

HORTICULTURAL RESEARCH 501 



case of tobacco that any indications of toxic action are still 

 visible and then only in the most highly heated soil. 



Though the general results with all the plants examined 

 were similar to those here described, it was noticeable that the 

 toxic action was much less potent in the case of grasses than in 

 that of the other plants (tomatoes, tobacco and spinach), the 

 beneficial after-effect coming into evidence earlier and to a 

 greater extent. 



These results fully justify the conclusion that the oxidisable 

 substance in heated soils which is toxic towards seeds, hindering 

 their germination, is toxic also towards plant-growth. 



The experiments were further extended so as to establish 

 the identity of the action on trees with that on the other crops 

 mentioned : in the case of trees, if the trees are grown in the 

 ordinary way, the soil being fully exposed to air, owing to the 

 extended time required for growth nothing is observed but the 

 beneficial effects of the heating : but when the experiment is so 

 modified as to limit the access of air considerably, the toxic effect 

 is even observable. Small trees were grown in soil contained 

 in bottles the necks of which were closed, except for two open- 

 ings into which tubes plugged with cotton wool were inserted ; 

 the results of a series of experiments made in this way showed a 

 small increase of vigour of growth, not exceeding 10 per cent., in 

 the case of soils heated to temperatures up to 100 s but a decrease, 

 up to 35 per cent., in the case of soils heated to 125° and 150°. 



The connexion between the toxic action of heated soil and 

 the toxic action of grass on trees and other plants cannot 

 be said to have been established yet but there are one or two 

 facts which point to a possible identity. The soil which is toxic 

 while the grass is growing in it does not behave normally 

 as soon as the grass is removed ; but after it has been exposed 

 to the air, just like heated soils, it is more favourable to plant 

 growth than ungrassed soil and contains a larger amount of 

 soluble organic and nitrogenous matter. Another somewhat 

 remarkable point of similarity has been noticed : soils which 

 have been heated contain some oily or resinous substance which 

 renders them more difficult to wet than unheated soil. This 

 peculiarity becomes more marked as the temperature of heating 

 is higher; different soils vary considerably in this respect. The 

 peculiarity was so marked in one case that the soil could not be 

 thoroughly wetted after it had been heated, even when left 



