46 CLAY LIMESTONE 



alkaline solutions tend to favour the diffusion of the colloidal clay and 

 will even destroy the flocculation produced by acids or salts. 1 



It is this action of lime or salts upon colloidal clay which causes the 

 improvement in the texture of clay soils which is brought about by 

 liming, also the quick settling of the clay particles when a muddy river 

 flows into the sea, with the consequent production of deltas and bars. 

 The waters of rivers remain muddy for a long time if deficient in lime 

 compounds, while rivers containing very hard water soon clarify. The 

 persistent muddiness of the waters of some rivers, even in the lower 

 reaches, where the flow is very slow, is probably due to the presence 

 of alkaline carbonates. Frost also produces, to some extent, the 

 shrinkage and coagulation of colloidal clay. 



Clay possesses strong retentive powers both for water and for cer- 

 tain soluble substances (vide Chap. IV). 



It is to be noted that the characteristic properties of clay, especially 

 its plasticity, are dependent upon its being in the hydrated condition. 

 If the two molecules of water of hydration which it contains, be expelled 

 by exposure to a high temperature, the clay is greatly altered in pro- 

 perties and does not, afterwards, take up water again. Such a per- 

 manent change is familiar to everyone in the production of bricks, tiles 

 and earthenware of all kinds. Advantage was formerly taken of this 

 fact in the improvement of very heavy clay soils. Large quantities 

 of the clay were dried by exposure to air and sun, then mixed with 

 fuel brushwood or faggots piled into heaps and the fuel ignited. 

 The clay was, by this treatment, rendered anhydrous and any ferrous 

 oxide present was converted into red ferric oxide. The resulting mass 

 spread over the land and ploughed in, was found to exert a good in- 

 fluence on the physical properties of the soil. To be of much effect, 

 however, very large dressings of burnt clay were necessary 50 to 100 

 tons per acre were sometimes used. The practice, because of the 

 large amount of labour and expense involved, is now rarely adopted. 



Limestone. This term in connection with the constituents of a 

 soil, must be taken to mean the finely divided particles of calcium car- 

 bonate, which are present, acting, perhaps, in some cases, partly as a 

 cementing material to the quartz grains. As already stated, it furnishes 

 plane food by virtue of the calcium, magnesium and phosphoric acid 

 which are always present in it. Its action in the soil, however, is 

 more important than as a mere source of plant food. It acts upon 

 the colloidal clay in a manner already described as characteristic of 

 lime and other salts and thus modifies the physical texture of the 

 soil. Perhaps its most important function, however, is to act as a 

 weak base, with which acid products, formed by decomposition of the 

 organic matter in the soil, can readily unite and by which their 

 harmful acidity is destroyed. If such basic material be absent, the 

 soil becomes " sour," as it is called, and unfitted for the growth of most 

 crops. This sourness is generally due to the production of free or- 

 ganic acids of the humic acid type and is possessed by many peaty soils 



1 For a study of the comparative flocculating effects of various salts, acids, and 

 alkalies upon colloidal clay, see Hall and Morrison, Jour. Agric. Sci., 1907, 2, 244. 



