THE SOIL SOLUTION 277 



gards both water and nutrients, and losses by leaching, must 

 always be considered. In addition, the effect of tillage as 

 well as the common practices of adding farm manure, plow- 

 ing under of green-crops and applying fertilizers and lime, 

 are constantly effective in obstructing equilibrium adjust- 

 ments. 1 (See Fig. 50.) 



The soil solution is, therefore, markedly dynamic in char- 

 acter, constantly changing in composition and concentra- 

 tion. Its important control is absorption, the absorptive sur- 

 faces acting as a depository, in which active reserve nutrients 

 are held. As the solution is depleted in any constituent, 

 quicker adjustment takes place between the solvent and the 

 colloidal complexes than is possible between the solution and 

 the solution surfaces. Rapid adjustments, as far as the sup- 

 ply of nutrients for plants is concerned, is possible only be- 

 cause of the absorptive properties of the colloidal complexes 

 of the soil. 



145. Methods of studying the soil solution. — Questions 

 regarding the soil solution are difficult to answer because no 

 adequate procedure has been devised for extracting a repre- 

 sentative sample of the solution as it existed in the soil. More- 

 over, no wholly satisfactory method has been perfected for 

 its measurement in place. Various extractive methods have 

 been tried. Briggs and McLane 2 attempted to sample the 

 solution by the use of a centrifuge developing a force of two 

 or three thousand times that of gravitation. When the soil 

 contained a rather large quantity of capillary water, a small 

 amount of it could be removed in this way. 



*Bouyoucos has shown that even under controlled conditions the 

 equilibrium between finely ground minerals and water is not absolute 

 or real due to the complex hydration and hydrolysis which continually 

 occur. Bouyoucos, G. J., Bate and Extent of Solubility of Minerals 

 and Bocks under Different Treatments and Conditions ; Mich. Agr. Exp. 

 Sta., Tech. Bui. 50, July, 921. 



2 Briggs, Lyman J., and McLane, John W., The Moisture Equivalent of 

 Soils; U. S. Dept. Agr., Bur. Soils, Bui. 45, pp. 6-8, 1907. 



