Finding Win-Wins for Manure: Maximizing Soil Quality Benefits
This article is Part 2 of a two-part discussion on the value of manure. Part 1 focused on manure nutrient substitution for commercial fertilizer and appeared in the February 27, 2017 CropWatch.
Manure’s Soil Quality Value
Manure contains organic matter, which is mostly carbon and the source of energy for an active soil microbial community (Figure 1). Farmers using manure will often notice a greater earthworm population in manured soils. In addition, when manure is added to the soil, the soil is quickly colonized by millions of bacteria. These organisms derive their energy and nutrients from the organic matter and, during decomposition of the organic matter, produce large quantities of polysaccharides, the “sticky glue” that allows soils to form aggregates. Soils with organic matter levels on the low end of their typical range can benefit the most from manure applications that do not exceed the crop’s nitrogen requirements. Soils that crust over after a rain also benefit from manure’s organic matter.
The result of these improved soil aggregates will be increased infiltration of precipitation and irrigation water, greater water-holding capacity of the soil, and reduced runoff and erosion (Figure 2). Soils with these characteristics experience greater drought tolerance and pose less environmental risk. While the environmental risks of manure are often highlighted (typically a result of excessive application rates), the potential environmental benefits of manure are often ignored.
Charles Wortmann and Dan Walters, faculty in the Department of Agronomy and Horticulture, monitored erosion and runoff from multiple replicated corn and soybean plots receiving manure from 1998-2006. Composted beef feedlot manure described as Low-P Compost (cattle fed corn-based ration) and High-P Compost (cattle fed corn and distillers grains ration) was applied annually for three years to meet crop nitrogen (N) requirements. These plots were compared with a control using commercial fertilizer (no compost). Many soil factors, including erosion and runoff, were monitored during the years following manure application and for four years thereafter when no manure was applied.
The application of composted manure resulted in significantly less soil erosion and runoff (Figure 3) on these plots. Approximately 1.75 inches of additional rainfall infiltrated the soil annually and was potentially available for supporting the crop. In addition, erosion carried with the runoff was reduced by more than one-third, benefiting adjacent streams and lakes. Similar benefits persisted during the four crop years following the final year of manure application (Figure 4). This is one example of a large body of research demonstrating the runoff- and erosion-control benefits associated with manure application.
Three years of compost application at an N-based rate produced very high soil phosphorus (P) levels and the resulting loss of P from the compost sites exceeded that of the sites that did not receive compost. However, if manure application were conducted such that one application was followed my multiple years of no application, soil P levels might have been maintained at reasonable levels and P loss, soil loss, and runoff might have decreased below that of the no-compost site.
Additional soil quality benefits from manure have been observed including
- a liming benefit by increasing soil pH,
- increased diversity of soil organisms yielding benefits in recycling of nutrients, and
- soil-related disease suppression
Manure and Yields
While research on yield increases resulting from manure use is relatively limited and variable in terms of the yield boost provided, testimonials from farmers using manure will often focus on yield gains resulting from manure application. Five Nebraska on-farm research studies in eastern Nebraska in the late 1990s demonstrated variable results with several reports suggesting a yield increase and others showing no change. One report observed a yield decrease for soybeans following manure application. Farms considering use of manure for the first time are encouraged to make a comparison of manure versus commercial fertilizer fertility program. The Nebraska On-Farm Research Network can assist with attaining an accurate comparison.
Can Manure Be a Win–Win?
Management of manure will always carry risks. Odor and other community nuisances must be carefully managed with manure. Pathogens and other contaminants in manure require that we always pay attention to the connection between a field and surface and ground water. If fields for manure application are carefully chosen, the economic and soil quality benefits provided by manure can produce “win–win” opportunities. To maximize those benefits, consider these factors in selecting fields or cropping programs where economic and environmental benefits are achievable.
Achieving Higher Economic Wins
To achieve a higher economic “win” from manure fertility value:
- Prioritize application to fields with soil P concentrations of 20 ppm or less.
- Avoid annual manure applications on the same field. Wait for soil P concentrations to return to a 20 ppm target before reapplying manure, often delaying manure application for three to eight years.
- Apply manure ahead of non-legume crops benefiting from supplemental N.
- Conserve the ammonium N value of manure by incorporating the manure into the soil immediately. This is especially important for swine and dairy slurry manures while of little value for open lot beef manure.
- Credit the manure’s organic nitrogen for the second and third cropping years after a manure application. A soil test will not give credit to residual organic nitrogen in the soil. For estimating these credits, see the Nebraska Extension NebGuide, Determining Crop Available Nutrients from Manure.
- Target fields requiring potassium, sulfur, or micronutrients. Fields requiring potassium experience a much greater economic return from manure application.
- Apply manure at a rate less than the N requirement of the intended crop, and then side-dress the crop with commercial fertilizer based upon in-season soil or crop N measures.
Achieving Soil Quality Wins
To maximize soil and environmental quality “wins” from manure, focus on applying to:
- Fields with lower organic matter. High manure applications on low organic matter sands can lead to nitrogen leaching. Be cautious to carefully credit manure and all other sources of N (avoid leaving N in the soil at harvest) and apply manure as close to the growing season as practical.
- Fields with low soil P concentrations.
- Fields yielding greater runoff or experiencing “crusting” after a rainfall.
- Fields commonly experiencing drown out losses due to ponding.
- Fields with low pH.
- Fields with limited signs of biological activity (e.g., few observations of earthworms).
- Nebraska On-Farm Research Network, where you can search for "manure" in the Results Finder to view the results of farmer-led research on manure use in Nebraska.
Reviewers: Leslie Johnson, Extension Animal Manure Management Coordinator; Amy Schmidt, Extension Animal Manure Management Specialist; Jim Jansen, Extension Educator