Influences of management and the environment on phosphorus leaching in corn-soybean systems in Minnesota

University of Minnesota M.S. thesis. August 2018. Major: Applied Plant Sciences. Advisor: Daniel Kaiser. 1 computer file (PDF); 150 pages.Phosphorus is an essential nutrient in corn and soybean systems for maximizing yields and profits in Minnesota. The over-application of P fertilizer can lead to the impairment of freshwater ecosystems throughout the Midwest and United States. Excess P in freshwater can ultimately lead to eutrophication and detrimental species population shifts. To help mitigate such pollution here in Minnesota, research was completed to identify the risk of P loss to the environment through leaching. The objectives of this research were to 1) Evaluate the potential for water soluble P loss from top soils (0-15 cm) across a range of initial soil test phosphorus (STP) levels and timings of leaching events after fertilizer application (denoted as “leachtimes”); 2) Determine the effect of soil type, soil properties, initial STP level, various leaching intensities, and P fertilizer source on P leaching losses; 3) Assess the predictability of P loss through leaching using routine soil tests, leaching intensity, soil properties and sorption estimates. Six sites throughout the state of Minnesota under long-term P management were used for this study. Intact soil columns were taken from all sites to identify the risk of P leaching from various Minnesota soils with varying chemical and physical properties. Columns were leached at 10x field capacity to balance for various water holding capacities among sites. The influences of soil test P (STP), soil type, and timing of a leaching event after fertilization were evaluated for effects of P leaching as P leachate concentration, P load, and percent of P fertilizer leached. In addition, a second set of intact columns were collected from three of the six sites for further evaluation of leaching intensity and P fertilizer source on P leaching potential. Leaching intensity was applied at three rates 200, 400, 600 mL to evaluate effects on P leaching. Liquid P fertilizer, dry P fertilizer, manure, and liquid P fertilizer with the addition of AVAIL® were evaluated for influences on P leaching. Results from this study indicated P leaching is a substantial loss pathway with greater losses occurring at higher STP levels. Leachtime also significantly affected P leaching losses, with immediate leach events after fertilizer applications leading to losses up to 33 percent. Volume of the leaching event was found to be the driver of P load, as P concentrations remain consistent with volume applied, and concentrations were not diluted. Phosphorus fertilizer source also significantly impacted P leaching losses, with dry fertilizers leaching significantly more P than liquid or manure sources. Results indicated soil hydrology may be substantially influencing leaching, with preferential pathways leading to greater P leaching losses. Relating P leaching losses to soil test measures was moderately successful; however, relationships were not strong enough to lead to P leaching loss predictions. Relationships were strongest among the environmental soil tests, Water extractable P (WEP) and BioAvailable P (BAP), and the Olsen P (OP) routine soil test. Further research is necessary to fully understand P leaching especially at deeper depths