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Fluid Journal : Fluid Journal 1996-1998
1 Fluid Journal Fall 1998 Summary: Phosphorus lost in runoff from farmland and pastures may contribute to the eutrophication of lakes, ponds, streams, and other bodies of water. Loss of P is most likely where excessive amounts of P have been applied over long periods, or where P applications have been poorly managed. The EPA has identified agriculture as the leading source of water quality impairment in U.S. streams and lakes. Significant losses of P are most often the result of mismanagement of manure P, although mismanagement of fertilizer P may also contribute. The eutrophication of P sensitive surface waters can be accelerated by P in runoff water from agricultural land. The problem is often aggravated by proximity of streams and lakes to potential P sources. Phosphorus is lost from agricultural land in particulate and dissolved forms (Figure 1). The loss of P in dissolved forms may occur through surface runoff water or, in special cases, through leaching. The concentration of soluble P in water moving through the soil profile is usually low due to sorption of P by P -deficient subsoils. However, leaching losses may occur in soils with unusually high soil P levels resulting from excessive manure or fertilizer application, organic soils, or deep sandy soils with little sorption capacity. Corrective strategies Reducing dissolved P loss from leaching involves implementing practices that maximize crop P uptake and minimize input in excess of crop needs, reducing leaching by disrupting macropore continuity through tillage, or removing P from surface ditches after field discharge has occurred. Loss of P in particulate forms is usually associated with the erosion of soil mineral or organic particles. Particulate loss from erosion accounts for about 60 to 90 percent of P loss from cultivated land. We'll look at some proven strategies to reduce such P loss on cultivated lands. by Drs. W.M. Stewart, K.A. Janssen, and K. Polizotto What About P Loss? Researchers have and are developing strategies for keeping phosphorus in place. Management is the key. Vegetative buffer strips are an effective means of reducing surface runoff volume, trapping sediment and restricting the transfer of nutrients and pesticides to lakes and streams. Buffer strips tend to increase infiltration of water and dissolved constituents within the strip. Filter strips may also increase retention of sediment and associated adsorbed compounds by filtration and sedimentation, as well as retention of soluble constituents by sorption onto organic matter and vegetation. In a recent field study that used ryegrass filter strips of various widths, runoff water volume was reduced as much as 99.9 percent when compared to a control. The grass strips also reduced sediment and soluble P surface runoff by as much as 100 and 89 percent, respectively (Table 1). In another study, sediment retention from bromegrass strips ranged from 40 to 100 percent. Effectiveness of filter strips is usually reduced as runoff velocity increases. Reducing P solubility from manure applied to agricultural land may result in reduced dissolved P losses in runoff, and through leaching. Research in Arkansas has shown that treating poultry litter with amendments such as alum (A12[SO4]3) or slaked lime (Ca[OH]2) can reduce P solubility and ammonia volatilization by several orders of magnitude. Several compounds were tested for their ability to reduce soluble P levels in poultry litter. About 10 percent of P from this litter was water soluble. Water soluble P levels were reduced from greater than Table 1. Effect of grass filter strips on runoff volume, sediment, soluble P, and nitrate loss, Patty, et al., 1997. Site 1 Site 2 Site 3 Strip width(m) 0 6 12 18 061218061218 Runoff vol. (liter) 48027522030 4577312.3535713980 Sediment (mg) 20300 49354.43092985 Soluble P (mg) 2817223 4969268264562829 Nitrate (mg) 2958 1562 924 33 2460 377 62 .03 2577 365 139 88
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