xviii CONTENTS 



CHAPTER XXV 



PAGE 



HEATED AND TREATED SOILS ..... 249 



Effect on bacterial growth; effect on soluble matter present; 

 changes after treatment ; germination of seeds in heated soils ; 

 results not due to bacterial changes; action of antiseptics; 

 plant-growth in heated soils; trees grown in heated soils; 

 the bacterial and chemical explanations of the results. 



CHAPTER XXVI 



THE EFFECT OF GRASS ON TREES 259 



General characteristics of the action; independent of the age 

 of the trees ; action on different kinds of fruit trees ; on forest 

 trees ; of different kinds of grasses ; in different soils ; effect 

 on the roots; on the colouring, matter; limited action of 

 grass ; possible recovery from the effect ; grassing to different 

 extents ; gradual grassing. 



CHAPTER XXVII 



POSSIBLE CAUSES OF THE GRASS EFFECT . . . 275 



Aeration of the soil; carbon- dioxide contents of the soil; 

 soil-temperature ; bacterial conditions ; alkalinity ; physical 

 condition ; the water-supply ; evaporation from soil with and 

 without a surface crop ; water contents of grassed and tilled soil ; 

 pot experiments on the subject; the food-supply; feeding off 

 the grass ; enrichment of the soil by grass. 



CHAPTER XXVIII 



THE TOXIC ACTION OF ONE CROP ON ANOTHER . . 291 



Pot experiments; universality of the action; variation of the 

 effect ; action of plants on others of the same and of different 

 sorts ; recovery from the effect ; surface crop not deleterious 

 if the toxin from it is retained; the importance of drainage 

 explained by the removal of toxin ; effect of trees on grass and 

 other crops. The general question of toxin-formation ; relative 

 action of weeds and grass on trees ; richness of soil from grassed 

 land ; destruction of the toxin by aeration ; grassing trees to 

 different extents; possible beneficial action in some cases; 

 variability of the grass effect under different conditions; 

 gradual grassing; effect of depth of soil; the manner of 

 toxin-formation; the rapidity of the decomposition of the 

 toxin. 



