654 



Theories of Plant Nutrition. [nov., 



A convenient explanation is thus provided for the beneficial 

 action of rotation of crops and the sterility which results when 

 the same crop is grown continuously on the same ground. 

 In order to explain the unquestioned beneficial effects of fer- 

 tilisers, which the above hypothesis would seem to negative, 

 it is supposed that they act either on the distribution of the 

 soil moisture, or as antidotes to the toxic substances. 



In discussing this hypothesis Dr. Russell in the first 

 instance points out that the evidence is insufficient to establish 

 the primary assumption that the soil solution is constant for 

 all soils. Figures are quoted to show that the concentration, 

 while showing little absolute variation, yet relatively differs 

 considerably. The difference between 5 parts per million and 

 30 parts per million, when stated in concrete figures, is 

 small, but relatively it is equivalent to a difference of 500 per 

 cent. Moreover, the irregular distribution of the figures, 

 showing the number of results for each degree of concen- 

 tration, suggests that the variation is outside the ordinary 

 experimental error. 



In the next place, Dr. Russell contests the view that 

 variations in the soil concentration do not influence the rate 

 of plant growth, and consequently are without influence on 

 fertility. He points out that, while there is always a limiting 

 factor which controls the situation, the amount of growth in 

 nutrient solutions tends to increase as the concentration of 

 salts is increased. 



With regard to toxic substances, it is pointed out that it is 

 not surprising that some of the numerous organic compounds I 

 in the soil should have a toxic action when supplied to the 

 plant through the medium of a water culture. It does nor 

 follow that such substances retain their toxic properties when 

 added to the soil. In regard to the phenomena associated 

 with "sour" soils, the writer is, however, prepared to admit 

 that toxic substances may be present. 



In regard to the excretion of toxic substances during the; 

 normal processes of growth, Dr. Russell is able to bring 

 forward the results of his own work at Rothamsted, and to 

 show that there is no evidence in support of this hypothesis. 

 Plants grown continuously in water, sand and soil cultures 

 for as many as six generations, have shown no signs pi 



