128 AGRICULTURE ON THE PACIFIC SLOPE 



Loss of humus. Again, in lands very well cultivated 

 it may happen that because the crop was all taken 

 off, and the weeds always kept down, the soil humus_ 

 has all been burnt out by the sun and air. Then the 

 land becomes difficult to cultivate, andT crops become 



poor. The remedy will then be to grow green crops, 



^^MVM^ 



preferably legumes, such as clover, alfalfa, peas, or 

 vetches. When fully grown, plow them under and let 

 them rot in the soil to form humus. This is called 

 greenmanuring. When we have plenty of stable ma- 

 nure this will answer the same purpose even better. 



Greenmanuring should always be done with legumes 

 if possible, because we then not only put back into the 

 soil what was taken from it in the growth of the crop, 

 but also add to it the nitrogen such plants always take 

 from the air by means of the root bacteria (see page 

 141 and figure 75, page 142). Such greenmanuring is 

 in so far a real fertilization. Other plants, such as 

 grasses (rye, barley, pasture grasses), give- back to the 

 soil only what they took from it (except \he humus, 

 which came mainly from the air). 



Allowing weeds to grow. Sometimes the land has 

 been allowed to become too full of weeds to give the 

 crop a chance to grow well. Tjiisjiappens most com- 

 monly with crops that are not cultivated while grow- 

 ing, such as grain sown broadcast. If then the weeds 

 go to seed before the grain is cut, the weed seeds will 

 next season get the start of the grain and greatly hurt 



