Characterizing Soil Aeration Under Changing Soil Moisture Conditions for Bean Growth

An attempt was made to evaluate different soil aeration indices as related to plant growth during fluctuations in soil moisture content. Beans (Phaseolus vulgaris var. ‘Dubbele Witte’) were grown in soil at different aeration conditions obtained by changing bulk density (1.30 and 1.38 g cm-³), irrigation frequencies (3 to 11 days between irrigations), and O₂ concentrations at the soil surface (11% vs 21%). Frequent measurements were taken O₂ and CO₃ concentrations of the soil air, oxygen diffusion rate (ODR), and redox potential and a continuous record was kept of soil air content by pot weighings. After each irrigation, the O₂ concentration of the soil air decreased to less than 10% and 5%, respectively, for O₂ concentrations at soil surface of 21% and 11%. No complete recovery to the soil surface values was obtained, especially with frequent irrigations. The CO₂ concentration of the soil air was never high (6.5% maximumb) because of the high solubility of CO₂ in the irrigation water. The redox potentials measured did not show any relationship with air content or with 0₂ concentration. The ODR measurements were quite variable, the mean values per pot all dropped below 0.2 µg cm-²in-¹ after irrigation. Vegetative growth and dry matter production were affected strongly by bulk density and irrigation frequency (yields ranged 4 to 16 g dry matter per plant). Total root production was hardly influenced, but the depth of root penetration was affected strongly by these aeration treatments. Lowering the O₂ concentration at the soil surface from 21% to 11% had no significant effect on plant growth. Comparing the measured aeration indices (0₂ concentration, ODR, and air content), the mean daily air content gave the best correlation with plant growth (r = 0.82). A continuous record of soil air content seems good index for evaluating aeration under changing soil moisture conditions.