4 o TROPICAL AGRICULTURE CHAP, iv 



elements necessary for the food of the plants, and whether 

 these elements exist in an active condition. If not, he knows 

 what to add to the soil, in the form of manure, to make it 

 fertile ; or, as we shall see later on, he finds out that tillage 

 or rest will develop in the soil all the necessary substances 

 in sufficient quantity and in a soluble form ready to be taken 

 up by the roots of the plants. 



