1 68 SOIL CONDITIONS AND PLANT GROWTH 



plants or micro-organisms some kind of equilibrium may be 

 attained under definite conditions : this equilibrium is the 

 " wilting-point " of the physiologist. On this view the other 

 constants and critical points that have been indicated by 

 various investigators are all equilibrium points and do not re- 

 present breaks in the condition of water in the soil. Further 

 discussion will be found on p. 218. 



Per cent, of water. 



FIG. 21. Curves showing rate 01 evaporation of water from : I. Ignited garden soil 

 over cone. H 2 SO 4 ; II. Arable soil (Hoos field, dunged) over cone. H 2 SO 4 ; III. 

 Arable soil (Hoos field, dunged) over 55-4 per cent. H 2 SO 4 . 



The Influence of Colloids on the Reactions Taking Place 



in the Soil. 



It is obvious that a reaction taking place in a colloidal 

 medium possessing properties like selective adsorption must 



