The action of sodium nitrate upon the infection of lucerne root-hairs by nodule bacteria

The fact that nitrates and ammonium salts harmfully affect nodule formation and the subsequent nitrogen fixation in legumes is well known, and the problem has formed the subject of an extensive literature, refer­ences to which are quoted by Giöbel (1926), by Fred, Baldwin, and McCoy (1932), and by Allison and Ludwig (1934) and need not be given here inextenso. The technical difficulties involved in analysing the action of nitrate upon the association of host and nodule organism are considerable; moreover, the great variation in cultural methods used has made it difficult to compare the results of different workers and has sometimes produced apparent discordance between them. The following facts appear, how­ever, to have been established: 1. Nitrates in sufficient concentration reduce the number of nodules formed and may even entirely inhibit their appearance. Doses of from 5 to 175 mg of nitrate nitrogen per litre have been found entirely to stop nodule formation in agar cultures of legumes (Hiltner, 1900; Flamand,1903-4; Marchal, 1901; Hills, 1918). In sand and soil cultures the inhibiting doses have been found to range from 21 to 488 mg of nitrogen per litre of soil moisture or of solution in the sand (Fred and Graul, 1916; J. K. Wilson, 1917; Hills,1918; Strowd,1920; Ohkawara, 1928). The highest of these figures in both agar and pot cultures are probably excessive, since they refer to initial doses, whereas nodule formation may sometimes have occurred when a weaker concentration, initially inhibitive, was further reduced by assimilation. RESP-108