EXPERIMENT STATION BULLETINS. 499 



quires moisture for its existence, the soil water performs a double func- 

 tion,— it furnishes the moisture to the plant, and holds in solution the 

 mineral and the organic compounds. The solutes that are present assist 

 by dissolving some of those constituents of the soil otherwise insoluble. 

 The character or type of soil, e. g., the size and composition of soil 

 particles, Avill determine to a large extent, what the soil solution con- 

 tains. Treatment of the soil as to cultivation, drainage, fertilizer and 

 cropping, have a marked influence upon the amount and kind of solutes 

 present. 



Gedroitz^ says that the greatest part of the soil compounds have a very 

 low degree of solubility. The solubility of the soil compounds is not a 

 constant quantity, but depends upon temperature and pressure and as 

 stated previously upon the solutes present. Practically- every soil con- 

 tains all the common rock-forming minerals, all of which are more or 

 less soluble in water, the amount in solution depending upon such factors 

 as the temperature, pressure, etc. Not only these mineral compounds, 

 but the organic compounds formed from decaying plant and animal 

 tissue are found in solution. 



The soil solution furnishes material from its immediate surroundings 

 to the plant. The upward movement of the water brings material from 

 below some of which the plant absorbs through its roots. The plant 

 will, on dying, deposit this material on the surface. 



HOW ADSORPTION AFFECTS THE SOIL SOLUTION. 



A very important factor was brought out by Gola' in regard to the 

 colloidal character of the soil particles and its effect upon the soil solu- 

 tion. On account of the complexity and colloidal nature of some of the 

 soil particles, it is claimed that the colloids adsorb some of the salts of 

 the solution. The amount adsorbed depends upon the moisture con- 

 tent. Presence of a large amount of water sets up a dialysis which 

 causes the hydrogels to give up the salts adsorbed. Reverse action takes 

 place at first with a diminution of the moisture content, but finally the 

 imbibition water of the hydrogels evaporates and the gel is precipitated 

 as an amorphorous mass, the bonds uniting the salts with the colloidal 

 molecule are broken, and the water set free is able to dissolve more salts 

 than it can before the loss of the adsorbent properties of the soil. 



REAL IMPORTANCE OF THE SOIL SOLUTION. 



The study of the soil solution is of a great importance from various 

 points of view. The very complexity of the soil and the constant change 

 which the soil is undergoing introduce a countless play of factors. This 

 complexity inherent in the subject should not discourage investigation. 

 It must be admitted that the soil solutions from difl:erent fields or from 

 different parts of the same field will not be exactly the same nor even 

 will the soil solutions present at any one spot remain constant. But the 

 same could be urged against chemical analyses of the soil which likewise 

 vary, yet which on the whole give a general idea of the field conditions. 



The soil solution, at the outset, may be considered in its relation to 



'Gedroitz : "Solubility of Soil Compounds." Russian Journal of Expt. Landw. 7, 190G, 

 D 529 



' ^Canieron, F. K. : Soil Solution, 1911, p. 32 ; Cameron, F. K. and Bell, J. M. The Min- 

 eral Constituents of the Soil Solution, Bui. 30, Bureau of Soils, U. S. D. A., p. 9. 



"F. Cavers' Review of Gola's Osmotic Theory of Edaphism. Jour, of Ecology, Vol. 2, No. 

 4, 1914. pp. 209-231. 



