THE COLLOIDAL PROPERTIES OF SOIL 163 



the movements of water and of air, leading often to swamp 

 conditions. The effect on vegetation becomes so marked that 

 in agricultural practice the pan has usually to be removed, 

 often at considerable trouble and expense. 



The conditions determining the formation of pan seem to 

 be a supply of organic matter, permeability of soil, low content 

 of soluble mineral matter, and absence of calcium carbonate. 

 These conditions occur most frequently on light sandy soils 

 where for some reason the water is held sufficiently near the 

 surface. 



Pans are best seen when the sand is overlain by a deposit 

 of peat. The sand is then bleached to a depth of 5 to 60 cms. 

 Suddenly there comes a change : a coloured layer of solid 

 rock occurs which may vary in colour from yellow to black 

 and in thickness from 10 to 60 cms. : on closer inspection this 

 is seen to consist of particles of sand cemented firmly together. 

 This is the pan : ^ underneath it lies the sand proper. But 

 pans are by no means confined to peat : they often occur in 

 forests, on heaths and on certain cultivated soils. 



Chemical analysis shows that the pan is much richer in 

 organic matter, iron, alumina, and especially material dis- 

 placeable by ammonium nitrate solution than either the sand 

 above or the sand below. Table XLIV, gives typical results 

 showing the amounts of material soluble in HCl. 



The process of pan formation, therefore, involves a con- 

 centration of these substances in a certain layer of the soil. 



On the older view humic acid was supposed to be formed 

 from the organic matter present, and to dissolve iron, alumina, 

 and other substances, forming soluble humates which were 

 washed down into the soil and precipitated at the point where 

 pan formation occurred. 



Various hypothesis were advanced to account for these 

 changes. 



» 



1 In German the pan is called " Ortstein " and the bleached sand *' Bleisand," 

 '* Bleichsand," or occasionally " Grausand ". When the formation, instead of 

 being sand, is clay, the white soils are called " Molkenboden ". Some of these 

 are described in Internat. Mitt. Bodenkunde, 1914, 4, 105-137. See also (24). 



II* 



