124 NATURE TEACHING 



organisms are those which most conduce to its fertility. 

 Practical agriculturists long ago discovered these facts, 

 and scientific workers have now supplied the explana- 

 tion. 



As plants require their nitrogenous food to be in 

 the condition of nitrates it will readily be understood 

 that nitrate of soda is a valuable source of plant food, 

 for it can be used at once without any change. Other 

 substances containing nitrogen are slower in action in 

 proportion to the time they require for the necessary 

 changes to take place. Such bodies are converted into 

 nitrates, or, as we say, nitrified, at very different rates. 

 Sulphate of ammonia is very quickly changed, whilst 

 horn and leather are very slowly altered and are of less 

 immediate use as plant food on account of the slowness 

 with which they are nitrified. Most vegetable sub- 

 stances change with moderate facility, hence stable- 

 manure, decaying grass, weeds, and leaves are valuable 

 sources of nitrogen. Certain animal substances are also 

 useful, such as blood and refuse from slaughter-houses, 

 the refuse from fish-curing establishments, as well as 

 fish themselves, when caught in greater abundance than 

 required for food. 



One fact demands notice. Many substances, when 

 mixed with soil, are so firmly held by it that they are 

 not readily washed out by rain and carried away in the 

 drainage water. This is the case with phosphates, 

 potash and ammonium salts. With nitrates, however, 

 it is different ; over these, soil possesses little holding 

 power, and they are easily washed out and lost. As 

 all nitrogenous matters eventually pass into the form of 



