162 AGRICULTURAL ECONOMICS 



2. The calcium carbonate and phosphate (the latter being usually 

 in much smaller amounts), and organic matter derived from marine 

 or other organisms deposited simultaneously with the soil. 



3. The soil water, a dilute solution of carbonic acid containing 

 small quantities of any soluble soil constituent. 



4. The residues of plants that have grown since the soil occupied 

 its present position, consisting of the mineral plant food taken up 

 from the soil water and of part of the complex organic matter. As 

 the source of energy this may be regarded as the distinguishing char- 

 acteristic of soils. 



By the method of mechanical analysis the particles of soil can be 

 sorted out into fractions, each falling within certain specified limits 

 of diameter; those adopted in Great Britain are as follows: 



Fine gravel Above i mm. in diameter 



Coarse sand i to o. 2 mm. 



Fine sand o. 2 to o. 04 mm. 



Silt 0.04 to o.oi mm. " 



Fine silt o.oi to 0.002 mm. " 



Clay Below 0.002 mm. " 



So far as is known, all coarser particles are chemically inert. The 

 clay fraction, on the other hand, stands out in sharp contrast, both in 

 composition and in chemical and physical properties. 



The clay fraction. Clay may be regarded as a plastic colloid, but 

 its special properties are seen only when a certain amount of water 

 is present. If it is well rubbed with water it becomes very sticky and 

 absolutely impervious to air or water; it is also highly plastic. It 

 shrinks very much on drying and absorbs heat; on moistening again, 

 however, there is a considerable swelling and evolution of heat. The 

 separate particles of clay are so small that, when placed in water, they 

 sink only very slowly, in spite of their high specific gravity. Small 

 quantities of acids or salts cause the temporary loss of plasticity, 

 impermeability, and the property of remaining long suspended in 

 water without settling; the clay is now said- to be flocculated. The 

 change can be watched if a small quantity of any flocculating sub- 

 stance is added to the turbid liquid obtained by shaking clay with 

 . the minute particles are then seen to unite to larger aggrn^u es, 

 which settle, leaving the liquid clear. There is, however, no per- 

 manent change; deflocculation takes place and the original properties 

 return as soon as the flocculating agent is washed away. Alkalies 



