54 MOVEMENTS OF DISSOLVED SUBSTANCES 



acidic properties possessed by ferric oxide and alumina, leading to the 

 formation of salt-like compounds analogous to spinel, MgO.Al 2 3 . 



It must be remembered that complete absorption never occurs, 

 but that a small proportion of the substance always remains in the 

 solution. 



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, etc. It 

 possesses great absorbent powers for ammonia. Whenever a soluble 

 salt, particularly of a strong acid, 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 

 interaction 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 superphosphate ; with 

 dung, some of the phosphoric acid is carried into the second or 

 even the third 9 in. With potash, although the uppermost 9 in. 

 contains the largest quantity of the unused fertiliser, a considerable 

 amount penetrates to, and is retained by, the second and third 9 

 in. 1 



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 it- 

 self. 



1. Diffusion is the phenomenon in which a dissolved substance 

 passes from a greater to a less concentrated portion of the sol- 

 vent. It is shown in different degrees by different substances, 

 Colloidal bodies have the slowest rate of diffusion. The diffusi- 

 bility 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 : 



Acid radicals. Metals. 



Chloride Potassium 



Nitrate Ammonium 



Sulphate Sodium 



Carbonate Calcium 



Magnesium 



Diffusion is, under any circumstances, a slow process and has been 

 shown by the experiments of Miintz and Gaudechon ' L to lead to very 

 little lateral movement of soluble manures in soils. A crystal of 

 sodium nitrate, placed on the surface of wet soil, dissolved, but dif- 

 fusion did not carry the dissolved salt laterally or vertically more than 

 1 in. in thirty days. Even after rain, the soluble salt was confined 

 to a steep-sided vertical cone of soil, below the crystal. 



In field trials of manures on grass land, it has been noticed that 

 repeated applications of manure, year after year, to one plot has practi- 



] Dyer, Proc. Hoy. Soc., 1901, 11. 



"Cornpt. Rend., 1909, 148, 253; Jour. Chem. Soc., 1909, Abstracts, ii. 259, 



