210 SYSTEMS OF PERMANENT AGRICULTURE 



sap most of the nourishment upon which the bacteria live. The 

 bacteria, on the other hand, take nitrogen from the air, contained 

 in the pores of the soil, and cause this nitrogen to combine with 

 other elements in suitable form for plant food, which is afterward 

 given up to the legume for its own nourishment. 



Another illustration of remarkable parasitism, if not, indeed, 

 one of true symbiosis, is found in the common lichens living upon 

 rocks and trees. The lichen is not a single plant, but two plants, 

 one an alga, which lives upon the wood or stone, and the other a 

 fungus, which lives upon the alga. Algae also live in the free state 

 separate from fungi, and the present opinion of botanists seems to 

 be that when the two are associated in the form of lichens, this 

 association is not detrimental, but rather beneficial, to the alga, 

 as well as to the parasitic fungus. If this is true, then it is another 

 case of true symbiosis. (It is now known that some fungi have 

 power to feed upon atmospheric nitrogen, and probably those 

 in lichens furnish combined nitrogen to the algae upon which they 

 live.) 



In the symbiosis of legume plants and nitrogen-fixing bacteria 

 we have a partnership or relationship of immeasurable value to 

 agriculture. Here is a class of plants (legumes) that are capable 

 of consuming or utilizing nitrogen in quantities larger than could 

 possibly be obtained from ordinary soils for any considerable 

 length of time. They have no power in themselves of taking 

 nitrogen from the atmosphere, and to them the symbiotic relation 

 with this low order of plants (the nitrogen-fixing bacteria, Pseu- 

 domonas radicicola), is especially helpful, and for the best results 

 it is absolutely necessary. 



INOCULATION FOR NITROGEN FIXATION 



While it is true that nitrogen-fixing bacteria are essential to the 

 most 'successful growing of legumes, it is also true that, as a very 

 general rule, the proper bacteria for the ordinary legumes are 

 already present in the most common soil, especially where the 

 particular legume has been grown in the vicinity for several years, 

 or where manure made from the legume has been applied. This 

 applies especially to alfalfa in the alfalfa country of the West, to 



