COVER CROPS. 287 



worm., Heterodera radicicola, so common in southern 

 regions. 



These bacteria are commonly known as nitrogen 

 fixers, because of their ability to collect and store the free 

 nitrogen of the air in the tubercles formed by their action. 

 This nitrogen eventually becomes available to the plants 

 growing on the soil. The tubercles are most in evidence 

 during the earlier period of growth of the legumes. The 

 greater number of them decay and become shrunken later 

 in the season, their nitrogen contents having been taken 

 up by the plants. 



Rye, oats, crab-grass, sand-spurs and other grass 

 plants do not collect nitrogen, but when returned to the 

 soil, give back only that which they have withdrawn. 

 However, they add fibre to the soil, increasing the humus 

 content and have a general ameliorating effect. 



The benefits derived from cover crops may be ren- 

 dered entirely negative, unless proper care is exercised in 

 handling them. In the hot climate of the citrus districts, 

 they should not be plowed under while green, but should 

 be allowed to become dry and partially rotted before being 

 incorporated with the soil. Unless this plan be followed, 

 an acid condition of the soil, inimical to the growth of 

 citrus or any other fruit trees, may result. When com- 

 mercial fertilizers are used, the potash content should be 

 increased in proportion to the amount of nitrogen returned 

 by the cover crop and only pure chemical fertilizers should 

 be applied. None containing organic source of nitrogen 

 should be used, else in many localities, die-back will but 

 too frequently result. 



VALUABLE COVER-CROP PLANTS FOR THE SOUTH. 



The legumes most serviceable as cover crops in Flor- 

 ida are Beggarweed (Desmodium tortuosum DC.) and 



