1 62 SOIL CONDITIONS AND PLANT GROWTH 



nearly expressible by an absorption curve.^ Pickering (226(2) 

 has demonstrated an absorption in the case of clay. 



Comber (69) has observed an interesting difference in the 

 flocculation of silt and of clay. Silt behaves like other sus- 

 pensions of particles insoluble in water ; it is flocculated by 

 small quantities of electrolytes, and the flocculation proceeds 

 most rapidly at the iso-electric point. Flocculation is hindered 

 by the presence of hydroxyl ions which increase the negative 

 charge, and therefore calcium nitrate is a more potent floccu- 

 lator than lime. This is the normal behaviour of such sus- 

 pensions under the conditions of the experiment. 



Clay, on the other hand, behaves differently ; flocculation 

 by small quantities of electrolytes is most rapid in slightly 

 alkaline solutions, away from the iso-electric point. Lime is 

 a better flocculator than calcium nitrate. In order to account 

 for the difference Comber assumes that the clay particles are 

 coated or protected with a layer of emulsoid silica which 

 impresses emulsoid characters on the whole system. On 

 this view the OH ion "peptises" the protecting emulsoid and 

 the Ca ion then flocculates the clay. Support for this view 

 was found in the fact that addition of colloidal silica to a 

 ferric oxide suspension made the system behave like clay, 

 flocculated more easily in alkaline than in neutral solutions. 



These observations throw important light on the well- 

 known difference between clay and silt^ (p. 107). 



Oden discusses the protective effect of humic acid on the 

 coagulation of clay.^ 



Pan Formation. 



A pan is a layer of hard impermeable rock that gradually 

 forms at the usual water level below the surface of the soil 

 under certain conditions. Its effect is to cut off the soil above 

 from the material below and therefore to modify profoundly 



1 Linder and Picton, Trans. Chetn. Soc, 1895, 67, 63. For further work see 

 H. Freundlich, Zeit.f. phys. chetnie, 1910, 73, 385-423. 



2 See also Otto M. Smith, J. Amer. Chem. Soc, 1920, 42, 460-472. 



3 Sven Od6n, y. Landw., 1919, 67, 177-208. 



