i68 



SOIL CONDITIONS AND PLANT GRO WTH 



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. H2SO4 ; II. Arable soil (Hoos field, dunged) over cone. H2SO4 ; III. 

 Arable soil (Hoos field, dunged) over 55-4 per cent. H2SO4 . 



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 



