FORTY-SECOND ANNUAL. REPORT. 



LEGUMES FOR COVER CROPS. 



L. M. HUTCHINS., FENNVILLE. 



(Third Prize Address.) 



The place of cover crops in soil improvement has long been recog- 

 nized. The immense importance of the legumes in this connection is 

 of more recent development. The legumes themselves, that is the 

 individual vetch, clover and alfalfa plants are not in themselves so 

 superior to other cover crops. Their great value lies in their ability 

 to serve as hosts for nitrogen fixing bacteria, the influence of which 

 may readily be seen at a glance at these two samples of alfalfa taken 

 from adjoining plots of three square feet each of sandy loam soil, the 

 one inoculated and the other not. A comparison of the two shows the 

 inoculated specimen to be more than twice the height of the uninocu- 

 lated and more than ten times the quantity.. The explanation of this 

 phenomenal difference of growth under identical conditions is found 

 in the presence of nodules of free nitrogen fixing bacteria on the root 

 hairs of the inoculated specimen. This species of bacteria, known as 

 Pseudomonas radicicola, is found only on the roots of legumes and is 

 distinctive in possessing the ability to take nitrogen gas from the air 

 in the soil with which to construct its richly nitrogenous cell pro- 

 toplasm. The organisms live in the interior of the cells of the root 

 hairs, forming galls or tubercles where they act in a symbiotic rela- 

 tion with the host which furnishes sugars and starches to the bacteria, 

 receiving in return the albuminous bodies of dead bacteria, killed by 

 the plant and changed to bacteroids, which are the richest form of plant 

 food and ready for immediate absorption. 



After elaborate experimentation and much careful work with this 

 organism, isolating it from the nodules to artificial culture, it was 

 found that it could be grown in pure culture on many kinds of media. 

 However if it is to preserve its nitrogen fixing power it must be grown 

 on nitrogen-free media, since it will take the required amount of that 

 element for forming its protoplasm from the media, if available, in 

 preference to fixing atmospheric nitrogen. Further work with rein- 

 oculating disinfected seeds growing in sterile N-free agar proved con- 

 clusively that strains of the organism have developed which require 

 particular species of legumes as hosts. Hence it became necessary to 

 isolate and grown in pure culture organisms from nodules of each of 

 the legumes, the alfalfa organism from nodules on the alfalfa plant, 

 the vetch organism from nodules on the vetch plant and similarlv with 

 all. 



For commercial use the organism is grown on nitrogen-free agar in 

 small bottles (sample shown). The surface of this media is soon covered 

 with a thick growth of the organism. They are then ready for shipment. 



Many methods of distributing the organism in the field have been 

 tried out. Inoculating with soil from an adjoining field known to con- 



