THE SOIL AND THE PLANT 



i47 



When we turn to the question of plant excretions we can 

 find no evidence whatsoever in favour of their existence. At 

 Rothamsted plants have been grown continuously in water 

 cultures, in sand cultures and in soils without showing any 

 sign of suffering from excretions of previous generations. Six 

 crops of rye were grown in succession in sand to which only 

 nutrient salts were added so as to maintain the food material 

 at a constant amount. A seventh crop was then taken and 

 at the same time a crop was grown on perfectly fresh 

 sand on which nothing had grown before, supplied with an 

 equal amount of the same nutrient salts. There was no 

 significant difference in the two crop yields, they agreed within 

 the 5 per cent, experimental error. A similar experiment was 

 made with buck-wheat, another with spinach, and a parallel 

 series was made in soil cultures. In all cases the result was 

 the same ; the 1910 weights were as follows : 



Both sand and soil contained 2 per cent, of calcium carbonate. 



If either the rye, buckwheat or spinach excreted any toxin 

 the amount accumulating during the growth of six successive 

 crops was insufficient to cause any appreciable depression in 

 yield in the crop ; the exceptional result given by buckwheat 

 in sand could not be confirmed. 



Thus we are once more up against a question of fact : it is 

 asserted by the American workers that the continued growth 

 of one and the same crop on the same soil leads to a low crop- 

 production, whilst we on this side are unable to obtain any 

 evidence to this effect. 



But again the experimental difficulties are considerably 

 greater than they appear at first sight. Water cultures do 

 not long remain sterile but soon contain a bacterial population 

 which clearly produces some change in the solution, because 

 the nitrates quickly disappear. Sand cultures rapidly develop 

 a vigorous growth of algae and bacteria ; here also nitrates 



