MAINTAINING FERTILITY OF LAND 195 



necessary capital to follow very intensive methods, even 

 if such methods could be shown to pay when once estab- 

 lished. In many cases, the still less intensive farming 

 would pay better, that is, raise lumber. 



MAINTAINING THE NITROGEN SUPPLY OF THE SOIL 



125. Fixation of atmospheric nitrogen. If a soil is 

 kept well supplied with organic matter, it usually has 

 plenty of nitrogen, because most of the nitrogen of the 

 soil is in the organic matter. The ultimate source of 

 nitrogen is the air. Bacteria working on the decaying 

 vegetable matter are able to take nitrogen out of the soil 

 air, and so fix it for plant use. Bacteria working on the 

 roots of legumes also fix nitrogen.. Adding organic matter 

 in any way, keeping the land in sod, or growing legumes are 

 the chief ways of encouraging the fixation of nitrogen. A 

 leguminous sod is usually better than a cultivated legume. 



At the Rothamsted Experiment Station in England, two 

 fields have been allowed to run wild since 1881. Nothing 

 has been removed or added to the land. On one of these 

 about one-fourth of the plants are legumes. During the 

 past thirty years, this field has gained in nitrogen at the 

 rate of 90 pounds per acre per year. On the other field 

 where practically no legumes grew, the gain averaged 60 

 pounds per acre per year. Most of this gain was unques- 

 tionably due to the fixation of nitrogen by soil organisms 

 living on the organic matter in the soil. In the former 

 case, we do not know how much was due to legumes, 

 because this soil contained more lime, and the lime favors 

 the organisms that act independently of legumes as well 

 as those that act on legumes. 



Of course, the nitrogen supply may be maintained by 

 the addition of farm manure or commercial fertilizers, 



