78 THE PRINCIPLES OF AGRICULTURE 



and by stable manures and other direct appli- 

 cations. 



2. Cropping Resources 

 2a. The kinds of green -manures 



108. The stubbles of grain, clover, grass and 

 sowed corn add considerable humus to the 

 soil, and there is also much vegetable fiber 

 left in the ground in the roots ; and the refuse 

 left from potatoes and garden crops is often 

 important. Sometimes the stubble and roots 

 are nearly as valuable for ameliorating the 

 soil as the part which is removed from the 

 land. This is especially true in clover, par- 

 ticularly if it is not cut close to the ground. 

 Roberts reports that a second -growth of clover, 

 two years from seeding, gave 5,417 pounds per 

 acre of top and 2,368 pounds of roots in the 

 upper eight inches of soil ; and the roots usu- 

 ally extend to three or four times that depth. 



109. Humus is often secured by growing 

 crops for that particular purpose ; that is, by 

 the practice of green -manuring. Green -manure 

 crops are of three categories : (a) regular or 

 full -season crops, which occupy the land for 

 one or more seasons before they are plowed 

 under, or until they have reached nearly or 

 quite their full growth ; (b) catch -crops, which 

 are grown in the seasons between other crops; 



