Soil-plant system response to lime and phosphorus amendments of an andisol of the Andean highlands of Ecuador
Strong phosphorus sorption limits the agronomic productivity of volcanic soils (Andisols). It is understood that in acidic Andisols, P sorption occurs due to reaction with variable charge mineralogy developed from weathering of volcanic ejecta, soil organic matter content, and aluminum and iron (hydr)oxides. It has been demonstrated that because of the extremely high P sorption rates in these soils there is little residual effect of phosphorus fertilizer carrying over from an initial application into a second crop. As a result, managers of these soils, which are frequently found in semi-subsistent farming systems, find themselves obligated to apply high rates of P with every cycle: a situation much the opposite of those who are currently trying to reduce excess phosphorus runoff and leaching from their nutrient-laden fields in other parts of our world. There have been numerous studies conducted on the P sorption mechanism of Andisols, and there have also been studies conducted on how to reduce phosphorus sorption in these soils with the obvious potential to benefit agronomic production, however, these studies remain inconclusive in their evidence on how to treat Andisols with high P-sorption which is attributed to the methods of study used, interpretation of results, and also because of the variable nature of the soils themselves. Andisols have variable OM content and mineralogy depending on stage of weathering and parent material, and phosphorus sorption response is highly dependent on these factors. Therefore, it is important to conduct further studies on the nature of soil and plant response to P-sorption in order to develop a technique for managing Andisols for better agronomic production. The main goal of this study was to examine the interactive effects of lime and P fertilizer amendments on P sorption and plant P availability in a moderate pH, moderately low soil test-P Andisol from the central Andean highlands of Ecuador. A two-way factorial consisting of three rates (0, 90, or 180 kg ha-1) of P2O5 and three rates (0, 3, or 6 tons ha-1) of calcium carbonate (CaCO3) were mixed with equal volumes of soil and seeded with barley (Hordeum vulgare, L.) in a greenhouse experiment. Soils were sampled two times during an initial incubation with lime and two times after fertilizers were added and barley seeded. Aboveground plant tissues were harvested at 53 days of growth. To determine the treatment effect on phosphorus sorption in the soil, Mehlich-3 and Olsen extractable P, Al, and Ca and soil pH were determined on soil samples taken before liming and at 21, 32, 46, and 85 days after liming. The 46 and 85 day marks coincide with the 14th and 53rd days after planting (dap). Additional laboratory work was done in University Park, PA, to build sorption and desorption isotherms, study P fractionation, and identify soil mineralogy. To quantify the impact of the treatments on plant P uptake and growth, biomass and physical response, as well as tissue P and other nutrient content were determined on whole barley plants harvested at 53 dap. Observations show that barley grown in this Andisol responded strongly to P fertilizer additions, and had smaller responses to lime, however there were no notable lime and P interactive effects. Lime did not cause any change in P uptake or P concentration with liming, however it did improve plant nutrition and increase biomass production. These findings suggest lime could be a beneficial addition to management practices for subsistence farmers in the Andean Highlands, however additional research should be done to further develop this information and to evaluate the cost-benefit ratio of a liming effect on production.