Assessing Phosphorus Dynamics in Calcareous Soils Amended with Organic Matter.
Paul Grossl, Utah State Univ, Dept of Plants, Soils, and Biometeorology, 4820 Old Main Hill, Logan, UT 84321-4820, Richard Koenig, Washington State Univ, Johnson Hall 201, PO Box 646420, Pullman, WA 99164-6420, and Stephen Trolove, Crop and Food Research, 265 Lawn Road, Hastings, New Zealand.
Tests used to assess Phosphorus (P) status in calcareous soils amended with organic matter (i.e. manure) are often inaccurate and underestimate the amount of potentially mobile P. This can lead to over applications of manure and an increase in mobile P which potentially threatens the quality of receiving waters. In this study, P dynamics in calcareous systems were examined in order to understand and develop a test that would accurately predict the fate of P in these environments. Our objective was to measure the sorption behavior of P on different soil constituents with or without exposure to soluble organic matter. We hypothesized that soluble organic matter from manure inhibits the sorption of P and consequently influences soil test P levels. Batch and kinetic studies using mineral phases found in calcareous soils such as calcite, brushite , and the iron oxide ferrihydrite were performed. Humic acid was used as an analogue for manure. Specifically, solutions containing 10 millimolar phosphate and calcium chloride were equilibrated with the soil mineral phases mentioned above. Experiments were kept at pH 7.5 and humic acid obtained from the International Humic Substances Society was added as the source of soluble organic carbon. Solutions were sampled over time. At termination, solids were collected and P extracted with water, calcium chloride, and sodium bicarbonate. Sorption of P was greatest on ferrihydrite and was not inhibited by soluble organic matter. Very little of this sorbed P was removed via extraction from the ferrihydrite surface. Humic acids sorbed onto all soil constituent surfaces. Calcareous systems with humic acid had lower levels of sodium bicarbonate extractable P than without humic acid. Humic acid completely inhibited the kinetics of P sorption for calcareous systems. Sorption without humic acid for these systems was also slower than systems containing ferrihydrite, suggesting that P sorption in calcareous systems involved the formation of less soluble calcium phosphates. Further work will use geochemical modeling to verify our findings and assist in developing an accurate P indicator test.