28 



THE PLANT FOOD SUPPLY 



effect of green manures upon plant food already present in the 

 soil is thus similar to that of animal manures. 



Although a crop of weeds growing on idle land may be made 

 to serve as green manure when plowed under, vegetable growers 

 usually cannot afford to let land be idle and depend upon nature 

 to furnish the plants for the green manure. Usually some crop 

 is used for green manuring that mil make sufficient growth for 

 that purpose after the regular crop of vegetables has been har- 

 vested and before it is necessary to prepare the land for the next 

 crop. Rye is a good crop for green manuring because it can be 



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Fig. 12. 



Heavy growth of cow peas to be plowed under for green manure. 



sown at any time land is cleared of vegetable crops from August 

 1 to October 15 in temperate climates, and will live through winter 

 and fnake a strong growth in spring before time for planting any 

 but the very earliest crops. If sown as early as August 15 it may 

 be plowed under in the fall instead of spring, and thus not delay 

 the planting of even the earliest crops. 



Plants for Green Manuring. — Weeds and rye add no plant 

 food to soils. They merely give back to the soil the food they have 

 taken from it during their growth, and also in the process of their 

 decomposition help liberate some of the mineral elements of the 

 soil and make them available to other plants. There is, however, 



