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The Truth About Sludge as "Free Fertilizer"

(Published December 2004)

Sewage sludge and poultry litter are commonly touted as “free fertilizer.” In fact, all forms of animal waste are highly inefficient forms of fertilization, guaranteeing considerable groundwater and surface water pollution.

The Department of Conservation and Recreation (DCR) is responsible for nonpoint source pollution in Virginia. DCR’s 1995 “Virginia Nutrient Management Standards and Criteria” lists “Estimated Nitrogen Mineralization Rates for Biosolids” in Table 6-1 (p. 64). Mineralization is the release of inorganic nitrogen and phosphorus from the organic animal waste by microbial activity. In the case of lime-stabilized sewage sludge, Table 6-1 indicates that 30% of the applied nitrogen is available to the crop the first year, 15% the second year and 7% the third year. If corn, requiring 120 ponds of nitrogen per acre, is being grown, sewage sludge would be applied at a rate of 400 pounds of nitrogen per acre (400 * 0.3 = 120). It is assumed that the farmer reduces application of chemical nitrogen fertilizer by 15% the second year and by 7% the third year. Presuming that these regulations are followed, approximately half [100 – (30 + 15 + 7) = 48%] the nitrogen in lime-stabilized sludge is not used by crops. Most of the nitrogen not consumed by crops will be oxidized to nitrate and enter the groundwater or contribute to runoff. The United States Geological Survey estimates that half the 50 billion gallons of water that reaches Chesapeake Bay each day is groundwater, discharged underground directly to rivers and waterways. We know that groundwater today typically contains high concentrations of nitrate from agricultural and homeowner practices, so the excess nitrogen from the sludge constitutes additional pollution. Land application designed to fertilize a spring crop will result in mineralization that continues long after the crop has matured and is drying in the field. Late summer temperatures, especially if there is abundant rainfall, will expedite mineralization. High nitrate pollution the summer after land application is guaranteed as nitrate and phosphate are released from the sludge by microbes but not taken up by the crop. Contrast this inefficient form of fertilization with conventional application, ideally split application to row crops, especially in environmentally sensitive soils as DCR recommends. Conventional fertilization minimizes the amount of fertilizer applied to times when it is used by the crop, thus reducing loss to the environment as much as possible. Animal waste fertilization, in stark contrast, is inefficient and results in extensive nitrogen pollution.

All forms of animal waste are rich in phosphorus (P). Only small amounts of phosphorus enter the groundwater (unlike nitrogen), and most phosphorus pollution takes place as the result of runoff, especially if soil is lost. For common soil types, the following table summarizes phosphorus recommendations (from DCR documents) for corn, soybeans and wheat:

Soil Test Level (ppm P)      P recommendations (pounds P/acre)-

        Low             (0 - 6)                  80 - 120

        Medium    (6 - 18)                  40 - 80

        High           (18 - 55)                  20 - 40

        Very High   (55+)                       0

Most soils in the Chesapeake Bay watershed are already High or Very High in phosphorus according to soil tests, and already contain sufficient phosphorus to support crop growth. If VDH regulations were being enforced (12VAC5-585-550.A “The applied nitrogen and phosphorous content of biosolids shall be limited to amounts established to support crop growth.”) sewage sludge could not be applied to most soils. Rates of land application are currently regulated, despite the law quoted above, only by the nitrogen needs of the crop. Most of the excess phosphorus added to the soil as the result of land application of animal wastes is not released to the groundwater as rapidly as is nitrogen, but “banking” phosphorus in the soil guarantees slow long-term release and makes catastrophic loss of P-laden soil much more likely. The phosphorus content of typical sewage sludge is about 2/3 the nitrogen content. Land application of sewage sludge at 400 pounds of nitrogen per acre to meet the nitrogen needs of corn with an expected yield of 120 bushels/acre (it is convenient that about one pound of nitrogen per acre is needed to grow a bushel of corn per acre) will result in application of about 250 pounds of phosphorus per acre. Note that this massive application of phosphorus exceeds any conceivable crop need. Excess phosphorus in most Virginia soils makes Best Management Processes that prevent runoff from fields, and prevent soil from entering waterways, especially critical and in need of strict enforcement.

Farmers who choose to use sewage sludge or poultry litter instead of conventional fertilizer must recognize the inefficiency of this form of fertilization and their role in exacerbating nutrient pollution of Chesapeake Bay. They must recognize that their fields are being used as landfills to dispose of unwanted animal waste in the guise of “free fertilizer.”


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