REASONS FOR ROTATION OF CROPS 



305 



becomes usable in the soil as plant food. Nitrogen in the 

 bodies of dead plants and animals also is acted upon by 

 the bacteria which cause decay, and is eventually made 

 into soluble mineral substances called nitrates. The 

 changes through which 

 nitrogen passes are too 

 complex for us to ex- 

 plain at present, but we 

 should remember that 

 in all the world it is in 

 circulation, from the 

 bodies of living plants 

 and animals, to the soil, 

 there to be changed into 

 soluble nitrates which 

 may be absorbed by the 

 roots and used by the 

 plants again. 



Reasons for rotation 

 of crops. It is com- 

 mon knowledge among 

 boys and girls who have 

 had gardens that the soil 



" wears out " after a while, and that it becomes necessary 

 either to add manure, which contains nitrogen in a usable 

 form, or some artificial fertilizer which contains nitrates, 

 phosphates, or potash. By raising peas, alfalfa, or clover, 

 we can often bring back the soil to its former state of 

 fertility. A boy who wishes to get the most out of his 

 garden will rotate the crops, or make a combination plant- 

 ing. This enables him to get much more out of his garden 

 than if he planted only one crop. A good combination for 



H.-WHIT. CIV. SCI. IN THE HOME 2O 



