14 SYLVICULTURE IN THE TROPICS pt. i 



fallow for a sufficient time the toxic matter in the soil 

 will be removed. The formation of humus in a forest 

 soil would have a similar effect to artificial manure on 

 texture and on toxic substances. 



These toxic substances which exist in the soil are 

 said to be largely due to excretions from plant roots, 

 and it is explained that the plants do not poison them- 

 selves because the roots are constantly on the move 

 searching for food, and fresh areas are constantly being 

 tapped. It is the tip of the root which absorbs the 

 solution, and as it is only capable of performing these 

 functions for about four days, it must extend itself in 

 order to be able to continue these functions, and thus 

 the poison -impregnated soil is left behind. 



The toxic substances excreted by a plant are said 

 not to be necessarily toxic to other species. It is held, 

 on the contrary, that the cultivation of other crops will 

 sweeten the soil and render it capable of again rearing 

 those which had been excluded owing to the excremental 

 toxins which they had given out. 



These arguments, although they are admitted by 

 English experts to help in the true solution of the 

 functions of the soil towards the plants, are by no means 

 accepted in their entirety. For example, as regards 

 plant excretions, no evidence has been found to prove 

 that they exist. At Rothamsted l wheat has been 

 grown continuously in one plot since 1843, and the young 

 plants show no signs of poisoning ; and other experiments 

 failed to show any such effects on crops grown several 

 times in succession in the same soil. As regards soils 

 which were grown in rotation and with one year fallow, 

 the wheat certainly gave astonishingly good results, but, 

 on the other hand, turnips did not. Some other causes 

 for the diminution of soil fertility must, therefore, be 

 looked for. 



Dr. Russell 2 disagrees with American experts that 

 the soil solution is the same in all soils, and says : 



1 Dr. E. J. Russell, loc. cit. p. 148. 

 2 lb. p. 150. 



