Fig. 2. Nitrogen fixation (measured as the difference between final plant N content and initial seed N content) as related to the efficiency of Ca use (the percentage of the Ca supplied that was taken up by the plants) at different degrees of Ca saturation of a clay mineral. Data from Albrecht (1939). Vertical dotted line is 65% Ca saturation. Scales on each of the axes presented as in Albrecht (1939).