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  • Publication
    Managing legacy soil phosphorus in grassland soils for agricultural productivity and environmental quality: a review
    Phosphorus (P) is a lithophile element that tends to accumulate in the solid phase at the Earth’s surface and has a low water solubility. As P is a limiting nutrient for plant growth in most terrestrial systems, P in fertilizers has been a major factor underpinning global agricultural production in the 20th and early 21st centuries, including that from grassland. However, P is a costly farm input and it is also a finite mineral resource. Best agronomic practice is to maintain soil P levels at optimum over the medium-to-long term by managing P application and offtake. However, in some cases, soil P levels have been built up in excess of agronomic optimum due to P application driven by organic “waste disposal” or with the intention of building up a “bank” of soil P for future use. This has been associated with P losses to surface waters and impacts on water quality. Legislation, policy and best management practice advice in many countries has attempted to affect these legacy high P soils through a range of measures. In Ireland, for example, the Good Agricultural Practice measures, introduced in 2006 under the Irish Nitrates Action Plan, attempt to impose P deficits on soils with high P. National data shows that P fertilizer use declined by 55% on grassland soils between 2003 and 2008 and would suggest that soils with high soil P levels dropped from 30% in 2007 to 22% in 2011. This paper presents a review of the international literature on legacy excessive P in grassland soils, management practices and policy measures to manage them, and changes in soil P in response to such measures. Consideration is given to both agronomic and environmental concerns. There are a number of factors in grassland production systems, and particularly dairy production systems based on grazed grass, that differ from other agricultural production systems. For example, offtakes are typically lower than in tillage and the recycling of P, either by animal deposition or spreading of manures, gives less control to the farmer. Important questions addressed include: how quickly do grassland soil P levels decline under situations of negative P balance?; what fractions of P control soil P decline?; what grassland management practices are important in determining where and how fast soil P levels decline?; and what scale is appropriate to implement practice change and monitor effects?
  • Publication
    Managing Legacy Soil Phosphorus to Sustain Agriculture and Protect Water Quality
    A central tenet of modern nutrient management planning is the need to maintain soil phosphorus (P) in a range that optimizes crop production and protects water quality. Decades of research have identified the soil test P (STP) critical values needed for economically optimum crop yields, leading to well-established recommendations for efficient use of inorganic and organic P sources as soil amendments. However, in many areas of the USA and other countries, long-term over-application of animal manures and fertilizers has led to soil P accumulations to values that are considerably above agronomic optima and of concern for surface water quality. These soil P accumulations are a legacy of historically inefficient P management and present serious challenges to our efforts today to prevent nonpoint P pollution of surface waters. The fundamental issue identified in most research has been that it can take years, even decades, to decrease soil P values from “excessive” to “optimum”. Thus, even if P inputs to “high P” soils are restricted or eliminated, environmentally significant P losses to water may continue. For example, in Delaware, statewide summaries show ~60% of soils tested have STP values more than twice the critical value (~30 mg P kg-1 , Mehlich 3); in the intensive poultry producing regions, >30% of soils have STP values more than six times the critical value. A recent long-term (11 yr) cropping (corn-soy) study we conducted at two sites with initial Mehlich 3 P values of 98 and 70 mg kg-1 found that ceasing P applications decreased STP by 43% and 27%, with no negative effects on crop yields, providing guidance for emerging strategies for management of “high P” soils. Although similar long-term studies are somewhat rare, we have analyzed the findings of > 25 studies from the US and Europe investigating the relationship between P management, cropping system, and changes in amount and form of P in “high P” soils. Our presentation summarizes the findings of long-term P depletion studies in Delaware and our quantitative analyses of similar studies conducted in settings varying in initial STP, soil type, cropping system, and climate. We present strategies and policies to address the “legacy” P issue in overfertilized soils, toward a goal of sustaining soil P values in ranges optimum for crop production and protection of water quality.