176 THE SOIL: INORGANIC MATTER 



or diffusion may remove the soluble compounds. The re- 

 action, then, continues to go from left to right, and phos- 

 phorus is made soluble. 



But suppose the supply of carbon dioxide diminishes, 

 organic matter for some reason does not decompose very 

 rapidly or not at all; or suppose no plant removes the sol- 

 uble phosphate; or suppose that a solution of monocalcium 

 phosphate passing through the soil comes in contact with 

 considerable calcium bicarbonate, or, more than all this, 

 suppose fresh supplies of calcium bicarbonate are being 

 continually added to the solution from other places; under 

 these conditions, then, the reaction goes from right to left 

 and phosphorus is absorbed or fixed in the soil. Of course, 

 in all cases there must be enough water present to allow of 

 reactions in solution. 



Starting with a fixed amount of the reacting substances 

 the reaction will proceed in a direction dependent upon the 

 masses until an equilibrium is reached, or until the masses 

 on one side of the equation balance in reacting velocity those 

 on the other side. This, however, is something that rarely 

 happens in a soil, for the composition of a solution is con- 

 stantly changing. Reactions in a soil are always in a state 

 of change. They are essentially dynamic and not static. 



This reversible reaction and the resultant solubility or 

 absorption of plant food is applicable just as well in the case 

 of the reactions given for potassium in Sections 135, b, and 

 138, and for phosphorus and iron compounds in Sections 

 135, a, and 138. 



140. Physical Absorption. — Compounds of the plant- 

 food elements are retained by the soil to a greater or less 

 extent in an unchanged form. Certain solid substances in 

 the soil attract and hold on their surfaces compounds in 

 solution. This process is called adsorption and may be 

 defined as the existence in soil moisture (or other solutions) 

 of two different concentrations of dissolved substances, the 

 greater lying immediately adjacent to the surface of a solid. 

 For example, iron and aluminium hydrated oxides have a 

 very decided affinity for potassium sulphate. Adsorption 

 is a purely physical phenomenon in that the compounds 



