Purpose

Livestock producers dealing with animal mortalities may opt for composting as a biosecure on-farm carcass disposal method. The composting process accelerates the decomposition of animal remains, stabilizes nutrients, and, when executed correctly, subjects the carcasses to elevated temperatures capable of eliminating pathogens. Nevertheless, the use of compost derived from animal mortalities may introduce potentially harmful nutrients, heavy metals, pharmaceuticals, or pathogens to cropland when applied as a soil amendment (Sims and Kleinman, 2005).

At the same time, mortality compost represents a potential soil health amendment due to its high carbon content. With carbon being an important building block for organic matter in the soil, the soil will have improved structure and water-holding capacity if carbon content is elevated. There will also be increased microbial activity adding to the soil’s microbial diversity and nutrients present.

This study aimed to confirm these findings and to determine the balance of positive and negative impacts of mortality compost application in Eastern Nebraska by exploring key biological and chemical risk factors in soil receiving swine mortality compost over the course of one growing season.

What Did We Do?

This experiment was conducted at the University of Nebraska Rogers Memorial Farm, located 11 miles east of Lincoln, Nebraska. The study site was comprised of silty clay loam soil that had been cropped using a long-term no-till management system with controlled wheel traffic. Background soil and compost chemical results are portrayed in Table 1. Corn was grown during the previous season, and soybeans were grown during the period of this study. Eight plots (15’ x 15’) were established and randomly assigned to either a 20-ton/ac application of swine mortality compost or no application (control). The compost, made with swine mortalies and a bulking agent of wood chips, was applied to the surface one week after planting.

In-season sampling. Two weeks following treatment application, and every two weeks during the growing season thereafter, soil from each plot was collected from the top 0-4” of the soil profile by random core sampling using a 2 in diameter hand probe. A roughly 200 g composite sample of soil from each plot was used for subsequent analysis. Soil temperature was also recorded for each plot on sampling days at two random locations to depths of 2” and 4” at each location using a hand temperature probe.

Soil samples were assessed in the UNL Schmidt Laboratory in the Department of Biological Systems Engineering for moisture content by drying soil for 48 hours at 221°F, and for the mean weight diameter of wet-stable aggregates by wet-sieving for 10 min at a rate of 30 vertical oscillations per minute. Several biological properties of the soil were also examined, including E.coli prevalence, determined by the proportion of positive samples following enrichment of eight 1-g subsamples of soil in LB broth (Miller) for 8 h at 98.6°F followed by culturing on ChromAgar E.coli selective media for 24 h at 98.6°F. Microbial respiration was measured for two 20-g samples of air-dried soil per plot placed into a 33.8 fl oz glass jar containing a 0.5 fl oz vial of 0.5 M potassium hydroxide (KOH). The soil was re-wet with 0.24 fl oz of deionized water before jars were sealed and incubated at 77°F for 4 days, and the mass of CO2 released during the incubation was determined using the difference in electrical conductivity in the trap material. Finally, metabolic functional diversity was observed for the soil microbial populations by determining the oxidation rates of 31 different carbon substrates using Biolog® EcoPlates following a 48-hour incubation at 77°F of a 10-4 dilution of a 3 g soil sample. Soil microbes in the EcoPlate wells cause oxidation of the carbon species in the plates and results in a color change, which is measured by a microplate reader at 590 and 750 optical density (OD) units. The overall average color intensity, a measure of general population size and activity, as well as the proportional activity by metabolic type (amino acids, carbohydrates, carboxylic acids, polymers, and amines/amines), were considered as ecological soil health indicators in this study.

Harvest and post-harvest sampling. Grain yields were determined by hand harvesting a row length equal to 1/1000 ac from each plot. Soybeans were dried and weighed, and yield values were then converted to bu/ac using a standard 15% moisture content for the soybeans.

Following harvest, soil from each plot was retrieved according to the previously detailed methodology and sent to a commercial laboratory to determine end-of-season values for pH, sum of cations, soluble salts, calcium, organic matter (%), nitrate-N, phosphate (P2O5), potassium (K2O), sulfate, sodium, magnesium, zinc, iron, copper, manganese and heavy metals (arsenic, lead, and chromium) in the top 0-4” of the soil profile. Bulk density was also determined for two locations per plot at depths of 0-2″ and 2-4″.

Table 1. Initial chemical characteristics of compost and soil

Chemical	Compost	Soil
pH	7.1	6.6
Soluble salts (mmho/cm)	11.4	0.13
Zinc, ppm	57.6	1.12
Iron, ppm	1477	44.8
Copper, ppm	13.4	0.73
Manganese, ppm	100.6	9.2
Arsenic, ppm	1.807	5.971
Lead, ppm	2.09	14.46
Chromium, ppm	7.48	35.85

What Have We Learned?

The application of the compost treatment significantly increased the prevalence of E. coli in the soil samples, but only early (4 weeks) in the growing season (Figure 1). This is likely influenced by the compost’s organic matter and microbial diversity, which serve as a carbon source and support microbial population growth. However, as the season progressed, the difference in the prevalence of E.coli in soil that had or had not received compost application narrowed, potentially due to other factors impacting microbial survivability (such as temperature or moisture content) becoming dominant factors. Regression analysis comparing E.coli prevalence to soil moisture and temperature did not show a strong relationship (R-squared values of 0.48 and 0.18, respectively), which indicates that the microbial population is being impacted by other, more complex factors not included in this analysis.

No other soil biology, chemistry, or physical properties that we tested proved to be significantly impacted (a ≥ 0.05) by the application of mortality compost to the soil, nor was the soybean grain yield. This indicates that while the soil health impacts of this single-season compost application were negligible, there is also little risk to water quality associated with the application of 20 ton/ac swine mortality compost in crop production areas that are well-managed with soil conservation best practices.

Future Plans

The results suggest that there is little risk of prolonged elevated E. coli prevalence in soil when using swine mortality compost in row crop production areas. However, precipitation producing runoff may pose a risk to nearby surface water bodies if experienced within six weeks of compost application. Future research would be required to fully understand the risk of this occurring, but previous research conducted at the same farm determined that a 12.2 m (40 ft) setback of bare soil was sufficient to prevent most chemical and biological pollutants from leaving a field via runoff after receiving surface application of manure (Gilley et al., 2017).  This is an encouraging and valuable guideline for producers who are generating compost as part of their operation and must find suitable sites for application.

The negligible soil health improvements from mortality compost application during this single-season study could dissuade crop producers from seeking out this material if it were available in their vicinity. However, where organic matter is needed to improve soil health over time, this product should not be discounted as a valuable soil carbon amendment. While we did not observe any positive soil health impacts from a single 20 ton/ac application of compost in this study, other studies have seen single season effects. Several other studies found significant impacts of applying a single season of organic amendment on soil microbial biomass (Lazcano, et al., 2012; Leytem, et al., 2024; Crecchio, et al., 2001) and on C:N ratio, which were not tested in this study. Thus, future research could explore alternative rates of application, frequency of sampling, or testing methodologies.

Another possible explanation for the lack of significant soil health impacts was that the field used in this study has been under long-term conservation (20+ years of no-till) practices. As a result, we suspect that the soil health improvement gap (e.g., the difference between soil health status and potential soil health status under ideal management) may be quite minimal. Soil sampled from our plots prior to treatment application revealed an average organic matter (OM) concentration of 3.8%, which exceeds the average 2 to 3% OM concentration for this soil type (Magdoff et al, 2021). However, other soil health factors such as bulk density, microbial population richness, and organic nutrient availability were in line with reports for similar soil types (Oregon State University Extension Service., 2019; Chau et al., 2011; University of Florida., 2015). This likely indicates that future applications of this sort should avoid fields with elevated soil organic matter, as they will not greatly benefit from the addition of organic amendments where soil carbon is already sufficient to the needs of the soil ecosystem.

Authors

Presenting author

Jillian Bailey; Undergraduate Researcher; Department of Biological Systems Engineering; University of Nebraska-Lincoln

Corresponding author

Amy Schmidt, Professor, Department of Biological Systems Engineering, University of Nebraska-Lincoln, aschmidt@unl.edu

Additional author

Mara Zelt, Research Technologist, Department of Biological Systems Engineering, University of Nebraska-Lincoln

Additional Information

Castro, G., Schmidt, A. (2023). Evaluation of Swine Cadaver Disposal through Composting and Shallow Burial with Carbon (poster presentation). ASABE AIM. Omaha, NE. https://publuu.com/flip-book/818714/1802503

Crecchio, C., Curci, M., Mininni, R., Ricciuti, P., & Ruggiero, P. (2001). Short-term effects of municipal solid waste compost amendments on soil carbon and nitrogen content, some enzyme activities and genetic diversity. Biology and Fertility of Soils, 34(5), 311–318. https://doi.org/10.1007/s003740100413

Gilley, J. E., Bartelt-Hunt, S. L., Eskridge, K. M., Li, X., Schmidt, A. M., & Snow, D. D. (2017). Setback distance requirements for removal of swine slurry constituents in runoff. Transactions of the ASABE, 60(6), 1885–1894. https://doi.org/10.13031/trans.12310

Lazcano, C., Gómez-Brandón, M., Revilla, P., & Domínguez, J. (2012). Short-term effects of organic and inorganic fertilizers on soil microbial community structure and function. Biology and Fertility of Soils, 49(6), 723–733. https://doi.org/10.1007/s00374-012-0761-7

Leytem, A.B., Dungan, R.S., Spiehs, M.J., Miller, D.N. (2024). Safe and sustainable use of bio-based fertilizers in agricultural production systems. In: Amon, B., editor. Developing Circular Agriculture Production Systems. 1st edition. Cambridge, UK: Burleigh Dodds Science Publishing. p. 179-214. https://doi.org/10.19103/AS.2023.0120.16

Magdoff, F., Es, Harold van. (2021) (2024, July 18). CH 3. Amount of organic matter in soils – SARE. USDA Sustainable Agriculture Research and Extension. https://www.sare.org/publications/building-soils-for-better-crops/amount-of-organic-matter-in-soils/

Oregon State University Extension Service, Horneck, D. A., Sullivan, D. M., Owen, J., & Hart, J. M. (2019). Soil Test Interpretation Guide. In EC 1478. https://extension.oregonstate.edu/sites/default/files/catalog/auto/EC1478.pdf

Sims, J. T., & Kleinman, P. J. A. (2005). Managing Agricultural Phosphorus for Environmental Protection. In J. T. Sims, & A. N. Sharpley (Eds.) Phosphorus: Agriculture and the Environment (Vol. 46, pp. 1021-1068). American Society of Agronomy. https://doi.org/10.2134/agronmonogr46.c31

University of Florida. (2015). Urban Design – Landscape plants – Edward F. Gilman – UF/IFAS. (n.d.-b). https://hort.ifas.ufl.edu/woody/critical-value.shtml

Acknowledgements

Funding for this study was provided by the Agricultural Research Division (ARD) of the University of Nebraska-Lincoln through an Undergraduate Student Research Program grant award. Much gratitude is extended to collaborating members of Rogers Memorial Farm, Stuart Hoff and Paul Jasa, and to the members of the Schmidt Lab – Alexis Samson, Logan Hafer, Maddie Kopplin, and Carol Calderon – for their assistance with sample collection and analysis.

 

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