of seeds as a means of 

 investigation. It does not follow, of course, that a substance 

 which is toxic towards the germination of seeds is toxic also 

 towards plant-growth but the results indicate that this is probably 

 so in the present case. 



When soil is heated, the amount of soluble organic matter 

 and soluble nitrogenous matter is increased ; at the same time, 

 it becomes toxic, as shown by its effect on the germination of seeds. 

 The extent of this toxicity depends on the temperature ; different 

 seeds are affected to different extents and the results naturally are 

 also influenced by the nature of the soil dealt with. On heating 

 the soil to 1 50 C, the soluble organic matter is sometimes 

 increased over tenfold and the time which some seeds take to 

 germinate in the soil increased five or sixfold. When the soil is 

 heated to a lower temperature, the soluble matter and also the 

 toxic effect on seeds rapidly diminishes but the latter is recog- 

 nisable in soil heated to as low a temperature as 6o° ; from the 

 general form of the curve obtained on plotting the various 

 results, it is probable that some such action exists (though it 

 may not'be measurable) in soil which has been heated only by the 

 sun, that is, to a temperature of about 30 , so that even so-called 

 unheated soil probably contains some of this toxic substance. 

 This conclusion is further supported by the fact that various 

 soils behave differently towards germinating seeds and that in 

 nearly every case the seeds do not germinate so readily in soil 

 as they do in pure silica moistened with water. 



It was ascertained that the treatment of soils with antiseptics, 



