NITROGENOUS MATTER. 119 



the soil. It will be observed that these con- 

 ditions are those which have been repeatedly 

 referred to as the objects aimed at in good 

 tillage and cultivation ; that is to say, the pre- 

 sence of moisture, air, and lime, together with 

 vegetable matter which contains the nitrogen 

 to be acted upon. Speaking generally, there- 

 fore, we find that those operations and condi- 

 tions which render the soil best suited for the 

 life and growth of the nitrifying organisms 

 are those which most conduce to its fertility. 

 Practical agriculturists long ago discovered 

 these facts, and scientific workers have now 

 supplied the explanation. 



3. As plants require their 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. 



