138 SCIENCE PROGRESS 



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 Department of 

 Agriculture which we shall now proceed to set out. 



The Hypotheses of the United States Bureau of Soils 



The foundation of these hypotheses is the recognition of the 

 vitally important functions of the soil moisture. The films 

 of water 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 solution 

 and the plant has been elucidated by a study of plant growth 

 in various culture solutions. Perhaps the most striking con- 

 clusion is that the rate of plant growth is relatively constant 

 over wide variations in the concentration of the solution ; in 

 Breazeale's experiments [5] J 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 constituent 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 

 constant 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 

 P0 4 = 7*6, N0 3 = 5'5, Ca= 117, K = 227. Whitney and Cameron 

 [28] accept these figures which, they consider, "represent closely 

 the average figures for the great majority of cultivable soils 

 as regards these several constituents." It is since admitted 

 that the nitrate figures are subject to considerable fluctuations 

 and indeed that nitrates ought hardly to be regarded as mineral 

 but as organic constituents of the solution, as they arise from 



1 The numbers; in square brackets refer to the bibliography at the end of the 

 paper. 



