1 86 THE FIRST BOOK OF FARMING 



free nitrogen from the air which other plants can- 

 not do. This enables the farmer to grow nitrogen 

 which is very expensive to buy. 



THE TIME FOR GROWING GREEN MANURE CROPS 



Green manure crops may be grown at any time 

 that the soil is not occupied by other crops, pro- 

 vided other conditions are suitable. Land which is 

 used for spring and summer crops often lies bare 

 and idle during fall and winter. A hardy green 

 manure crop planted after the summer crop is har- 

 vested will make considerable growth during the 

 fall and early spring, and this can be plowed under 

 for the use of the following summer crops. If there 

 is a long interval of time during spring or summer 

 when the land is bare, that is a good time for a 

 green manure crop. 



Green manure crops are often planted between 

 the rows of other crops such as corn or cotton at 

 the last working of the crop for the benefit of the 

 crop which is to follow. 



It is advisable to arrange for a green manure 

 crop at least once in three or four years. 



LEGUMINOUS GREEN MANURE CROPS 



Cowpea. (Field pea, stock pea, black pea, black- 

 eyed pea, clay pea, etc.) (Fig. 79.) 



The cowpea is perhaps the most important legu- 

 minous plant grown for soil improvement in the 



