20 PHILIPPINE AGRICULTURE 



Mold. When plants and animals die, their bodies 

 become a part of the soil. Where vegetation is very 

 dense, as it is in heavy forests, a large part of the soil 

 is formed in this way. Soil formed mostly by the decay 

 of plants and animals is called mold. Mold is always a 



FlG. 22. Fertile alluvial soil in a valley 



rich soil, because it is made up of what has already been 

 plant food. 



Humus. In marshes the organic matter the bodies 

 of plants and animals sometimes cannot decay, be- 

 cause there is no air in the soil. Such soil becomes 

 sour, and therefore useless. A soil which is sour 

 because it contains much insufficiently decayed matter 

 is called humus. 



Most plants cannot live in sour soil ; but if humus 

 is well aired, it will become an excellent place for them. 



