Manchester Memoirs, Vol. Iviii. (1914), ^Vc;. (J. 3 



ment of Ajrriculture. Leaving out of account for the 

 moment the nitrogen compounds, they argued that all 

 soils are known to contain much the same compounds of 

 phosphoric acid and potash, in the former case combina- 

 tions with lime, iron and alumina, while potash finds its 

 place in certain complex hydrated double silicates of 

 alumina and the alkalis and alkaline earths. All these 

 compounds possess a low solubility, so low that the 

 amount of the solids present in any soil is always sufficient 

 to saturate the water contained in the soil. It may be 

 considered as established that plants draw their nutri- 

 ment only from the soil solution thus formed, and are not 

 capable of feeding directly upon solid compounds of 

 phosphoric acid and potash. Thus it would be a matter 

 of indifference to the plant whether the soil contained 

 0'2% or 0"05% of phosphoric acid ; in either case the soil 

 solution would be saturated with phosphoric acid, i.e., it 

 would contain the maximum amount in the dissolved 

 state that is in equilibrium with the solid compounds in 

 the soil, which ex JiypotJiesi in all soils are identical in kind 

 though not in quantity. All soils then should give rise 

 to a soil solution of equal concentration in phosphoric 

 acid and potash, and should have as regards these con- 

 stituents equal feeding powers for the crop. Moreover, 

 the addition of soluble phosphates or potash compounds in 

 the form of fertilisers should not sensibly disturb the com- 

 position of the soil solution, for the added soluble material 

 will, as is known, at once react with the bases in the soil and 

 pass into compounds identical with those already existing 

 in the soil, thus effecting no permanent change in the 

 concentration of the soil solution. The cardinal feature of 

 Whitney and Cameron's theory, then, is that all soils 

 must give rise to a soil solution of practically constant 

 composition. Furthermore, they argued on the basis of 



