THE FOOD OF PLANTS 31 



system of cultivation consists in clearing a new field 

 whenever the old one ceases to yield well. This system 

 is very expensive in the amount of labor required and 

 very bad for the land. 



Rotation of Crops. Different crops do not need the 

 different foods in equal amounts. Therefore, a soil 

 which will not produce more good crops of the kind 

 already grown may still prove fertile, if a different crop 

 is planted. It is good agriculture, when working with 

 crops which do not live many years, to change the crop 

 frequently on each piece of ground. A regular succes- 

 sion of crops chosen so as to prevent the exhaustion of 

 any particular food from the soil, is called rotation of 

 crops. 



In the garden, fertilize one half of a plot of maize, and one half of a 

 plot of some kind of bean, with manure, putting one kilo on each square 

 meter. Leave the other half of each plot without any fertilizer. Cul- 

 tivate the two halves in exactly the same way. Note the difference in 

 the amount of the crop, and in the time it takes it to mature. This ex- 

 periment should be made by the whole class rather than by the individ- 

 ual students. 



