The high concentration of organic carbon (C) and plant essential nutrients in manure and organic biosolids make them excellent fertilizers. However, manure is greatly underutilized as a fertilizer; only about 22% of the manure produced worldwide is applied as fertilizer (FAO 2018). This underutilization can yield regional nutrient imbalances when inorganic fertilizers are imported to meet crop nutrient needs that locally produced fertilizers could supply. However, with this challenge comes opportunity as the campaign to improve agricultural soil health has gained momentum among conservationists and researchers worldwide. Thus, a comprehensive assemblage of outcomes from manure and soil health-related research studies is important. Particularly, the identification of knowledge gaps is an important step to direct future research that informs soil health improvement outreach programs.
What did we do?
We conducted a systematic literature review based on peer-reviewed studies that evaluated the effect of livestock manure and organic biosolids on soil health properties. Soil health properties included in the review are shown in Figure 1. All studies included had to be replicated field trials written in English where manure or organic biosolid application was the only differing factor between treatments. Additionally, included data had to be statistically analyzed to compare organically amended treatments to a control. A total of 163 studies met all criteria.
What have we learned?
Overall, manure and biosolid applications have the potential to improve the health of agricultural soils. These organic amendments add significant amounts of organic C to soil, which has positive effects on other soil health metrics. When compared to inorganic fertilizers, soils that have had application of livestock manure or organic biosolids have the following properties:
| Physical |
|---|
| decreased bulk density |
| more resistant to compaction, especially when wet |
| increased water holding capacity (WHC) in fine-textured soils with no effect in course textured soils |
| varied effect on aggregate stability |
| increased saturated hydraulic conductivity and infiltration |
| Biological |
| increased microbial biomass C and microbial biomass N |
| increased bacterial and fungal populations but no effect on diversity as measured by PLFA |
| increased microbial respiration and potential N mineralization |
| no change in microarthropod population and diversity |
| increased earthwork population |
| Chemical |
| increased soil organic C and soil organic matter, in general |
| varied effect on soil NPK; depends on study methodology (application rate and timing, amendment type, etc.) |
| varied effect on pH; depends on initial soil pH, amendment pH, and application rate |
| increased cation exchange capacity due to increased soil organic C |
Future Steps
The evaluation of the impact of manure and biosolids on soil health properties is difficult to do based on current literature because 1) there are inconsistent research methodologies between individual research studies, and 2) there are few comprehensive studies that have included all soil health properties. Improvements in research methodologies needs to be improved to fill substantial knowledge gaps identified with this review. Specifically, future research should: (1) quantify soil biological metrics, (2) investigate the short- and long-term effects of a single application of manure or biosolids, (3) study nutrient application balance on an annual or multi-year basis, and (4) discuss how research findings translate into management decisions relevant to agricultural crop producers.
Authors
Corresponding Author: Linda Schott, Assistant Professor/ Extension Specialist- Nutrient and Waste Management, University of Idaho, lschott@uidaho.edu
Other Authors: Amy Schmidt, Associate Professor/ Livestock Bioenvironmental Engineer, University of Nebraska-Lincoln; Humberto Blanco-Canqui, Associate Professor, University of Nebraska-Lincoln
Additional Information
FAO. (2018). Nitrogen inputs to agricultural soils from livestock manure. New statistics. Integrated Crop Management (Vol. 24). Rome, Italy
More information on this project can be found at: https://soilhealthnexus.org/resources/manure-and-soil-health/
Acknowledgements
This project was supported by funding from the North Central Region Water Network and the Soil Health Institute. The authors would also like to thank Mara Zelt and Ashley Schmit for their assistance.
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