ORIGIN AND COMPOSITION OF SOILS 21 



Sulphur plays an important part in the nour- 

 ishment of plants, being found in them both as 

 sulphuric acid and in organic compounds; it 

 is an essential constituent of both animal and 

 vegetable protein and is thus intimately associated 

 with one of the most important classes of foods. 



Hydrogen is a colorless, invisible gas, without 

 taste or smell. It occurs free in small propor- 

 tions in certain volcanic gases, and in natural 

 gas, but its most common form is in combination 

 with oxygen as water (H^O), of which it forms 

 11.19 per cent by weight. It also occurs in com- 

 bination with carbon to form the hydrocarbons, 

 such as the mineral oils (petroleum, etc.), and 

 gases. Hydrogen is of no importance to agri- 

 culture in a free state, but water is the most 

 important and necessary of all plant foods. 



Chlorine occurs free in nature only in limited 

 amounts in volcanic vents. Its most common 

 form is in combination with hydrogen, forming 

 hydrochloric acid, or with the metals to form 

 chlorides. It combines with sodium to form 

 sodium chloride or common salt (NaCl), which 

 is the most abundant mineral ingredient in sea 

 \vater and which can usually be detected in rain 

 and ordinary terrestrial waters. In this form, 

 also, it exists as extensive beds of rock salt, which 

 is mined for commercial purposes. 



Chlorine is found uniformly in plants and may 

 be regarded as a common constituent thereof. 

 Common salt applied to a soil modifies its power 



