Sewage Sludge as Fertilizer

It is very difficult for farmers to be good stewards of Chesapeake Bay. Chemical fertilization is not 100% efficient, and it has been known since the Corps of Engineers study of 1977 that agricultural practices are a major polluter of Chesapeake Bay. Farmers cannot control the weather or commodity prices and simply seek to make a decent living off the land. Sewage sludge is commonly touted as “free fertilizer” and some farmers accept it in hopes of improving their bottom line. In fact, all forms of animal waste, including poultry, diary and swine manure, and municipal sewage sludge (“sanitized” by use of the word “biosolids”), are inefficient forms of fertilizer, guaranteeing considerable groundwater and surface water pollution by both nitrogen (N) and phosphorus (P).

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. For lime-stabilized Class B sludge, the most common material spread in the Northern Neck, 30% of the nitrogen is available to plants the first year, 15% the second and 7 % the third. Summing, 30 + 15 + 7 = 52, meaning that 48%, nearly half, of the nitrogen is not used by the crop. 40% of the nitrogen in poultry manure is not used by the crop (Table 5-3). Most of the nitrogen not used by the crop will be oxidized to nitrate and enter the groundwater or contribute to runoff, and then discharge to a nearby waterway or stream. If land application, say for corn requiring 120 pounds of nitrogen per acre, takes place in spring, sludge is applied at a rate of 400 (400 * 0.3 = 120) pounds of nitrogen per acre. Mineralization (release of inorganic nitrogen and phosphorus from the organic material in the sludge) will continue after the crop has matured and is drying in the field, expedited by high summer temperatures. Nutrient pollution is guaranteed as nitrate and phosphate are released from the sludge by microbes but not used by the crop. Contrast this inefficient form of fertilization with conventional application of chemical fertilizer, ideally split application to row crops 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.

A recent publication, Agriculture and Phosphorus Management: The Chesapeake Bay (A. N. Sharpley, ed., 1999) states “…much of the crop land in the Chesapeake Bay watershed is now considered “optimum” or “excessive” in phosphorus from an agricultural perspective and hence needs little additional phosphorus, from any source, to ensure that economically optimum crop yields are attained (p. 66).” All forms of animal waste are phosphorus rich. Sewage sludge commonly contains about 2/3 as much phosphorus as nitrogen and in poultry manure the amounts of nitrogen and phosphorus are similar. Sewage sludge applied at a rate of 400 pounds of nitrogen per acre would add over 250 pounds of phosphorus per acre. For “Soil Management Group IIA, a common soil type in the Northern Neck, the following table summarizes phosphorus recommendations (from DCR documents) for corn, soybeans and wheat:

Soil Test Level (ppm P)
P Recommendation (lbs/acre
Low (0-6) 80-120
Medium (6-18) 40-80
High (18-55) 20-40
Very High (55+) 0

For soils typical of the Northern Neck that already are High or Very High in phosphorus, the massive over-application of phosphorus is obvious.

Because of its low solubility, little phosphorus enters the groundwater (unlike nitrogen), and most blatant phosphorus pollution takes place as the result of runoff, erosion and accompanying loss of soil. High phosphorus in soil particles makes Best Management Processes that prevent soil from entering waterways especially critical and in need of strict application and enforcement. In the long term, however, even though the solubility of phosphorus is low, excess phosphorus will be “weathered” out of high-phosphorus soils and enter the groundwater or surface water. The current practice of ignoring phosphorus in determining animal waste application rates guarantees low-level phosphorus pollution of our waterways and Chesapeake Bay far into the future. Virginia Department of Health (VDH) biosolids regulations state (12VAC 5-585-550A) “The applied nitrogen and phosphorus content shall be limited to amounts established to support crop growth.” If this law was being enforced, sewage sludge could not be applied to most local soils. Sludge obviously is being applied, in violation of common sense and, in the case of phosphorus, the law. Why does this practice continue? Because, as VDH told the Board of Health, more restrictions would impose “…additional costs on the generators, appliers, and users of biosolids.” Never mind local waterways and Chesapeake Bay.

Farmers who choose to use sewage sludge instead of conventional fertilizer must be aware of their role in exacerbating nutrient pollution of Chesapeake Bay. They are violating the common-sense premise “if the crop doesn’t need it, don’t apply it.” Sludge may be free fertilizer for the farmer, but it is also free landfill for the generators and appliers. Farmers also need to recognize that there are other things in municipal sewage sludge that they are getting for free, like heavy metals, pathogens and pharmaceuticals.

Previous NAPS Stewardship Tips have addressed the sewage sludge issue, including high nitrate concentrations in local groundwater and the potential for gulls to transport pathogens to local waterways, many already restricting for the harvesting of shellfish because of high bacterial levels.

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