SOILS AND MANUEES 



other substances are occasionally present. The chlorine 

 and fluorine are always combined with other elements ; 

 the remainder occur in the form of oxides. Some of these 

 oxides are further combined together in the form of salts, 

 but, as it is difficult to determine exactly how the various 

 combinations are arranged, they are usually stated separ- 

 ately as simple oxides. The proportions of the various 

 ingredients vary like that of the total ash. The averages 

 in certain typical cases are shown in the following table : 



AVERAGE COMPOSITION OF THE ASH OF PLANTS. 



It will be seen that all the products are rich in potash 

 and that the grains are rich also in phosphoric acid. The 

 turnip leaves, bean straw and clover contain more lime, 

 and the barley and meadow grass more silica than the 

 others. It has been said that sodium, silicon, chlorine and 

 fluorine are commonly present in the ash, but they do not 

 appear to be essential for the growth of the plant. They 

 may, however, be useful in other ways. For example, the 

 outer ^husk of cereals, which forms a protective covering 

 to the grain, is largely composed of silica. Chlorine, on 

 the other hand,, is not only of no use whatever, but, in large 



