MANURING AND WORN-OUT SOILS 329 



TWO KINDS OF GREEN MANURE 



Almost any herbaceous plant has some value 

 when plowed under as a green manure. The 

 weeds that get a foothold in the garden and corn- 

 field in late summer serve one useful purpose in 

 this way. But the trouble with weeds for green 

 manure is that we cannot depend upon them. 

 They come in where they have a mind to, not as we 

 desire. Usually they are rank in the hollows, 

 where the soil is rich and needs no humus, but shun 

 the knolls, where the soil is hard and needs humus 

 badly. Sometimes weeds may be turned to good 

 account for green-manuring, but usually a special 

 crop must be grown. 



There is a distinction between crops for this pur- 

 pose. They may be * ' leguminous ' ' plants or ' ' non- 

 leguminous." A leguminous plant is one that, 

 among other characteristics, bears its seeds in a 

 certain kind of a pod, called by botanists a 

 " legume." Peas, beans, clovers, vetches, alfalfa, soy 

 beans, cowpeas, are examples of leguminous plants 

 commonly used for green-manuring. If it is 

 known that the soil is more or less lacking in the 

 )lant food, nitrogen, a leguminous crop should be 

 p*own for plowing under in preference to a non- 

 eguminous crop, like rape, buckwheat or rye. 

 rhrough the little warts or nodules on their roots 

 < eguminous plants may feed upon the nitrogen that 

 is in the soil air, instead of drawing upon the supply 

 that is in the soil. When these plants are plowed 

 under, therefore, the soil is enriched with the 

 nitrogen that they have gathered. The plants 

 themselves are richer in nitrogen, and have a nigher 

 feeding value, when the nodules are on their roots. 

 This wonderful process of "nitrogen-fixing" has 



