62 THE REACTIONS OCCURRING IN SOILS. IV. 



The humus in a soil also possesses in a high degree the 

 physical retentive power of all porous, bulky substances, and 

 in addition it acts as an acid and forms insoluble humates with 

 lime, magnesia, &c. It possesses great absorbent powers for 

 ammonia. Whenever a soluble salt, of a strong acid particu- 

 larly, is applied to a soil, interchange of base occurs to some 

 extent ; thus even sodium nitrate or chloride will cause the 

 formation of potassium or calcium nitrate or chloride by inter- 

 action with the silicates of those metals present in the soil. 



Phosphoric acid is mainly retained by the uppermost layers 

 of the soil, especially if it be applied in the form of super- 

 phosphate ; with dung some of the phosphoric acid is carried 

 into the second or even the third 9 inches. With potash, 

 although the uppermost 9 inches contains the largest quantity 

 of the unused fertiliser, a considerable amount penetrates to 

 and is retained by the second and third 9 inches.* 



The distribution of dissolved matters in a soil is regulated 

 partly by diffusion, i.e., motion of the dissolved substances 

 without that of the water as a whole and partly by motion of 

 the liquid itself. 



I. Diffusion is the phenomenon by which a dissolved sub- 

 stance tends to pass from a greater to a less concen- 

 trated portion of the solvent. It is shown in different 

 degrees by different substances. Colloidal bodies have 

 the slowest rate of diffusion. The diffusibility of a salt 

 depends partly on its acid and partly on its metal. 

 The common acids and metals stand in the following 

 order, starting with the most diffusible : 



Acids. Metals. 



Chloride Potassium 



Nitrate Ammonium 



Sulphate Sodium 



Carbonate Calcium 



Magnesium. 



II. The liquid in the soil moves from various reasons : 

 (a) The water, as a rule, exists as a liquid film upon the fine 

 particles, and its position is partly determined by its 

 surface tension or the force which tends to cause it 



Dyer, Proc. Royal Soc. , 1901, 11. 



