SYMPOSIUM ON COLLOIDS 63 



of each land plant and, so far as many processes are concerned, 

 the more interesting, if the less well understood, half. Floccu- 

 lation, a process by which clay soils are rendered more penetra- 

 ble and given a better oxygen supply, is a typical reaction be- 

 tween suspensoids and suspensions and electrolytes. It is well 

 illustrated in delta formations at the mouths of rivers where 

 the silt and clay constituents of the fresh water are brought 

 into contact with the salts of the sea water and flocculated. The 

 effect here is mainly due to the cations of the salt acting on the 

 negatively charged soil particles. Deflocculation, brought 

 about by the addition of excess of flocculating salts, is a well 

 known reaction of suspensions and suspensoids included un- 

 der the term peptisation (1 : p. 293-295). 



The dissolution and removal from the soil of lime carbon- 

 ate, its natural sweetener or neutralizer, and the final accumu- 

 lation of acid in it, is in part at least due to flocculation. The 

 positive ions of the inorganic salts are absorbed by the organic 

 and inorganic negative soil particles and the latter are floccu- 

 lated, leaving free in the soil the strong inorganic acids corre- 

 sponding to the negative ions of the salt. The soil particles thus 

 flocculated may be of microscopical size, or suspensions, or of 

 ultramicroscopical size — true colloids ; but the principle of the 

 reaction is the same in either case. Flocculation, in part at 

 least, explains the mysterious humic acids of the soils that have 

 been so much discussed without certain identification and so far 

 as it does account for the acidity shows that the acids are 

 largely not organic but inorganic. 



Daikuhara (29) has lately brought evidence to show that 

 the injurious effects of adding fertilizer salts to many soils of 

 Japan and Korea, poor in lime, are due to the fertilizer salts 

 freeing complex acid salts of aluminium and iron from ad- 

 sorbed condition on the colloids of the soil. If lime accompany 

 the fertilizers very beneficial effects results from them. He im- 

 plies that the same may be true in America where acid soils 

 are so prevalent. 



Litmus paper as a qualitative test for soil acidity has its 

 short-comings in the differential adsorption of the two ions 

 of the organic salts of the litmus by the suspensions and sus- 

 pensoids of the soil on one hand, and by the colloids of the 

 paper on the other — typical adsorption processes of suspen- 

 soids. The common quantitative tests for soil acidity are ren- 

 dered unsatisfactory on similar grounds (30). 



Of course, no one will claim that the total loss of lime car- 

 bonate from the soil (about 800 lbs. per acre per year) along 





