PLANT FOOD AND MANURES 129 



manuring has to be done on a large scale it is not always 

 easy to procure sufficient quantities of these substances, 

 and recourse must then be had to artificial manures. 



Guano is the excretion of sea-birds, deposited in 

 rainless, tropical regions. It contains all the essential 

 constituents of plant food; that is to say, nitrates, 

 phosphates, and potash in a condition in which they are 

 most readily assimilated by crops. The nitrogen 

 exists in various forms, part ready to be used at once 

 by the plant, part requiring to be changed before use. 

 Guano is thus both lasting and rapid in its effect. 

 Rich nitrogenous guano is becoming a scarce com- 

 modity, and much of that now collected and sold 

 contains comparatively little nitrogen, but a considerable 

 quantity of phosphates. These phosphatic guanos are 

 very inferior in value to the rich nitrogenous ones. In 

 order to increase their usefulness and value, nitrogenous 

 substances are frequently mixed with them by the 

 dealers, but, even then, they are by no means equal to 

 guanos naturally rich in nitrogen. Guano, when stored, 

 must be carefully protected from the rain, as it readily 

 spoils. 



Green dressings have already been described. They 

 are a very valuable means of adding organic matter, and 

 the various constituents of plant food to the soil. In 

 particular they supply that most costly and most easily 

 wasted substance, nitrogen. 



Nitrogenous Manures. 



Sulphate of ammonia. This is obtained as a by- 

 product in the manufacture of gas from coal, in the form 



