Introduction. 11 



some species of plants obtain their necessary nitrogen from the 

 air. It will be seen in the later chapters that, although plants 

 are surrounded by air, rich in free nitrogen, combined nitrogen 

 is one of the essential and most valuable constituents of manures. 

 A large part of the nitrogen in the food consumed by man and 

 animals is eliminated as simple compounds in the excreta and un- 

 fortunately, especially in our cities, sent down the sewers and 

 rivers and finally discharged into the sea. To agriculture this 

 valuable combined nitrogen is therefore wasted. This element is 

 the most expensive of those necessary for plant growth and is 

 among those liable to be most deficient in our soils. No other ele- 

 ment takes such an important part in agriculture or in life pro- 

 cesses. 



Sulphur (S) is found both free and combined in nature. The 

 free element is found in volcanic districts, while in the combined 

 state it occurs as hydrogen sulphide in mineral waters and as sul- 

 phides of many metals, as for example iron, lead, and zinc. The 

 sulphide of iron, known as iron pyrites, is often mistaken for gold 

 because of its yellow color ; sulphur also occurs as sulphate of cal- 

 cium, in which form it is very widely distributed in soils, and is 

 the main source of the sulphur for crops. 



The element sulphur (brimstone) is a yellow, brittle substance 

 and very inflammable. It burns in air with a pale blue flaflBi 

 forming the suffocating gas, sulphur dioxide. Such forms of sul- 

 phur are very poisonous to plants and animals, while sulphates 

 are not only harmless, but necessary. Sulphur is presenlj in the 

 proteins of both plants and animals and when putrefaction of 

 these substances occurs is often liberated as hydrogen sulphide. 

 This substance is perceptible by its disagreeable odor as one of 

 the chief products of the decay of eggs. 



There is generally less than 0.10 per cent of sulphur trioxide. 

 as sulphates in ordinary soil, and it is now known that the amount 

 required by crops is considerable ; for this reason it may be neces- 

 sary to use certain sulphates occasionally as fertilizers and as 

 sources of sulphur for the growing crops. 



