224 SYSTEMS OF PERMANENT AGRICULTURE 



planted on certain pots every year after the wheat was harvested, 

 the legumes being turned under before sowing wheat for the next 

 year. 



From a study of Table 98, it will be seen that practically no gain 

 has been made except where nitrogen has been supplied, either 

 directly in commercial form or indirectly by means of legume 

 treatment. It should be borne in mind that no legume treatment 

 preceded the 1902 wheat crop. The catch crop of cowpeas which 

 was planted after the 1902 wheat crop and turned under later 

 in the fall, produced a marked effect upon the 1903 wheat crop. 

 This effect became more marked in 1904 and 1905, when every pot 

 receiving legume treatment outyielded the pot receiving lime- 

 nitrogen treatment. Previous to 1905, the addition of phosphorus 

 to nitrogen or legume treatment always increased the yield. The 

 addition of potassium still further increased the yield more or 

 less. The effect of both phosphorus and potassium has been less 

 where decaying organic matter has been provided in the legume 

 treatment than where the nitrogen has been supplied in commercial 

 form (dried blood) carrying but little organic matter. 



The last line in the table gives the yield from a pot of virgin 

 soil collected from a piece of unbroken virgin sod land adjoining 

 the cultivated field from which the soil in all the other pots was 

 taken. 



NITROGEN FIXATION BY NONSYMBIOTIC BACTERIA 



Aside from the fixation of free nitrogen by the bacteria living in 

 symbiotic relationship with legume plants, there are at least three 

 groups of bacteria that have nitrogen-fixing power without this 

 relationship. 



First, and possibly of greatest importance, are the legume 

 bacteria themselves, which continue to fix nitrogen in pure cultures 

 entirely separated from legume plants, and very probably also 

 continue thus to fix some nitrogen in the soil, even after the 

 legume plants have been destroyed, the bacteria drawing their 

 nutriment from the decaying organic matter. 



Second, is the anaerobic group of bacteria discovered by Wino- 

 gradsky in 1893, and called Clostridium; but these have little 



