December 2013

SportsTurf provides current, practical and technical content on issues relevant to sports turf managers, including facilities managers. Most readers are athletic field managers from the professional level through parks and recreation, universities.

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FieldScience | By Dr. Gwen K. Stahnke* Phosphorus availability in turfgrass rootzones after organic and synthetic N fertilizer apps O RGANIC FERTILIZERS have increased in popularity over the past 10 years due to the belief they are more environmentally sound to use than synthetic fertilizers. Most fertilizers derived from organic materials contain phosphorus as well as nitrogen, so use may be affected in states that legislate the application of P to lawns. States are considering exempting organic fertilizers from their zero-P legislation, as Wisconsin did, because it is thought that P from organic sources is less likely to be lost in leachate or runoff. Fertilizers are applied on turfgrasses as needed based on N form and content. Many organic fertilizers contain as much P as N in their formulations, and therefore similar amounts of P and N are applied with each application. Soil tests in native soil and a fairway sand and peat mix used in the Pacific Northwest showed that organic fertilizers applied at rates to provide adequate N for acceptable turf increased soil Bray-1 P levels from 16 to 18 mg/kg to 23 to 66 mg/kg within 3 years. Oxalate extractable Fe, Al, and P was determined for all treatments in both soils and used to calculate phosphorus saturation (PSI). PSI values from sand treated with one organic fertilizer source were significantly higher than measured in other treatments, indicating future risk of P loss with repeated applications of this organic fertilizer. Because of concerns about phosphorus effects on eutrophication of surface waters, local and/or state governments New Jersey, Maine, Florida, Wisconsin, Minnesota, and Washington have adopted restrictions on residential use of phosphorus-containing fertilizers. Urban and suburban lawns pose a specific concern for potential P loss, because managed turfgrass often abuts imper- 10 SportsTurf | December 2013 meable surfaces such as sidewalks, driveways, and curbs, which provide a direct conduit for P transport to storm drains and surface water. Increased recycling of organic waste streams into organic slow-release fertilizers has led to increased availability and popularity of these materials. Many homeowners and professional landscapers use these natural organic slow-release fertilizers to limit the loss of nutrients from lawns through leaching and runoff. Some phosphorus-restriction legislation is considering exempting organic fertilizers based on the premise that risk of P loss is reduced with these materials. However, many natural organic-based fertilizers (particularly manures and municipal biosolids) supply an excess of P when applied at rates to meet plant N needs. When high-P organic fertilizers are applied repeatedly, excess P accumulates in soil, potentially increasing the risk of runoff and leaching loss. The risk of loss of P from natural organic sources depends on the availability as well as the concentration of P in those sources. Although P from organic sources is generally less available to leaching and runoff than synthetic P sources, P availability varies widely by source. Biosolids P tends to be less available than manure P, but even among biosolids sources P availability can vary widely. Understanding the effect of repeated applications of natural organic lawn fertilizers on soil test P can provide guidance for the suitability of these materials in P sensitive areas. If P availability is low enough in organic fertilizers, it could be possible to use them without increasing the risk of water quality degradation. Evidence shows that the risk of soluble P loss occurs at much higher soil test levels than those needed for agronomic sufficiency. Researchers have proposed alternative soil tests to assess environmental risks, such as phosphorus saturation (PSI), dissolved P index, or water extractable P. No environmental soil P test is widely recognized and in common use. Agronomic tests also have some value as environmental indicators. Another factor is the effectiveness of P fertilizers in changing Table 1. Fertilizer products applied to soil and sand root zones at WSU-Puyallup, RL Goss Research Facility in Puyallup, WA, 2008-2011. a Organic 6-7-0 was originally labeled as 5-4-0, but analysis form 2008-2010 showed that it consistently contained 6% N and >7% P2O5. The label was changed to 6-7-0 to reflect that analysis in 2010.

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