Science Reconsiders the Value of Land Application

New guide offers additional management practices to ensure farm productivity in the northeastern United States

The Guidelines for Application of Sewage Biosolids to Agricultural Lands in the Northeastern U. S.offers the best science available and seeks to protect the land resource base, emphasizing agricultural productivity in perpetuity. Scientists from Cornell, Rutgers, the University of Massachusetts, Penn State, and the University of New Hampshire developed the document, which is an updated version of a 1985 document, Criteria and Recommendations for Land Application of Sewage Sludge in the Northeast.

More than 20 years later, new scientific evidence and regulatory policy have emerged, and the use of sewage biosolids on agricultural soils has expanded. The multistate research committee of faculty from land grant universities convened in 2000 to reconsider the topic. Guidelines was published by Rutgers University this year.

It’s a matter of soil
The range of soil conditions, farming practices, and demographic factors in the Northeast led the scientists to conclude that the Code of Federal Regulations Section 40 Part 503 may not be adequately protective under all agricultural use scenarios in this region. The purpose of the new guidelines is to inform potential users of sewage biosolids (including landowners, farmers, and their advisers) of situations in which additional management practices beyond current regulations can be implemented to promote sustained farmland productivity and to better protect public health and the environment.

Soil conditions, farming systems, and the population density of the Northeast dictate a cautionary approach to land spreading of sewage biosolids. Relatively acidic, shallow, low-organic matter soils are common. The chemistry and hydrology of these soils contrast sharply with the deep, high-organic matter mollisols of the Corn Belt in the Midwest or the calcareous aridisols of the West, for example. Northeast soils have been shown to have lower heavy metal sorption capacities than do soils with higher pH or more organic matter. This situation places the Northeast in a position of having some soils that are less suitable for biosolids application.

Dairy farming is the primary agricultural system in most Northeast states. On many dairy farms, applications of farm manure supply more nutrients than required for on-farm crop production (due to feed importation), resulting in the potential for excess nutrients polluting groundwater and surface waters. The addition of sewage biosolids in such systems may exacerbate this nutrient imbalance. Certain elements in sewage biosolids (such as sulfur and molybdenum) have the potential to result in copper deficiency in cattle (as well as sheep and goats) if there are dietary imbalances.

Fruit and vegetable production is another important agricultural system and both are sensitive commodities with regard to public perception. A decision to land-apply sewage biosolids on a fruit or vegetable crop can directly affect a farmer’s livelihood, because some supermarket chains, food processors, and wholesalers do not accept produce from farms where sewage biosolids have been used. A summary of the guideline recommendations follows, with fuller explanation and references available on the Internet at

Metal Recommended maximum soil concentration (mg/kg)1
  Sand & laomy sand Sandy loam to silt loam Silt to Clay
Iron (Pb)2
Zinc (Zn)3
1 The values in this table represent total elemental concentrations. The recommendations apply to soils maintained at pH of 6 or greater.
2 This is the plant health Ecological Soil Screening Level established by the U.S. Environmental Protection Agency.
3 Limit to prevent phytotoxicity to less than 10 percent yield reduction. Higher concentrations can be tolerated in calcareous soils with pH greater than 7.

Guidelines summary
These guidelines refer to bulk application of both “exceptional quality” (EQ) and non-EQ sewage biosolids.

1. Any farm considering the application of sewage biosolids should construct a farm-scale nutrient balance to ensure a demonstrated need for the additional nutrients.

2. Apply the highest quality sewage biosolids available. Suggested maximum concentrations (expressed as parts per million, ppm) are 18 ppm arsenic, 8 ppm cadmium, 160 ppm chromium, 1,100 ppm copper, 180 ppm lead, 3 ppm mercury, 33 ppm molybdenum, 70 ppm nickel, 15 ppm selenium, and 1,500 ppm zinc, on a dry weight basis. These are approximately the 95th percentile values for several different data sets representing northeastern U.S. sewage biosolids as of 2005. These are not risk-based values or regulatory limits but rather provide reference points to determine general quality of the material.

3. Apply material that has consistent quality. Nitrogen concentrations in sewage biosolids from one wastewater treatment plant can be highly variable, making accurate calculation of appropriate agronomic application rates difficult. Some wastewater treatment plants produce sewage biosolids with relatively low trace element concentrations in comparison to other facilities. The best strategy is to choose sewage biosolids from facilities that have consistent nutrient and trace element concentrations and also low trace element concentrations. For trace elements, a rolling average, including at least four analyses per year, is recommended for sewage biosolids from each wastewater treatment plant source. Seasonal and operational variations need to be taken into account.

4. Maximum recommended, cumulative soil trace-element concentration limits for sites to which sewage biosolids are applied are intended to address and protect the agricultural productivity under soil conditions and for farming practices and demographics, some of which are unique to the Northeast (see table).

5. Farmers should test soils before application and again when it is estimated that the soil trace element concentrations have reached approximately one half of the recommended maximum soil concentration.

6. The potential for induced copper deficiency in ruminant animals due to molybdenum, iron, cadmium, and sulfur in sewage biosolids requires careful consideration of livestock dietary intakes.

7. Alkaline-stabilized sewage biosolids should not be applied at rates greater than the soil liming requirements, and the nutrients present in the product (nitrogen, phosphorus) must be accounted for.

8. Wherever possible, sewage biosolids should be incorporated (injected into, mixed into, or turned under the surface) in the soil within 24 hours of application. When stockpiling material, caution is advised to minimize odor and leaching problems.

9. Care is needed to spread sewage biosolids uniformly and avoid creating “hot spots” in a field where the material is over-applied locally.

10. Soil pH of 6 or above should be maintained as long as the land to which sewage biosolids have been applied is used for crop production.

11. Use of sewage biosolids on soils used to grow vegetable or fruit crops is not recommended.

12. Farmers should keep records of the source, quantity, and quality of materials applied. Records should be kept of when, how, and by whom sewage biosolids are applied, as well as any concerns (such as odors) noted during application.

13. Farmers using sewage biosolids may wish to obtain written assurance from the supplier that any sewage biosolids being land applied are of appropriate quality (pollutants, pathogen reduction, vector attraction reduction) and have been properly treated and that the application procedures meet federal and state regulations.

The table contains the recommended maximum soil elemental concentrations because these can be measured at any point and are what the plants respond to. Although only a portion of the total trace elements in the soil are bioavailable, the research on which these recommendations are based was developed primarily using total trace element data, and thus the relative bioavailability already was accounted for in these data.

Based on the typical concentrations of various trace elements in sewage biosolids, copper is frequently the element that would first reach recommended maximum soil concentrations and thus limit cumulative application. Molybdenum may be the limiting element in some situations.

If sewage biosolids containing 548 mg copper/ kg of sewage biosolids (which is approximately the median copper concentration for sewage biosolids in the Northeast) are used, the guidelines suggest that the total quantities of sewage biosolids that could be applied (before reaching the recommended maximum) range from approximately 79 dry tons/acre for a coarse textured soil to 186 tons for fine textured soils. These quantities drop to between 40 and 93 tons/acre, respectively, if the sewage biosolids applied have a higher copper concentration of 1100 mg/kg (which is approximately the 95th percentile concentration for copper in the Northeast).

This article originally appeared in the 10/01/2007 issue of Environmental Protection.

About the Author

Ellen Z. Harrison is director of the Cornell Waste Management Institute.

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