SOIL FERTILITY AND PLANT GROWTH 243 



proving the soil. Green manures improve the soil both by 

 adding to its fertility and bettering its physical condition. 

 If the soil is light and sandy, green manure prevents it from 

 drying out. On heavy clay soil green manure has quite the 

 opposite effect, because of admitting the air, loosening the 

 soil, and improving its drainage. 



Hogs in clover. After pasturing, this clover will be turned 

 under as green manure. 



Green manure crops. For most purposes the best 

 green manure crops are the nitrogen gatherers already dis- 

 cussed the clovers, alfalfa, cow-peas, soy-beans, the vetches 

 and other legumes. Rye, buckwheat, rape and turnips are 

 among other crops used as green manures. This group, 

 however, lacks the advantage of gathering nitrogen. 



4. Commercial Fertilizers 



Although proper systems of manuring and 'rotation of 

 crops will insure fertility on naturally good soil for many 



