CH. ii] The Two States of Clay 21 



iron, aluminium, calcium, magnesium, sodium, potas- 

 sium, etc., forming substances known as silicates. Some 

 of these, like sand, are very resistant to the action of 

 water and weather so that they remain as relatively 

 coarse particles and behave agriculturally like sand. 

 Others, however, are more easily acted upon with two 

 important results. Instead of being inert, like sand, 

 they are reactive, i.e., they will act upon various sub- 

 stances that may be brought in contact with them, such 

 as superphosphate, siilphate of potash, etc. The action 

 of water and weather not only rounds off any edges 

 they may have possessed, but reduces them so much in 

 size that they become extraordinarily small and form 

 the particles which tail off from 0-002 mm. (jjioo ^^O 

 in diameter to much smaller dimensions. Substances of 

 this nature are the essential constituents of clay, indeed 

 the agricultural chemist regards them as the true clay, 

 any larger inert particles being simply admixtures. 



Clay is remarkable in that it can exist in two states ; 

 one being sticky and the other crumbly or flocculated. 

 A number of other substances are known that do the 

 same and they are included in the class known as colloids 

 a word that means "like glue." Clay is such a domina- 

 ting substance that it impresses its properties on the soil 

 to a considerable extent, hence when the clay is in the 

 sticky state the whole soil becomes sticky, and con- 

 versely, when the clay is in the crumbly state the whole 

 soil becomes crumbly. 



Practical men have long since learned that the 

 crumbly state is good for plants while the sticky state 

 is not, and they have also discovered how to change one 

 into the other. Addition of lime, chalk or limestone 

 causes the change to take place rapidly : organic matter 



