Evaluation of root growth limiting factors in spodic horizons of Spodosols

Spodic horizon in Spodosols often restricts growth of plant roots. Generally the Spodic horizon is acidic with high levels of aluminum (Al). In this study soil solution Al was characterized in the Spodic horizon and the respective surface horizon of three Spodosols in the major citrus production region in Florida. The soil pH ranged from 4.92 to 5.14, and 4.82 to 5.50 in the Spodic and top soil samples, respectively. The modified aluminon analytical technique was used to measure the concentrations of Al in the soil solution as monomeric Al which includes both organic and inorganic monomeric Al complexes, and as reactive Al (which excludes most of the organically complexed Al). The organically complexed Al was estimated by the difference between the above two concentrations. The concentrations of total Al in soil solutions were measured by the aluminon technique. The polymeric Al was estimated by the difference between the total and monomeric Al concentrations. Further, the activities of different monomeric Al species were calculated by MINTEQA2 thermodynamic speciation model. The concentration of reactive Al varied from 29.5 to 51.9 and 27.5 to 62.7muM in soil solutions from the top soil and Spodic horizon samples, respectively. The corresponding values for the organically complexed Al were 10.3 to 32.5 and 15.6 to 51.0muM, and those for the polymeric Al were 11.6 to 54.8 and 25.4 to 46.0muM. The shoot and root dry weights of soybean seedlings were lower in the Spodic horizon samples as compared to that of the seedlings in the respective top soil samples, in all three soil series. The difference in growth of citrus seedlings in the Spodic horizon soil as compared to that in the respective surface soil horizon was dependent on each of the soil series. The root growth reduction in the Spodic horizon soil sample as compared to the growth in the surface horizon soil of the respective soil series, was marginal (17%) in the Wabasso sand, as compared to 34 to 38% reduction in the Ankona and Nettle sand soils.