46 Agricultural Chemistry. 



tity of aluminum silicate containing more water than the rest, 

 to which the plasticity and tenacity of clay is due. If this con- 

 stituent is fully swollen with water the clay is impervious and 

 sticky, while if it is shrunken or coagulated the clay becomes 

 more friable and less plastic. Calcium compounds are partic- 

 ularly effective in inducing such coagulation and it is to this 

 cause that the improvement in texture of heavy clays by lime 

 applications is due. 



Lime stone. Calcium carbonate is present in the soil in a fine- 

 ly divided state distributed ^among the other constituents, but in 

 addition there may be larger fragments which are classed with 

 the "sand." The finely divided material is the one of import- 

 ance. It furnishes plant food, for the plant must have calcium, 

 but it also plays other important functions. It modifies the 

 plasticity of clay in the manner described above and in addition 

 neutralizes any acids accumulating in the soil. Acids are pro- 

 duced by the decay and fermentation of vegetable matter, and 

 if allowed to accumulate, will render the soil unfit for max- 

 imum crop production. Such soils are spoken of as "sour" and 

 can best be restored to fertility by the application of quick lime 

 or ground limestone. Limestone performs another important 

 function by acting as a basic material necessary for the process 

 known as nitrification, to be explained later. 



Limestone also serves an important function in those soils 

 which have received applications of the commercial fertilizer, 

 ammonium sulphate. It prevents the accumulation of sulphuric 

 acid, which otherwise would make the soil sour, by its power to 

 neutralize this acid. The neutral salt formed calcium sulphate 

 partly runs off in the drainage water. 



Humus is the brown or black organic matter of surface soil. 

 It is the product formed by partial decay of organic matter and 

 is the material that gives the rich black appearance to some soils. 

 It is formed from the residue of plants previously grown on the 

 soil or from added organic matter in farm manures or commercial 

 fertilizers. It is a mixture of many ill-defined bodies. Besides 



