THE CONSTITUTION OF THE SOIL 59 



have a profound effect on these properties ; small quantities of acids or 

 salts cause the temporary loss of plasticity, impermeability, and the pro- 

 perty of remaining long suspended in water without settling ; the clay is 

 now said to be flocculated. The change can be watched if a small quan- 

 tity of any flocculating substance is added to the turbid liquid obtained 

 by shaking clay with water ; the minute particles are then seen to unite 

 to larger aggregates which settle, leaving the liquid clear. There is, 

 however, no permanent change ; deflocculation takes place and the 

 original properties return as soon as the flocculating 1 agent is washed 

 away. Alkalis (caustic soda, caustic potash, ammonia and their car- 

 bonates) deflocculate clay, causing it to remain suspended in water for 

 long periods. Clay is thus an electro-negative colloid, its reaction prob- 

 ably being conditioned by a trace of potash liberated by hydrolysis. 

 It shows the general properties of electro-negative colloids las elucidated 

 by Schulze (254) and Hardy (125) : thus it is flocculated only by a solu- 

 tion containing ions or particles of opposite electrical sign, and the 

 extent of flocculation increases rapidly with the valency and concentra- 

 tion of the ion. No quantitative relationships, however, could be found 

 by Hall and Morison (120). 



A remarkable change sets in when clay is heated beyond a certain 

 point, and it permanently loses all its special properties. 



These clay properties are of great importance to the fertility of the 

 soil, and no constituent is more necessary in proper proportions, or 

 more harmful in excess. Clay impedes the movement of water in the 

 soil and keeps it in the surface layers within reach of the plant roots, 

 thus making the soil retentive of water. Excess of clay, however, inter- 

 feres too much with the water movements, making the soil water- 

 logged in wet weather and parched in dry seasons even though the 

 permanent water level is near the surface ; it also impedes the move- 

 ment of air to the roots and lowers the temperature of the soil. The 

 adhesive properties of clay cause the soil particles to bind together 

 into those aggregates on which " tilth " depends ; soil without clay 

 would be very like a sand heap. Here also, however, excess of clay 

 does harm and makes the soil so adhesive that it' sticks to the tillage 

 implements and retards their movements ; it also tends to form large 

 clods unfavourable to vegetation. These effects are intensified in wet 

 weather ; the soil becomes sticky or " poached " and must not be 

 worked or the tilth is injured fpr a long time. Another effect of a 

 large amount of clay is to make the soil shrink very much on drying, 

 so that large cracks appear in the fields in summer time. These harm- 

 ful effects are reduced by flocculation effected by dressings of lime or 



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