| Abstract: | This study was to determine if diffusion of soil ammonium may explain why many sandy soils have greater nitrogen (N)–supplying capacity to rice than clay soils. A laboratory procedure using transient-state methods measured the linear movement of soil ammonium (NH4) in tubes packed with five field soils under aerobic conditions. Ammonium diffusion was measured by sectioning tubes after 48 h of equilibration and then measuring NH4 by steam distillation. Effective diffusion coefficients, De, and NH4 diffusion distance, d, per day ranged from De = 4.6 × 10?5 cm2 d?1 and 1.5 cm d?1 for Katy sandy loam to De = 2.9 × 10?7 cm2 d?1 and 0.11 cm d?1 for League clay. Ammonium diffusion distance d was strongly related to soil clay content and hence was predicted by d = Y × {100/(% clay)] ? 1}, where Y is set to 0.1. Predicted d and measured d were highly related (R2 = 0.99). |
