COAGULATION OF CLAY 33 



meability of the soil to water is much diminished. If a 

 treatment with acid has preceded the washing with distilled 

 water, the results just mentioned will be especially apparent. 

 One reason of the accumulation of clay in a subsoil is doubtless 

 the washing out of the clay in the surface soil by rain water. 

 The amelioration of clay soils by dressings of lime or chalk 

 is a well-known practice ; the effect of such dressings is most 

 marked, the clay losing much of its stickiness, becoming more 

 friable, more pervious to water, and being more easily cul- 

 tivated. A farmer once told the writer that dressing his 

 heavy land with chalk had enabled him to plough with two 

 horses instead of with three as formerly. 



Sachsse and Becker filled several wide glass tubes with 

 powdered kaolin and with a powdered heavy loam ; some of 

 the tubes were without lime, and some had 2 per cent, of 

 quicklime intimately mixed with the soil. Water was then 

 poured on the surface. Where no lime had been added 

 percolation did not occur, though a considerable head of water 

 lay on the surface. With lime, percolation proceeded regularly 

 in both cases. The kaolin tube without lime burst from the 

 excessive swelling of the clay. 



It may be laid down as a general rule that clay soils are 

 friable, and permeable to water, only when the colloid clay is 

 in the coagulated condition. 



The action of the various salts used in agriculture upon 

 a clay soil demands much further study. There appears some 

 evidence that salts which determine the coagulation of clay 

 when added to clay diffused in water may nevertheless be 

 capable of disintegrating the particles of a clay soil, and 

 liberating the clay, when used as a strong solution, or allowed 

 to crystallize in the soil. On some heavy soils a top-dressing 



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