Biomass accumulation of corn has been found to decrease with increasing clay contents in previous studies. It was hypothesized that anaerobic microsites would account for this decrease. The effects of above ground tissue nitrogen (AGTN), soil resistance (SR), water potential (WP), xylem stem sap O 2 concentrations, soil gas concentrations, redox potential and pore characteristics on corn grown on soils of increasing clay content, relative compaction and air-filled porosity were determined in controlled environment studies. Results identified limiting values of AGTN. Xylem stem sap O 2 concentrations, soil gas concentrations and redox potential measurements were poor indicators of anaerobic microsites. However, pore characteristics were used to define a volume fraction of pores that were considered to be anaerobic. This volume fraction of pores was significantly correlated with above ground dry weight and photosynthesis. However, none of the measured parameters were able to account for the effect of clay content on biomass accumulation.