Updating manure N and P credits: A growth chamber study

For a long time, farmers have realized the benefits of using manure as a nutrient source.  The ratio of various nutrients in manure, however, rarely matches the exact plant needs. Consequently, farmers must choose between overapplying some nutrients, or underapplying others and meeting the remaining needs with commercial fertilizers. Figuring out nitrogen (N) credits can be a difficult task since the total amount of N found in manure is not fully available the year of application, nor even after the second year of application. In addition, understanding P availability in manure is necessary because excess P can ultimately lead to eutrophication of surface waters. The amount of N that is available will depend on several factors such as animal species, bedding (if any), manure storage, and application method. We assume approximately 80% of the total manure P is available the first year, but even this can vary depending on soil texture, manure chemistry, and weather conditions. Current University of Minnesota recommendations help determine N and P credits for a variety of manures (Hernandez and Schmitt 2012). These recommendations were developed several decades ago and need an update since the diets of animals, storage of manures, and manure application equipment have changed over the years. Therefore, the primary purpose of this study is to estimate N and P mineralization from a variety of manures and soil types across different temperature regime. Our goal is to verify and/or update N and P credit recommendations from manure so that farmers are able to make better decisions when purchasing and applying commercial fertilizers in following years.  

What did we do?

Laboratory incubations were used to assess N and P release characteristics from a variety of manures in several different soil types. The incubation studies were a complete factorial with 4 replications and with manure type, soil type, and temperature as the main factors. We also included a control treatment that did not include any manure application to see how much nitrogen and phosphorus mineralized from the soils themselves. We tested 8 manures, including: dairy liquid (separated and raw [non-separated]), swine liquid (from a finishing house and a sow barn), beef manure (solid bedded pack and liquid from a deep pit), and poultry (turkey litter and chicken layer manure). Manure analyses to determine nutrient content were conducted on all samples prior to incubations. Soils for the incubations included a coarse textured soil from the Sand Plain Research Center at Becker, MN; a medium textured soil from a research field near Rochester, MN; and a fine textured soil from the West Central Research and Outreach Center in Morris, MN. Soils were collected from the top six inches of soil at each location in bulk and then air dried, ground down to pass a 2-mm sieve, and analyzed for nutrient and organic matter content.  

One liter clear glass canning jars were filled with 200 g of sieved soil and were kept at 60% of field capacity which was maintained by weighing every 4-6 days and adding deionized water as needed to replace the weight lost. We used the University of Minnesota guidelines and manure analysis results to calculate the appropriate application rate for each manure type. During the incubation study, the temperature inside the incubator was kept at either 25⁰C (77⁰F). We collected subsamples at 0, 7, 14, 28, and 56 days after the experiment had begun. Subsamples were destructively analyzed for potassium chloride extractable ammonium and nitrate and Bray-1 or Olsen extractable phosphate. Figure 1 shows the schematics of our experimental set-up and components.  

Figure 1: Growth chamber incubation study experimental set-up.
Figure 1: Growth chamber incubation study experimental set-up.

What have we learned?

At the time of writing, the experiment has only been run at one temperature, 25⁰C (77⁰F) and subsamples for days 0-28 have been collected. Ammonium and nitrate have been analyzed for subsamples for days 0-14. The remaining treatments will be completed later in 2019. Statistical analyses have not been conducted at this time.

The results of the initial soil and manure tests can be found in Tables 1 and 2, respectively. This will give an idea of the starting conditions of the soils and manures. For visual reference, Figure 2 shows the inorganic N (ammonium + nitrate) from each treatment from days 0-14 for the incubation at 25⁰C. The control samples showed that more inorganic N was present in the medium textured soil than the other soils. In general, the swine manure from both finisher and sow barns released the most inorganic N compared with other manures. Of the beef manures, the liquid deep pit manure tended to release more inorganic N than the bedded pack manure, likely due to the lack of bedding to tie up nitrogen. Of the dairy manures, the raw and liquid separated tended to release inorganic N similarly, except in the medium textured soil where the liquid separated manure released more inorganic N. Across soil types, the inorganic N release tended to be stable in the coarse textured soil, while in the medium and fine textured soil, it appears to have increased initially then slowly decreased. It is unclear why this may have happened but could be due to volatilization of ammonium, denitrification of nitrate, or immobilization of N into organic forms. More tests are needed and will be completed later in 2019.

Table 1. Initial characteristics of three soil types used in this study: coarse textured soil from Becker, MN; medium textured soil from Rochester, MN; and a fine-textured soil from Morris, MN.
Soil Characteristics Soil Textural Class
Coarse Medium Fine
Organic matter (%) 1.1 1.0 3.3
pH 5.1 5.2 7.9
Phosphorus – Olsen (ppm) 11 8 7
Potassium (ppm) 95 101 140
Magnesium (ppm) 42 49 570
Calcium (ppm) 274 310 3482
Ammonium (ppm) 3.4 2.8 8.6
Nitrate (lb/acre) 3.0 2.5 8.5
Table 2. Initial characteristics of eight manure types used in this study. The units of nutrients are in pounds per ton for solid manure and in pounds per 1000 gallons for liquid manure.
Species Type Manure Type Moisture Total N Ammonium-N Total P (as P2O5) Total K (as K2O) C:N Ratio
(%) (lbs per unit) (lbs per unit) (lbs per unit) (lbs per unit)
Beef Bedded Pack, Solid 60.5 13.43 2.37 9.59 18.01 22:1
Deep Pit, Liquid 86.6 56.72 36.7 23.43 30.83 9:1
Dairy Separated, Liquid 93.2 32.7 15.8 13.31 29.26 7:1
Raw, Liquid 88.9 33.17 15.66 13.08 31.29 13:1
Swine Finisher, Liquid 86.8 59.16 41.63 37.63 27.35 9:1
Sow, Liquid 99.3 16.5 15.69 1.38 11.34 1:1
Poultry Chicken Layer, Solid 48.6 55.51 14.39 35.78 25.91 7:1
Turkey Litter, Solid 53.0 28.2 13.16 26.69 28.65 12:1
Figure 2. The amount of inorganic-N (the sum of ammonium-N + nitrate-N) in soil mixed with various manure types in: a. coarse textured soil from Becker, MN; b. medium textured soil from Rochester, MN; and c. fine textured soil from Morris, MN.
Figure 2. The amount of inorganic-N (the sum of ammonium-N + nitrate-N) in soil mixed with various manure types in: a. coarse textured soil from Becker, MN; b. medium textured soil from Rochester, MN; and c. fine textured soil from Morris, MN.

Future plans

We plan to analyze all the 25 °C samples for nitrogen and phosphorus as well as samples from experiment at 15 and 5 °C this year. We also collected ammonia (NH3) gas samples from the headspace of each jars. We plan to analyze these samples to understand the effects of manure application on ammonia volatilization losses. In addition, on a separate set of experiments we deployed anion and cation exchange resins in each jar. These resins were replaced each week on average. We plan to extract these resins for N and P.


Dr. Suresh Niraula

Postdoctoral Associate

Department of Soil, Water, and Climate

University of Minnesota (sniraula@umn.edu)


Dr. Melissa Wilson

Assistant Professor and Extension Specialist

Manure Management & Water Quality

Department of Soil, Water, and Climate

University of Minnesota

(Corresponding author email: mlw@umn.edu)


This material is based on work that is supported by the Sugarbeet Research and Education Board of Minnesota and North Dakota as well as the Agricultural Fertilizer Research and Education Council of Minnesota.

Additional information

Hernandez JA, Schmitt MA. 2012. Manure management in Minnesota. Saint Paul (MN): University of Minnesota Extension [accessed 24 Nov 2017].

Pagliari PH, Laboski CAM. 2014. Effects of manure inorganic and enzymatically hydrolyzable phosphorus on soil test phosphorus. Soil Soc. of Am. J. 78(4): 1301-1309.

Russelle MP, Blanchet KM, Randall GW, Everett LA. 2009. Characteristics and nitrogen value of stratified bedded pack dairy manure. Crop Management 8(1). https://dl.sciencesocieties.org/ publications/cm/abstracts/8/1/2009-0717-01-RS.

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