In investigations on the effects of calcium carbonate and magnesium carbonate on nitrogen fixation by A. chroococcum in soils and in solutions, the following results have been obtained:
1. Calcium carbonate in mannite solution cultures acts only as a stimulant, never as a toxic agent at least up to and including a concentration of 2%.
2. Magnesium carbonate is sharply toxic in solution cultures in concentrations in excess of ·1% to ·2%.
3. Calcium carbonate in soil cultures is without effect up to 1·4% of the dry weight of the soil when it becomes slightly toxic in the Anaheim sandy soil.