The life-cycle of the nodule organism, bacillus radicicola (Beij.), in soil and its relation to the infection of the host plant

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Abstract
By means of a modification of Winogradsky''s staining technique the changes in morphology of Bacillus radicicola in soil were followed. A regular cycle was found, unbanded rods, cocci, and banded rods successively predominating in the soil. Increase in the percentage of cocci was associated with increased bacterial numbers and with the appearance of motile forms. By modifying the liquid used to suspend the inoculum added to the soil, the time of appearance of coccus predominance could be altered. In particular, inoculation of the soil with a bacterial suspension in milk +0.1% CaH4(PO4)2+2H2O hastened the predominance of cocci and increased the percentage attained. When the center of a petri dish 01 soil and sand is inoculated with a suspension of the bacteria, the latter, after a lag period, commence to spread radially at an approximate rate of 1 inch in 24 hours. The length of this lag period is apparently related to the time taken for cocci to predominate in the soil and is similarly effected by the nature of the inoculating fluid. The bacteria multiply rapidly in the soil into which they have recently spread, so that the nature of the inoculating fluid also exerts a remote influence on bacterial numbers. Thus inoculation of the soil with a bacterial suspension in milk containing 0.1% CaH4(PO4)2+2H2O results in a greater spreading of the bacteria through the soil and in greater multiplication at a distance from the point of inoculation than in the case when soil is inoculated with a suspension in milk alone. Lucerne plants grown from seed inoculated with a suspension of bacteria in milk containing 0.1% CaH4(PO4)2+2H2O showed a considerable increase in nodule numbers and in yield compared with plants from seed inoculated with a suspension in milk alone. This effect was confined to the deeper portions of the root and therefore increased as the plants became older and roots developed in the deeper soil. This suggests that the additional nodule formation is due to the known effect of the phosphate in increasing the spreading of the bacteria.

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