The present paper records some investigations on the growth of the clover- and pea-nodule organisms (Rhizobium trifolii Dangeard and Rh. leguminosarum Frank), and the nitrogen fixation of the pea, at low temperatures. Growth of the bacteria was studied with pure culture experiments in large flasks, in which the bacteria were grown on a solid gelatine medium. The amount of nitrogen in the bacterial mass thus obtained was used as the basis of comparison. The fixation of nitrogen by peas was studied with a pot-culture experiment in a relatively cold greenhouse. Inoculated plants were grown in quartz sand, and were supplied at an early stage with mineral nitrogen either as Ca(NO3)2 or (NH4)2SO4. The plants were harvested after flowering and analysed. The main results of these experiments were the following:

(1) Pure-culture experiments with five strains of clover-nodule bacteria and four strains of pea-nodule bacteria showed that at 6–13° C. the strains imported from more temperate climates grew distinctly less well than did the northern European strains. Even between the Swedish and Finnish strains there were clear differences: the latter formed the more bacterial matter in the cold. It may be appropriate to consider such differences when selecting cultures for inoculating leguminous crops.

(2) In the pot-culture experiment, the application of mineral nitrogen (30 mg. N per plant) to inoculated peas kept at relatively low temperatures had very little effect on the amount of dry matter produced. The slightness of this effect was similar to what has been previously observed in experiments at more favourable temperatures. Nitrogenous fertilization slightly lowered the amounts and percentages of total N present as protein and amino nitrogen, in the crop; but nitrate application somewhat increased the nitrate content of the crop. Hence, it may be concluded that, even under these low-temperature conditions, the application of artificial nitrogen has not had a favourable effect on the size or quality of the crop.