THE CONSTITUTION OF THE SOIL 61 



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 as elucidated 

 by Schulze and by 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 for 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 

 chalk (which become converted into calcium bicarbonate in the soil) 

 and by organic matter ; on the other hand, they are intensified by the 

 deflocculation resulting from the use of alkaline manures like liquid 

 manure, or by sodium nitrate, which leaves a residue of sodium car- 

 bonate in the soil. Further, as pointed out above, clay " fixes " and 

 retains the ammonia and potash supplied as manure. In general 8 to 

 1 6 per cent, is a satisfactory proportion of clay in a soil where the rain- 

 fall is 20 to 30 inches per annum. 



Fine silt (o'Oi to 0-002 mm. in diameter) has also great water- 

 holding power, ancl in excessive amounts (above 10 to 15 per cent.) 

 it increases the difficulty of working the soil, especially if much clay 



