Sewage Sludge vs. Nitrogen Fertilizer (research)

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Sewage Sludge vs. Nitrogen Fertilizer

Researchers: D.L. Binder and D.H. Sander
University of Nebraska-Lincoln Department of Agronomy

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Organic amendments like human and animal manure have always been useful as a nutrient source for crop production. Our country's founding father, George Washington, a farmer, knew this as well as anyone. He had manure from his barns and outhouses collected and piled in an area that was surrounded by a stone wall. The wall was built to keep the manure from washing down the hill and in to the Potomac River. 

Would George have put as much value on the manure if he had the convenience of inorganic fertilizer delivered to his farms? As recent as fifty years ago farmers didn't have to deal with that decision because inorganic fertilizers were not readily available. Today farmers are being asked to consider organic amendments as alternative nutrient sources. This is largely because livestock facilities that feed thousands of cattle and hogs, millions of chickens and cities where hundreds of thousands of people live do not have enough land available to properly apply the manure. 

On most farms, the distance animal manure can be hauled economically determines how it is applied. This is not the case for human manure mainly due to the regulations municipalities must operate under. In Nebraska, the most economical method of handling sewage sludge is usually either long-term storage in lagoons for small villages. For larger town and cities, some type of digestion process is used before it is land applied.

Research Study

In cooperation with the City of Lincoln, Public Works/Utilities Department, a study was started in 1995, to determine the amount of Theresa Street biosolids required to replace N fertilizer. Locations of plots consisted of six different farms in Lancaster county where different rates of sewage sludge and N fertilizer (34-0-0) was applied. Two sites were irrigated corn, one was dryland corn and the other three were dryland sorghum. The sludge and N fertilizer were incorporated by discing to minimize ammonia-N losses. Other nutrients, mainly phosphorus and zinc, were applied to the entire plot areas in an attempt to make N the only limiting nutrient.

1995 nitrogen rate

In 1995, crop response was limited by lack of rainfall at Sites 2 and 3 and by an early freeze at site three. The maximum grain yield at all three sites in 1995 were similar for both N sources (inorganic fertilizer and sewage sludge) (Fig. 1). Nutrients in organic amendments must be broken down and converted into available nutrients (mineralized) before a yield response is observed. Since temperature and moisture determine the rate of decomposition, it is likely that more sludge was required to maximize yields due to the dry year than it would have taken in a year with average rainfall. This is demonstrated by sludge being worth 0.5 lb of N fertilizer more per ton on the irrigated corn site compared to the dryland corn site (see Table 1, below). 

1996 rate

In 1996, there was little response to N fertilizer at all three sites. However the irrigated corn (Site 4) and dryland sorghum (Site 6) receiving high rates of sludge yielded more than that of N fertilizer. This increased yield above that of N fertilizer was possibly the result of other nutrients in the sludge. 

Table 1. Nitrogen fertilizer equivalency of sewage sludge based on corn and sorghum grain yields in 1995. 
Amount of N Fertilizer Required to Produce Equivalent Yield For
Sludge Rate Irrigated Corn Dryland Corn Dryland Sorghum
Tons per acre -------------- lbs. N per acre ----------------
5 12 18 12
10 36 36 30
15 63 49 37
20 94 47 26
  lbs. N per ton
Average 3.7 3.2 2.3

Fertilizer value: On average, in 1995, a ton of sludge was worth 3.7, 3.2 and 2.3 lbs of N fertilizer for irrigated corn, dryland corn and dryland sorghum. Sludge was worth more than N fertilizer on two of three sites in 1996. This study is continuing to better determine the value of sludge in terms of N availability during the current year of application as well as in succeeding years.

(Findings summarized for this site by Barb Ogg, PhD, Extension Educator, University of Nebraska-Lincoln Extension in Lancaster County)

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PHOTO Credit: Vicki Jedlicka

Lincoln's biosolids recycling program is a joint collaboration between the City of Lincoln, Public Works and Utilities Department and University of Nebraska-Lincoln Extension in Lancaster County with assistance from the University of Nebraska Agronomy Department, Lincoln/Lancaster County Health Department and the Natural Resources Conservation Service.

Contact Information

University of Nebraska-Lincoln in Lancaster County
Barb Ogg or David Smith
Web site:
444 Cherrycreek Road, Suite A, Lincoln, NE 68528 | 402-441-7180