4 Hall, Hoiv does the Plant obtain its luitriiiieiit ? 



recorded experiments that the growth of the plant is in- 

 dependent of variations in the composition of the soil 

 solution, that provided, for example, there is some phos- 

 phoric acid in solution the plant will obtain all it can utilise 

 whether the concentration is 3 or 300 parts per million. 

 From these considerations they concluded that neither 

 the amount of phosphoric acid and potash in the soil nor 

 the further supply in fertilisers arc direct factors in the 

 nutrition of the crop, as is usually supposed. Since, 

 however, it cannot be dein"ed that fertilisers ha\e some 

 beneficial effect upon the crop, another mode of action must 

 be found for them, and Whitney and Cameron had recourse 

 to a theory originally suggested by de Candolle that plants 

 excrete and leave behind in the soil certain substances 

 toxic to themselves that will depress the renewed growth 

 of the same plant in the same soil. From this it would 

 follow that a judicious rotation, by giving time for the 

 specific toxin to decay before the particular crop comes 

 round again, would be as effective as a fertiliser in main- 

 taining the productivity of the soil. The fertilisers act by 

 precipitating, or otherwise putting out of action, these 

 toxins. 



Man}' considerations make it difficult to accept so 

 novel a theory, but as the argument for a soil solution of 

 constant composition is a logical if too hard and fast a 

 deduction from accepted facts, it was decided to submit it 

 to the test of direct experiment. 



Experimental. 



The first line of attack was to test the nutritive power 

 of actual soil solutions derived from soils of known 

 origin, and accordingly solutions were made up from soils 

 selected from the wheat and barley plots at Rothamsted, 

 which had been growing these crops without break for 



