NOVEMBEE 6, 1908] 



SCIENCE 



623 



can hardly be regarded as valid in theory, 

 even if the conditions under which the 

 reagents exist in the soil are the same as 

 those which prevail in the laboratory when 

 such conditions of equilibrium between 

 sparingly soluble solids and water are 

 worlied out. It has no bearing whatever 

 on the amount of nitrates in the soil water, 

 since they come into a dissolved state as 

 fast as the nitrifying bacteria produce 

 them, and are not in equilibrium with any 

 store of undissolved nitrates in the back- 

 ground. As regards phosphoric acid the 

 theory assumes such an excess of bases that 

 all soils behave alike and immediately 

 precipitate the phosphoric acid, in prac- 

 tically the same form; while as regards 

 potash the argument seems to forget that 

 though the addition of a soluble potassium 

 salt may throw some of the other sparingly 

 soluble potassium compounds out of solu- 

 tion, the total amount of potassium re- 

 maining in solution is still greatly in- 

 creased. The function of the carbonic 

 acid in the soil water is ignored, as again 

 the fact that the processes of solution in 

 the soil must be in a constant state of 

 change, so that it is the dynamic rather 

 than the static solubility which is of im- 

 portance. The soil is too complex a mix- 

 ture to permit as yet of attaching great 

 weight to theoretical deductions as to the 

 actions taking place in it, and that the 

 state of affairs postulated by Whitney and- 

 Cameron does hold in the soil has not 

 however, been verified by experiment; the 

 analyses, given by the authors of the 

 theory, of the cold water extracts from a 

 number of soils show great variations in 

 their concentration, in nitrates, phosphoric 

 acid and potash; nor is any evidence 

 forthcoming that such concentrations are 

 not immediately raised by the addition of 

 fertilizers. Indeed, when the Rothamsted 

 soils, with their long-continued difference 

 in fertilizer treatment, are extracted with 



water charged with carbon dioxide, the 

 nearest laboratory equivalent to the actual 

 soil water, the amount of phosphoric acid 

 going into solution is closely proportional 

 to the previous fertilizer supply, and this 

 proportionality is maintained if the ex- 

 traction is repeated with fresh solvent, as 

 must be the case in the soil. In the field 

 it is not merely the initial concentration of 

 the soil water in plant food which de- 

 termines the supply of nutriment to the 

 crop, it is also the capacity of the soil 

 to keep renewing the solution as the plant 

 withdraws from it the essential elements. 



In one essential respect again the condi- 

 tions prevailing in the soil are very dif- 

 ferent from those of the laboratory; in the 

 soil all reactions are extremely localized, 

 since they take place in the thin film of 

 water normally surrounding the soil par- 

 ticles, in which movement of the dissolved 

 matter takes place very slowly and mainly 

 by diffusion. Of the extreme slowness of 

 the diffusion of soluble salts in the soil the 

 Rothamsted experiments afford some good 

 examples ; for instance, on the grass plots 

 only an imaginary line divides the pots re- 

 ceiving different fertilizers, the manure is 

 sown right up to the edge of the plot, a 

 screen being placed along the edge to pre- 

 vent any being thrown across the bound- 

 ary, then immediately on the other side of 

 the boundary the different treatment 

 begins. In two cases plots receiving very 

 large amounts of soluble fertilizer, e. g., 

 550 pounds per acre of nitrate of soda or 

 600 pounds per acre of ammonium salts, 

 march with plots receiving either no fertil- 

 izer or a characteristically different one, 

 yet in neither case is there any sign in 

 the herbage of the soluble fertilizer having 

 diffused over the boundary. Although the 

 treatment has been repeated now for 52 

 years, the dividing line between the two 

 plots remains perfectly sharp and the rank 

 herbage produced by the excess of nitre- 



