126 NATURE TEACHING. 



2. Guano is the excretion of sea birds, 

 deposited in rainless, tropical regions. It con- 

 tains 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 commodity, and much of 

 that now collected and sold contains compara- 

 tively little nitrogen, but a considerable quan- 

 tity of phosphate. These phosphatic guanos 

 are very inferior in value to the rich nitroge- 

 nous ones. In order to increase their useful- 

 ness 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. 



3. Green dressings have already been de- 

 scribed. They are a very valuable method of 

 adding organic matter and the various constit- 

 uents of plant food to the soil. In particular 

 they supply that most costly and most easily 

 wasted substance, nitrogen. 



