Environment Maryland Research and Policy Center
Phosphorus from manure applied to farmland is a major source of pollution in the Chesapeake Bay. Intensive chicken production, particularly on Maryland’s Eastern Shore, generates large volumes of manure. Growers and farmers often spread this manure on their fields as fertilizer, but when applied in excess, the nutrients that make manure useful for fertilizing crops also contribute to dead zones in the bay.
Pollution has taken a heavy toll on the bay.
• A “dead zone” covers a large portion of the bay each summer. Nutrient-fed algae blooms flourish briefly and then die, consuming vast amounts of oxygen as they decay. As a result, levels of dissolved oxygen in the water drop below the concentration needed to support fish, crabs and oysters.
• Algae blooms block sunlight, killing underwater grasses and destroying valuable habitat. The bay has less than half the acreage of underwater grass needed for a healthy ecosystem.
• Nutrient pollution, along with over harvesting and disease, has hastened the decline of major fisheries such as oysters and crabs.
Pollution from agriculture accounts for 41 percent of the phosphorus that enters the bay from Maryland. Manure contains high levels of nitrogen and phosphorus, making it useful as a fertilizer, but the ratio of nitrogen to phosphorus is such
that if manure is applied to meet the nitrogen requirements of a crop, phosphorus is over-applied. This excess phosphorus escapes from farm fields and into nearby waterways.
Large-scale chicken growing on Maryland’s Eastern Shore generates high volumes of manure that contain far more phosphorus than can be used by crops nearby.
• The 296 million broiler chickens raised in Maryland in 2007 generated approximately 550,000 tons of “chicken litter,” manure mixed with bedding and feathers.
• This chicken litter contained far more phosphorus than is required by crops in major chicken-producing counties in Maryland.
• Soil test data from 2002 show that more than 60 percent of soil samples from four Maryland counties—where tens of millions of chickens are raised annually—were saturated with phosphorus. At such high saturation levels, phosphorus is more prone to dissolve in water and pollute the bay.
Maryland’s current rules allow farmers to spread manure on fields where phosphorus is likely to run off and pollute the bay.
• Maryland requires farmers to use a test called the phosphorus site index (P-index) if soil is found to be overloaded with phosphorus. The P-index was meant to be a tool that would allow farmers to evaluate the water pollution risk of applying manure to the field.
• The P-index approach does not address the state’s long-term phosphorus problem, since it allows farmers to use some fields as dumping grounds for excess manure, even if they are already loaded with phosphorus.
• Maryland’s current P-index underestimates the extent to which phosphorus may escape from a field into nearby water bodies through subsurface water flows in some regions of Maryland, especially in the marshy Eastern Shore.
• Phosphorus may also escape from fields that have seemingly safe levels of phosphorus—fields below the P-index threshold—but Maryland’s current rules do little to control this source of pollution
• Application of the P-index appears not to be solving the problem of phosphorus pollution into the bay from agriculture. In at least one major chicken-producing region, water quality has not improved since Maryland adopted its current rules. In the Choptank River, phosphorus levels have risen by an average of 1.9 percent per year from 2000 to 2008. The Choptank drains parts of Caroline, Dorchester, Queen Anne’s and Talbot counties, where large-scale chicken production generates hundreds of
thousands of tons of excess manure and there is relatively little residential development.
Maryland produces far more phosphorus-laden manure than crops in the region can use. The state needs to keep
phosphorus out of the bay, and it needs a long-term solution for ending phosphorus build-up in soil.
• Maryland should end land applications of chicken litter that endanger the health of the bay by replacing the current inadequate rules for phosphorus application with more effective ones. Maryland should phase in standards that prevent more phosphorus from being applied to cropland than crops
• The volume of manure in Maryland that needs to be disposed of through land application must be brought into balance with the amount of nutrients that crops need. Options include processing more manure into pelletized fertilizer and transporting more manure out of the region.
• Any alternative plan for disposing of chicken manure should require poultry producers such as Perdue to take responsibility for the pollution that their activities produce.