28 PHILIPPINE AGRICULTURE 



Nitrogen, Phosphorus, and Potassium. The elements of 

 which agricultural soils are likely to contain less than 

 plants need are nitrogen, phosphorus, and potassium. 

 Therefore, these are the ones which should be used in 

 fertilizers. Sometimes all three of these are needed, but 

 usually some one is needed more than the others. Some 

 fertilizers contain all three, and other foods besides, while 

 other fertilizers contain hardly any of more than one. 



Decaying vegetation contains all the foods of the 

 plants that are decaying, and is therefore sure to contain 

 some of whatever foods are most needed by any plant. 



Manures. Stable manure also contains all the sub- 

 stances needed by plants, but contains more nitrogen 

 than plants do, and is therefore an especially good fertili- 

 zer for general use. Poultry manure is especially rich in 

 phosphorus. Because it is very soluble in water, poultry 

 manure is a strong fertilizer, and plants are sometimes 

 injured by too much of it. 



Bat manure and bird manure, which are called guano 

 when sold as fertilizers, are also rich in phosphorus. All 

 of these manures contain considerable potassium also. 



Bones and Ashes. Bones and fish-refuse are rich in 

 phosphorus. Ashes are usually rich in potassium, which 

 is called potash when used as a fertilizer. When a plant 

 is burned, the hydrogen, carbon, and nitrogen go into 

 the air, leaving all the other foods in the ashes. Ashes 

 are therefore an excellent fertilizer for plants which are 

 not in especial need of nitrogen; but for plants which 

 need this substance, they are useless. 



