Ill SOILS 21 



concerned in agriculture. Some of the elements are termed Organic 

 organic, because they were supposed, formerly, always to be ^^^'"^"^''• 

 found in organised beings — that is, in plants and animals ; 

 and the rest are called inorganic, as they are not necessarily inorganic 

 present in living things. But organic elements occur in ^ ^"^*^"^^- 

 enormous quantities in inorganic substances, and inorganic 

 elements are found in organised bodies, so that the terms 

 organic and inorganic elements are now used simply as 

 convenient expressions. 



The organic elements are oxygen, hydrogen, nitrogen and Elements 

 carbon, which disappear as vapour or smoke when an orga- '" ^^^ ^°'^- 

 nised body is burnt in the air. The first three are gases, 

 and the last is a substance, one form of which, charcoal, is 

 familiar to every one. Besides these, there are two other 

 substances, namely, stilphur and phospJioriis, which are 

 sometimes called secondary organic elements, because they 

 are frequently found in organised bodies. In all, then, there Eleven 

 are six non-metals, as they are termed, ever present in fertile demSu? 

 soils, and, in addition to them, there are five metals which 

 are always to be found in combination with other elements, 

 in lands suitable for cultivation. These are potassium, 

 sodium, 7tiagnesium, calcium and iron. 



Oxygen is an invisible gas which enters largely into the Compounds 

 composition of the atmosphere ; and in combination with °^ °^yg^"- 

 another gas, hydrogen, it forms water. It combines readily 

 with other elements to form different compounds called 

 oxides. If iron be exposed to air and moisture its surface 

 rusts, and this rust is simply a combination of iron and the 

 gas oxygen ; it is in fact an oxide of iron. Oxygen also Oxides, 

 combines with carbon to form carbon dioxide, usually called 

 carbonic acid, a compound gas that plays a very important 

 part in plant life. In combination with the element silicon, 

 oxygen forms the oxide silica, which is the basis of all sands ; 

 with potassium it iorvsis potash ; with sodium it forms soda; 

 with magnesium it forms magnesia; with calcium it forms 

 lime, and with aluminium it forms alumina, which is the 

 basis of all clays. 



