SOILS AND FERTILIZERS 529 



realized that there has actually been added to the soil a certain 

 definite amount of nitrogen in such form that the wheat can be 

 benefited by it. Such efficient users of the atmospheric nitrogen 

 are clover and peas and similar crops that they can actually live and 

 thrive in a soil that has not the first trace of combined nitrogen 

 within it. 



Inoculation. Experiments showed that leguminous plants ob- 

 tained nitrogen from some source and under conditions where it 

 was not available for the nutrition of the cereals, and, eventually, 

 that it was obtained from the atmosphere. It was suggested that 

 the tubercles observed on the roots of leguminous plants had a di- 

 rect relation to the appropriation of nitrogen, but most observers 

 looked upon them as abnormal and of no physiological significance. 

 The latest investigations, however, show beyond the shadow of a 

 doubt that these tubercles or nodules are the result of infection by 

 microbes and that the relation between the roots and the bacterial 

 organisms is a true symbiotic one, each developing more vigorously 

 at the expense of the other, and that free nitrogen is appropriated 

 by the microbes. (Y. B. 1902; Y. B. 1906; Va. Ex. Sta. B. 159.) 



The tubercles or nodules on the roots of clovers, peas, etc., had 

 been noticed for a considerable time (since 1687). Some thought 

 they were of parasitic origin, and others saw in them simply ex- 

 crescences or galls, and a few thought they were the normal growths 

 of the plant; and it was not until 1886 that two German scientists, 

 Hellriegel and Willfarth, showed that the development of the root 

 nodules was intimately connected with the growth of the whole 

 plant, and that the assimilation of the nitrogen of the atmosphere 

 by legumes was associated with the development of the nodules or 

 tubercles on the roots. Later investigators confirmed these results, 

 and these discoveries were quickly followed by the detection of the 

 bacteria in the nodules, their isolation and growth on artificially 

 prepared food and the ability of these cultivated organisms to pro- 

 duce other nodules when brought into contact with suitable legumes 

 growing in sterilized or germ free soil. 



About 1902 the study of the root-nodule organism was under- 

 taken by the Laboratory of Plant Physiology of the United States 

 Department of Agriculture, and they perfected a method by which 

 these organisms could be sent out to farmers and used for inoculat- 

 ing seed. Up to November, 1904, over 12,000 packages of inoculat- 

 ing material were distributed, and the reports of 3,540 experiment- 

 ers showed that 79 per cent of these were successful. There are 

 three methods of inoculating the crop: 



(1) By transfer of soil; (2) by treating the soil; (3) by treat- 

 ing the seed. In the first case, soil which is known to contain the 

 nodule-producing organisms is scattered over the land where it is 

 desired to grow a crop of legumes. In other words, the land is top- 

 dressed with soil from an infected field. This method has given 

 good results, but is expensive when the soil has to be brought any 

 distance, and in certain sections of the country such a practice is 

 fraught with danger, as weed seeds and certain diseases are apt to 



