142 SCIENCE PROGRESS 



tended that their action is much more complex than has hitherto 

 been suspected. Most writers have maintained that fertilisers 

 are simply foods ; Whitney and his colleagues, on the other 

 hand, consider their nutritive effect as relatively insignificant, 

 or at least as only one of several beneficent functions. As the 

 soil solution has the same composition in all cases there clearly 

 is no exhaustion of food constituents from the soil, and " ex- 

 haustion " or " poverty " must therefore be explained in some 

 other way. Great stress is laid on two functions, the physical 

 effect of fertilisers on the distribution of the soil solution ; and 

 their influence in removing toxins or, as Whitney puts it, in 

 soil sanitation. 



The Points at Issue 



We can summarise the hypotheses under seven headings : 

 (i) The soil is formed by disintegration of the rock-forming 



minerals, decomposition taking place to a relatively insignificant 



extent. 



(2) All normal soils are therefore chemically alike and the 

 soil solution has the same composition in all soils. 



(3) The soil solution forms the nutrient solution for plants ; 

 all soils are therefore equally well provided with plant food. 



(4) This nutrient solution is distributed over the surface of 

 the soil particles in accordance with the laws governing surface 

 attractions. The rate at which any disturbances are readjusted 

 depends on certain properties of the particles and is of prime 

 importance in determining the productiveness of soils. 



(5) But in many cases there must be some other factor 

 involved, because the aqueous extracts of soils, instead of behaving 

 all alike as they should according to (2), often show the same 

 differences in productiveness as the soils themselves. A toxin 

 must therefore be present in infertile soils. 



(6) Some toxins may arise normally during the decomposition 

 of the soil organic matter but some are probably excreted 

 by plants. 



(7) Fertilisers do not function primarily in the soil as 

 nutrients for the plants but they have a much more complex 

 action. They may alter the distribution of the soil solution 

 or throw out of action some of the toxins and probably they 

 serve other functions also. 



Discussion first centred round the relative importance of 



