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Manure and Soil Health
by LuAnn Rolling, NRCS District Conservationist
Ruth Blomquist, the Southwest Iowa Soil Health specialist for the NRCS, says she was recently asked about manure application and soil health. The question was, “Are we doing more harm than good if manure is applied with a high disturbance method like injection or if it is surface applied with incorporation?” She did some research and found that the answer isn’t a simple yes or no, and it depends on multiple factors.
Blomquist says that manure can positively impact soil organic matter levels, nutrient content, cation exchange capacity and soil pH. She cites several articles that positively link biological soil health indicators to manure application. In an article titled “The surprising power of chicken manure”, she found that research has shown that soils that had poultry manure applied could get more rainwater into the soil, and hold more rainwater for crops to use when the field was dry. “They also found that soybean yields were higher over a longer period of time when comparing poultry manure to synthetic fertilizers.”
According to Rick Koelsch, University of Nebraska – Lincoln, substituting manure for commercial fertilizer has positive societal benefits for food security (increased yields) and for environmental protection (less nitrogen loss and lower greenhouse gas emissions). He states, in an article titled “Manure’s Impact on Yield, Nitrogen, and Carbon”, that commercial fertilizers add mineral N to our soils (ammonium and nitrate-N). “Manures add a mix of organic and mineral N. Beef feedlot manure and poultry litter are about 90% organic-N. Slurry manures from swine and dairy operations may be roughly equal parts organic and mineral-N. Mineral N is more susceptible to environmental loss to the air and water.”
According to Koelsch, minimizing mineral N in soils while meeting crop nutrient requirement protects our water and air resources. His research found that replacing fertilizer with manure resulted in decreased ammonium (NH4) losses to air and N losses to surface and ground water by more than 25%. These benefits were observed regardless of crop, manure type or substitution rate. The authors attributed these environmental benefits to both improvements in crop N use efficiency and greater N storage as microbial biomass nitrogen that is released closer to crop’s N utilization timing.
Blomquist says manure is a fantastic source of carbon and adds organic matter to the soil. “In most cases it does increase soil microbial populations and microbial activity. This leads to aggregate stability, water infiltration, improved nutrient cycling, better water holding capacity, and more resilient soils.” She says it is important to note that the benefits of manure can vary, depending on the form or type of manure, how it is applied, the application rate, and conditions at application. “Issues like compaction, runoff, or excessive nutrients can occur if manure isn’t managed properly.”
Blomquist says it is always important to consider the soil health principles, and adds that manure fits right in. “It adds a diverse source of carbon and nutrients to the soil and provides a diverse food source for the soil biology. It is always important to consider the soil health principles as a whole system, and to figure out how to integrate manure into the system in a way that incorporates as many of the principles as possible.”
She says manure utilization will look different on every farm operation. “In my opinion, it will depend on different factors, but it is likely that utilizing injection or incorporation of manure still has the potential to improve soil health, but if a producer can find a way to incorporate more soil health principles, they will see greater benefits in a shorter amount of time. Using low disturbance application methods and incorporating cover crops with the manure would have a much greater positive impact on soil health.”
