March, 1912.] 



235 



Scientific Agriculture. 



of fertilisers are not in dispute, but it 

 is contended that their action is much 

 more complex than has hitherto been 

 suspected. Most writers have maintain- 

 ed that fertilisers are simply foods ; 

 Whitney and his colleagues, ou 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 com- 

 position in all cases, there clearly is no 

 exhaustion of food constituents from 

 the soil, and " exhaustion " or " poverty " 

 must therefore be explained in some 

 other way. Great stress is laid on two 

 functions, the physical effect of ferti- 

 lisers 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. 



W can summarise the hypotheses under 

 seven headings : — 



(1.) The soil is formed by disintegra- 

 tion of the rock-forming minerals, de- 

 composition 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 nutri- 

 ent solution for plants ; all soils are 

 therefore equally well provided with 

 plant food. 



(4.) This nutrient solution is distri- 

 buted over the surface of the soil parti- 

 cles in accordance with the laws govern- 

 ing surface attractions. The rate at 

 which any disturbances are re-adjusted 

 depends on certain properties of the 

 particles and is of prime importance in 

 determining the productiveness of soils i 



(5.) But in many cases there must be 

 "some other factor involved because the 

 aqueous extracts of soils, instead of be- 

 having all alike as they should accord- 

 ing to (2), often show the same differ- 

 ences 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 pri- 

 marily in the soil as nutrients for the 

 plants, but they have a much more com- 

 plex action, They may alter the distri- 

 bution of the soil solution, or throw out 

 of action some of the toxins, and prob- 

 ably they serve other functions also. 



Discussion first cantered round the 

 relative importance of the physical and 



chemical properties of soils, and more 

 recently around the alleged excretion 

 of toxins by plants, That the physical 

 properties of soil were of fundamental 

 significance in determining fertility was 

 recognised by all the older agricultural 

 chemists, although actual investigations 

 were made only by Schubeler [22], 

 Wollny [30], and a few others. It was 

 not till Witneys's first papers appeared 

 [24, 25 J that the relationships became 

 clear and definite ; and when later on, 

 Briggs [6] and King [13] published their 

 notable researches on the movements of 

 water in soils, chemists were put in 

 possession of useful working hypotheses, 

 and learnt how to connect up the 

 properties of a soil with the properties 

 of its component particles. It was 

 shown that the size of the particles and * 

 the extent of their surfaces were highly 

 important factors in regulating the 

 water and air relationships of soils ; fur- 

 ther, that the known agricultural pro- 

 perties of the soil could be correlated with 

 the amounts of the variously sized parti- 

 cles it contained, amounts readily deter- 

 mined by a mechanical analysis. These 

 ideas were introduced into this country 

 by Warington [23], and have so com- 

 mended themselves that mechanical 

 analysis is now a recognised part of the 

 work of a soil chemist, and indeed gives 

 a better basis for explaining the agri- 

 cultural properties than does a chemical 

 analysis fll]. 



But we are not prepared to agree that 

 the chemical properties of soils are 

 relatively insignificant in determining 

 fertility. If the coarser fractions of a 

 soil are analysed they aie found to be 

 almost entirely silica, but the finer 

 portions— below 0'005 mm. diameter — 

 which play a controlling part in soil 

 fertility are more complex and contain 

 besides the undeeomposed rock material 

 a quantity of weathered silicates and 

 decomposition products. The special 

 properties of the finer material are 

 associated by Whitney with their small 

 dimensions, approximating in some cases 

 to molecular dimensions and not with 

 any particular chemical compositions; 

 but evidence is accumulating that mere 

 smallness of size will not account for all 

 the observed proportions. The view 

 most commonly held by chemists is due 

 to van Bemmelen [1-4]. Under the con- 

 ditions obtaining in the soil a number of 

 the decomposition products are deposited 

 in a colloidal form, hence the soil 

 particles are coated with a colloidal 

 complex containing silica.aluminium and 

 feme hydroxides, alkaline bases, phos- 

 phoric acid and humus, not in true 

 chemical combination or even in any 



