March, 1912. 



233 



Scientific Agriculture. 



groups— those that contain calcium car- 

 bonate and those that do not. So great 

 is the distinction that the practical mau 

 has long since made use of a separate 

 name for the latter soils and calls them 

 "sour," a word which many writers 

 naturally but not altogether correctly 

 interpret as "acid." 



These then are the main relationships 

 which agricultural chemists are called 

 upon to investigate. Of the various 

 hypotheses as to the constitution of the 

 soil that have been framed during the 

 course of these investigations, the most 

 complete are those of Whitney, Cameron 

 and others associated with the Bureau 

 of Soils of the United States Depart- 

 ment ot Agriculture which we shall now 

 proceed to set out. 



The Hypotheses op the United States 

 Bureau op Soils. 



The foundation of these hypotheses is 

 the recognition of the vitally important 

 functions of the soil moisture. The 

 films of watar surrounding the particles 

 are looked upon as the nutrient solution 

 feeding the plant and constituting by 

 far the most important part of the soil. 

 This soil solution has been studied in two 

 respects — in relation to the plant and in 

 relation to the soil, with remarkable 

 and unexpected results in both cases- 

 The relationship between the soil solu- 

 tion and the plant has been elucidated by 

 a study of plant growth in various cul- 

 ture solutions. Perhaps the mcst striking 

 conclusion is that the rate of plant growth 

 is relatively constant over wide varia- 

 tions in the concentration of the solution ; 

 in Breazeale's experiments an increase in 

 the strength of the nutrient solution from 

 75 parts to 750 parts per million of total 

 soluble salts produced only, little effect 

 on the growth of wheat seedlings. 

 Variations in the relative proportions 

 of the constitutent, salts have a much 

 more marked effect, and not only lead to 

 changes in the rate of growth but also 

 to changes in the character of the plant, 

 i,e., the proportion of root to leaf, etc, 



Perhaps the most important property 

 of the solution in relation to the soil is 

 that its concentration is practically con- 

 stant in all ordinary soils. Taylor and 

 Mooney (v. [28]) concluded from a large 

 number of analyses that the amount of 

 solution associated with a million parts 

 of soil contained P0i = 7.6, No3= 5.5, 

 Ca = 11.7,K=22.7. Whitney and Cameron 

 [28] accept these figures which, they 

 consider, " represent closely the average 

 figures for the great majority of cultiv- 

 able soils as regards these several con- 

 stituents." It is since admitted that 

 the nitrate figures are subject to con- 

 30 



siderable fluctuations, and indeed that 

 nitrates ought hardly to be regarded as 

 mineral hut as organic constituents of 

 the solution, as they arise from organic 

 matter; hut the position is still main- 

 tained with regard to the phosphoric 

 acid, potassium, and calcium. 



The constancy in composition of the 

 soil solution is readily accounted for. 

 Most soils arise from a relatively small 

 number of minerals. It is supposed that 

 the process is mainly one of disintegra- 

 tion and not decomposition. The large 

 fragments of the soil are shown to 

 consist of unaltered minerals, and the 

 small fragments could probably all be 

 identified if suitable methods were avail- 

 able. Thus most soils are chemically 

 alike from the qualitative point of view ; 

 they are mixtures of the same complex 

 minerals, and differ only in the propor- 

 tions of the mixture and the size of the 

 fragments. In consequence the soil 

 solution in equilibrium with these par- 

 ticles has the same composition in all 

 cases. Further, it is argued that the 

 addition of a soluble potassium salt to 

 the soil cannot increase the concentra- 

 tion of the solution in K ions, since any 

 of the new ions going into solution would 

 simply throw others out of solution. 

 The solubility product, it is supposed, 

 must x'emain approximately constant 

 whatever soluble salts are added. 



The mineral particles of the soil con 

 stitute the reserve from which the soil 

 solution is reinforced as fast as soluble 

 material passes into the plant. Bat 

 they serve another purpose ; they form 

 the great framework over which the soil 

 solution arranges itself in accordance 

 with the laws governing surface attrac- 

 tions. Any local disturbance brought 

 about by the action of plants or other 

 causes is readjusted at a speed which 

 depends on the size and nature of the 

 particles, the presence of organic matter, 

 and various other factors. Whilst there- 

 fore all soils are alike in regard to the 

 composition of the soil solution, they 

 show great differences in respect to the 

 arrangement of the solution over their 

 particles and the speed of readjustment ; 

 in other words, in the speed at which 

 the nutrient solvent is supplied to the 

 plant. 



The fertility of the soil does not de- 

 pend on the chemical composition of the 

 soil for two very good reasons : the com- 

 position of the soil solution is the same 

 in all soils ; and even if ifc were not 

 variations in composition would be with- 

 out effect on plant growth. In most 

 soils fertility depends on the physical 

 factors regulating the supply of the soil 

 solution to the plant. 



