392 Journal of Agricultural Research vol. xx. no. $ 



The application of the foregoing conclusions would seem to indicate 

 that soil extracts should be made with a small proportion of water and 

 for a short period. It would probably be desirable to use not more than 

 i part of water to i part of soil, but in many cases this may be imprac- 

 ticable, so that i to 5 extracts must suffice. It is true that special studies 

 of soil equilibria must take into account long-continued solvent action, 

 but in attempts to gain some idea of periodic changes in the soil solution 

 the technic should be directed toward lessening the solution of material 

 not actually present in the soil solution. This aim is less possible of 

 attainment in proportion as the volume of water or time of contact with 

 the soil is increased. It is not evident that attempts to reach approxi- 

 mate final equilibrium by large excess of water or long shaking are likely 

 to result in more accurate knowledge of the soil solution as it exists at 

 any given moment. On the contrary, the increase in solutes is derived 

 from substances not actually present in the soil solution, and their 

 solubility is in part conditioned on the concentration of the soil solution, 

 the variable under investigation. 



In concluding this discussion it may be well to summarize briefly our 

 present point of view with regard to the soil solution based on recent 

 researches in this and other laboratories. All the evidence supports the 

 general views expressed by Cameron (5) a number of years ago to the 

 effect that soil phenomena must be considered as dynamic. His criti- 

 cisms of the older methods of study by means of hydrochloric-acid 

 extracts of soils, analyses of total quantities present in the soil, etc., are 

 found to be entirely justified. It is now generally recognized, however, 

 that Cameron's conclusions with regard to the nature of the soil solution 

 were not sufficiently far-reaching. It is certain that the soil solution is 

 not simply a solution saturated with respect to all the original mineral 

 components of the soil and tending to approach a constant composition. 

 The original soil minerals themselves doubtless have a very slight solu- 

 bility in pure water, but the soil solution of a normally occurring soil is 

 something quite different. The solvent is never pure water, but rather 

 a solution of salts and organic matter, accompanied by carbon dioxid, 

 oxygen, and other gases. The soil solution at any given moment is the 

 resultant of the cumulative effect of the continuously varying solvent on 

 the soil minerals. The actual concentration of the solution is governed 

 principally by the equilibria existing between the dissolved substances 

 and the immediately soluble or absorbed substances. It is possible that 

 these latter may be removed almost completely from the soil mass by an 

 excess of water. The soil solution in contact with the residual soil has a 

 very low concentration, and this is not readily increased by the solvent 

 action of pure water. To a lesser degree a similar state of affairs results 

 when the dissolved or immediately soluble components of the soil are 

 removed by a crop. This effect may be of long duration, or, on the other 

 hand, the concentration of the soil extract with respect to many solutes 



