Soil health gains from manure

Sarah Fronczak and Rachel Drobnak, Michigan State University Extension - July 07, 2023

Recent and ongoing research highlights the impact of manure applications on soil health.

A tractor and a manure applicator with draglines moving across a field. — A tractor and manure applicator with draglines moving across a field. Photo by Glen Arnold.

In February, the Livestock and Poultry Environmental Learning Community hosted a webinar, “Manure and Soil Health: Current Research and Future Directions,” which highlighted current and future research on the soil health impacts of manure. Two clear messages emerged from the webinar—current assumptions about swine manure and soil do not always hold true, and the variability of soil texture and climate influence the potential benefits that manure can provide.

Starting the webinar was a presentation by Extension specialist and assistant professor at the University of Idaho, Linda Schott. Her presentation, “Impact of Swine Manure on Soil Health Properties: A Systematic Review,” gave an overview of current research findings and gaps in the literature. Even though pork has the highest consumption globally—beating out beef and chicken—not much is known about swine manure’s influence on soil health. From the 40 peer-reviewed studies investigated in this review, Schott found that the type of manure applied influences changes in soil organic carbon. Soil organic carbon (SOC) is a measure of the carbon stored in soils and relates to a soil’s ability to retain nutrients, hold water, form soil aggregates and maintain a habitat for soil microbes. Results from the review are in Table 1.

Table 1. Comparison of consistency and application of manure as it affects soil organic carbon.
Soil organic carbon
Factor	Influence
Type of swine manure	Liquid	No influence
	Solid	Increase
	Solid + inorganic fertilizer	Increase
Application method	Surface applied	No influence
Application method	Incorporated	No influence
Duration of application	Less than 5 years	No influence
Duration of application	More than 5 years	Increase
Soil texture	Medium-textured soils	Greater increase

Soil nitrogen was also evaluated as a measure of soil chemical health and as a response to manure applications. However, swine manure and inorganic fertilizer in the studies used for the review were not balanced for nutrients, meaning an unequal amount of nitrogen, phosphorus and potassium (NPK) could have been applied. Results for soil nitrogen analyses are shown in Table 2.

Table 2. Comparison of consistency and application of manure as it affects soil nitrogen.
Soil nitrogen
Factor	Influence
Type of swine manure	Liquid	Increase
	Solid	Larger increase
	Solid + inorganic fertilizer	Largest increase
Application method	Surface applied	10% increase
Application method	Incorporated	18% increase
Soil texture	No influence

Schott’s study recommended that future research should include more detailed background information on soils, manure and fertilizer. Also, research should balance NPK nutrients between treatments to make them comparable. Because of these findings, the National Pork Board is funding a multi-state study, including Michigan, to better understand the relationship between swine manure and soil health. Taking place over 17 states, this project aims to standardize research protocol and treatments to gather more consistent and significant data.

The last presenter was Manobendro Saker from the University of Missouri. His presentation, “Analysis of Missouri Soil Health and Manure Application,” investigated the impact of manure on soil health using statewide soil samples. Results from this work indicate manure significantly increased soil nitrogen, phosphorus, active carbon and organic carbon more than inorganic fertilizer. The study did not differentiate between manures from different species. There was no significant difference between manure and fertilizer for bulk density and water holding capacity within this study. These soil physical results are consistent with Schott’s study as well, indicating that manure may not have as much of a physical impact on soil texture and water holding capacity as was previously believed.

To learn more about this webinar and to view the video recordings or to explore further manure management topics, visit www.lpelc.org. Michigan State University Extension also has resources on manure management, including a Manure Hauler Certification Program.

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