230 THE LURE OF THE LAND 



that a neutralization of these toxic substances by proper 

 antidotes is sufficient to restore the soil to its pristine 

 fertility. This theory of the Bureau of Soils has not 

 gained a much wider vogue than the original one in re- 

 gard to the sufficiency of all soils to supply the phos- 

 phoric acid and potash necessary for growth. This 

 theory was also experimentally tested by Hall, and as 

 a result of the investigations in both lines of study the 

 following conclusions were drawn: 



(1) The composition of the natural soil solution as regards 

 phosphoric acid and potash is not constant, but varies signif- 

 icantly in accord with the composition of the soil and its past 

 manurial history. 



(2) Within wide limits the rate of growth of a plant 

 varies with the concentration of the nutritive solution, irre- 

 spective of the total amount of plant food available. 



(3) When other conditions, such as the supply of nitrogen, 

 water, and air, are equal, the growth of the crop will be de- 

 termined by the concentration of the soil solution in phos- 

 phoric acid and potash which, in its turn, is determined by the 

 amount of these substances in the soil, their state of combina- 

 tion, and the fertilizer supplied. 



(4) On normal cultivated soils the growth of crops like 

 wheat and barley, even when repeated for 60 years in succes- 

 sion, does not leave behind in the soil specific toxic substances 

 which have an injurious effect upon the growth of the same or 

 other plants in that soil. 



The net result of these investigations is to restore the 

 earlier theory of the direct nutrition of the plant by fertiliz- 

 ers. The composition of the soil solution which determines the 

 growth of the plant is dependent upon the amount and the 

 mode of combination of the phosphoric acid and potash in the 

 soil, both of which are affected by the fertilizer supply, 

 though to what extent is not yet determinable. 



It is evident that the consensus of scientific opinion 

 supports the results of Mr. Hall's investigations, and 

 therefore any failure of a soil to produce a crop, aside 



