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Fertilizing with Biosolids

by Barb Ogg, PhD, UNL Extension Educator

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Modern cropping practices include adding fertilizers to meet the nutrient requirements of crops or to improve unfavorable soils. Increased understanding and use of fertilizers has contributed significantly to the tremendous increases in crop yield in the last century. There are three primary nutrients that are commonly supplied by fertilizers: nitrogen, phosphorus and potassium. Other important nutrients that may be added to deficient soils are sulfur, magnesium, calcium, iron, chlorine, boron, copper, manganese, molybdenum and zinc. 

Nitrogen and Phosphorus

Commercial Nitrogen Fertilizers: Commercial N fertilizer manufacturing plants capture nitrogen from the atmosphere in a chemical process which synthesizes ammonia (NH3). Commercial chemical processes convert ammonia to stable liquid or dry formulations that vary in nitrogen content and may include other nutrients. Even though most N fertilizer formulations are fairly stable, care must be taken to prevent volatilization of nitrogen from ammonia back to the atmosphere. Nitrate (NO3) is the form of nitrogen that plants can most readily use. Nitrates are soluble in water and readily leach into the soil profile with precipitation. Unless managed carefully, considerable losses of nitrogen can occur with commercial fertilizer use.

Organic Sources of Fertilizers: Organic sources of fertilizer include animal manures and biosolids, the processed organic solids from wastewater treatment plants. One advantage of using organic fertilizers is that much of the nitrogen is tied up in large organic molecules that must be converted through chemical processes to forms of nitrogen that the plant can use. Because of this "slow release" chemical process, the nitrogen in biosolids and other manures is more likely to be available during the growing season when plants need it. This is a major reason why organic fertilizers often outperform inorganic sources and produce outstanding crop yields. However, excessive amounts of organic fertilizers can also result in nitrate contamination of surface and groundwaters. 

Mineralization and Nitrification: The chemical processes that convert organic N to ammonia (NH3 -N) is called mineralization. Like other chemical reactions, mineralization rates increase with higher temperatures and moisture levels. Ammonia-N becomes nitrate N through the process of nitrification. The proportion of the total organic N that is estimated to become available to plants through mineralization/nitrification the first year is called the PAN (potentially available nitrogen). Mineralization rate studies using anaerobically digested biosolids suggest that about 15% of the total available nitrogen will become available the first year and lesser amount in subsequent years. Eventually, most of the organic N will mineralize to plant available forms and be used by subsequent crops. Crop producers who use manure or biosolids should understand that nitrogen will become available for several years after an application, and N fertilizer amounts should be adjusted downward to prevent excess nitrate buildup in the soil. 

Phosphorus in Biosolids: Phosphorus is a nutrient needed for plant growth and is found in every living plant call. It plays an essential role in energy transformations in plant cells and is a component of lipids, proteins and metabolic efficiency. Phosphorus deficient plants are stunted, have thin, short stems, purplish leaves and delayed maturity. Phosphorus deficiencies often occur in eroded fields because P does not leach as nitrate does, but stays in the topsoil. It is estimated that 20-30% of Nebraska soils are deficient in phosphorus. Soil tests of fields from our Lancaster County cooperators indicate that nearly 65% of applied fields are low in phosphorus, with Bray-P levels less than 16 ppm.

Other Nutrients in Biosolids: In addition to nitrogen, biosolids have a whole complement of other essential nutrients that are required by plants. Biosolids are also 60-65% organic matter which improves the soil tilth and structure of nearly any soil. The organic matter and complement of essential nutrients in biosolids cannot be duplicated by synthetic fertilizers. Experiments (University of Nebraska-Lincoln, Department of Agronomy) using Lincoln's Theresa Street biosolids show that yields average 5% greater when biosolids are used compared with inorganic sources of N-fertilizer plus phosphorus and zinc equivalencies. Additional experiments, currently in progress, will give important information about the multi-year crop response and economic value that farmers can expect using biosolids.  

Four-Year Biosolids Study.

From 1993 - 1996, an on-farm research study was conducted on a no-till field at Dave and Wayne Nielsen's farm. This study was part of the Nebraska Soybean and Feed Grains Profitability Project, a UNL Extension Program. The objective of this study was to determine and document the profitability of biosolids versus anhydrous ammonia fertilizer as a nutrient source. Yields of four subsequent crops were evaluated from a single biosolids application.

In a replicated experiment, biosolids and anhydrous ammonia were applied prior to the 1993 crop. Approximately 45 Tons per acre of biosolids were applied to the biosolids plots and not incorporated to this no-till field. Anhydrous ammonia was applied to the remaining plots at the rate of 120 lbs per acre. The crops grown in rotation after the biosolids application were corn, sorghum, soybeans and wheat. The results are shown in the following table.

Moisture (%)
Test Weight (lb/bu)
60.3 (NS)
Yield (bu/a)

* =significantly different at 90% confidence interval
** =significantly different at 95% confidence interval
***=significantly different at 99% confidence interval
NS =not significantly different

Results: Crop yields were significantly increased for all four subsequent years after the biosolids application in 1993. Results also showed that, in three of the four years, the biosolids treatment influenced grain moisture at harvest which may be an indicator of the physiological maturity of the crop. The test weight (pounds per bushel) significantly increased in the 1993 corn crop, but significant differences were not found in later crops.

What could have contributed to the increased yields? This rolling, upland field had low phosphorus levels (9.2 ppm Bray P), and the extra P contained in the biosolids may have played a significant factor in the increased yields. No attempt was made to add equivalent nutrients contained in the biosolids to the anhydrous plots, although a starter fertilizer (11N-52P-0K) was applied to all plots at wheat planting. Although physiological development of the crop was not measured, it was observed that plants fertilized with biosolids plots grew faster in the early season and, by the end of the season, were more mature than the plants in the anhydrous treatment. (Data courtesy Keith Glewen, Dave Varner, Ed Penas, Earle Raun, Soybean & Feed Grains Profitability Project, UNL Extension.) 

Environmental Concerns and Fertilizer Use: Environmental and health problems can result from excessive nitrogen fertilizer use. Water soluble nitrates moving into surface waters and groundwater pose a threat to the health of humans that depend on those water sources. The U.S. Public Health Service has set the drinking water standard to be 10 ppm nitrate-N. Levels above 10 ppm nitrate-N can be harmful to infants because nitrate oxidizes iron in the hemoglobin of red blood cells to form methemoglobin which lacks the oxygen-carrying ability of hemoglobin. This results in a condition called methemoglobinemia in which the blood cannot carry sufficient oxygen to individual body cells. Infants are particularly susceptible to this condition (called "blue baby syndrome") because their immature systems lack enzyme systems which convert methemoglobin back to oxyhemoglobin. Healthy older children and adults may not the adverse health problems that infants under six months of age do, although little is known about the long term health consequences of drinking high nitrate water.

Even though nitrate leaching can occur naturally in soils, it can occur from excessive use of nitrogen fertilizers. Farmers who regularly test their soil profile to determine residual nitrate levels and apply recommended rates will prevent excessive nitrates in the soil and still provide adequate nutrients for crops. Many of the soils in Lancaster County are deficient in phosphorus, and this is a major reason for biosolids use by cooperating farmers. Unlike nitrates, phosphorus is not water soluble and stays in the top few inches of the soil. However, phosphorus can move when soil is eroded. Phosphorus, moving with sediment, can promote the growth of algae in surface waters which depletes oxygen and kills fish. This is called eutrophication. It is especially important for farmers in the Biosolids Land Application Program to construct terraces, filter strips and other soil conservation measures to prevent loss of phosphorus into surface waters.

Scientists have become increasingly concerned about a large area of oxygen-depleted waters that develops seasonally each year in the Gulf of Mexico, near the mouth of the Mississippi River. This phenomenon is called "hypoxia", referring to water that has dissolved oxygen concentrations of less than 2 parts per million which is the limit for the survival and reproduction of most aquatic life. Many scientists believe that nutrient loading from the Mississippi River is the cause of this hypoxia. 

Some people are concerned about possible environmental or health consequences due to heavy metals in biosolids. Numerous studies have shown that heavy metals are relatively immobile in soils and remain in the topsoil. Again, it is particularly important for cooperators to use soil conservation measures to prevent soil erosion. In addition, studies have clearly shown that, when metals concentrations are within EPA guidelines, crops do not accumulate metals in amounts sufficient to be harmful. For more information about EPA regulations and heavy metals concentrations, check out Regulations for Safe Use

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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