LAND AND OTHER AGENTS OF PRODUCTION 163 



(caustic soda, caustic potash, ammonia, and their carbonates) defloc- 

 culate clay, causing it to remain suspended in water for long periods. 



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. Excess of clay, however, interferes too much with the water 

 movements; it also impedes the movement 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 unfavorable to 

 vegetation and to make the soil shrink very much on drying, so that 

 large cracks appear in the fields in summertime. These harmful 

 effects are reduced by flocculation effected by dressings of lime or 

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

 by the deflocculation resulting from the use of alkaline manures. 

 Further, clay " fixes" and retains the ammonia and potash supplied 

 as manure. 



Fine silt has also great water-holding powers, and in excessive 

 amounts (above 10 to 15 per cent) it increases the difficulty of working 

 the soil, especially if much clay is present. It does not possess the 

 marked plastic and colloidal properties of clay and is less altered by 

 lime; indeed, no method is known for making it tractable. The 

 coarser grade of silt appears to be very valuable, and constitutes 

 30 to 40 per cent of many of the loams most famous in the southeast 

 of England for carrying their crops well and not drying out. Prob- 

 ably silt plays a very important part in maintaining the even condi- 

 tions of moisture so desirable for plant growth. It is fine enough to 

 retard, but not to prevent, percolation, and it facilitates capillary 

 movement of water. 



Fine sand forms a considerable fraction usually 10 to 30 per cent 

 or more of nearly all soils. Although its dimensions are relatively 

 large, it still possesses cohesiveness and a tendency to cake together; 

 it has not, however, so great an effect as silt in maintaining a good 

 moist condition. Soils containing 40 per cent or more of fine sand 

 tend to form, after rain, a hard crust on the surface, through which 

 young plants can make their way only with difficulty until it has been 



