Manure is often viewed by many as an environmental liability.  However, if manure is applied at rates equal to or less than the nitrogen (N) requirement of a crop, can manure produce environmental benefits over commercial fertilizer? 

Benefits to Crop Yields
The average of all grain related trials demonstrated a 5% increase in yield for manured fields. It is suggested that manure increases soil microbial biomass N , the living component of soil organic matter. As organic material is decomposed, MBN is slowly converted to inorganic N at a time more in sync with crop N uptake, improving crop N uptake and crop yield.  In addition,it is suggested that the increased levels of other nutrients and improvements in soil physical properties also contributed to observed yield increases.

Peak increases in yield of 13% were observed when manure was applied at 50% - 75% of crop N requirements. Attempting to substitute more than 75% of the crop N requirement with manure often produced reduced yields. It is important to recognize the complimentary roles that manure and commercial fertilizer can play in a crop fertility program.

Reducing N Loss to the Environment
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.

Minimizing mineral N in soils while meeting crop nutrient requirement protects our water and air resources. 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.

It's reported that increases in water stable aggregates and cation exchange capacity for manure amended soils; this helps hold ammonium-N in the root zone until the crop can use it. Previous articles have summarized the benefits of manure for increasing soil aggregates and preventing runoff and erosion losses to surface water.

Raising Soil Organic Carbon
Highlighted are the benefits of manure for increasing soil organic C.  Much of this increase in soil C was in the form of soil microbial biomass C.  Manure provides an energy source for soil microbes, which significantly increases soil microbial activity and improves soil storage of both N and C in soil organisms. (See the Michigan State University Extension Publication Manure effects on soil organism and soil quality.)

Increasing soil C levels can increase the risk of greenhouse gas emissions. However, this study suggested there were no changes observed in emissions of potent greenhouse gases, nitrous oxide (N2O) and methane (CH4). However, some increase was observed in CO2emissions due to greater soil microbial activity. Overall, manured soils proved to be a sink for carbon in upland fields, thus benefiting the environment.

Bottom Line
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). Not addressed by this review is potential phosphorus losses to surface water. Manure is known to reduce erosion and runoff losses but phosphorus loss can increase if soil P levels are allowed to increase above optimum levels for crop production (see previous article).

Thus, a strategy for increasing yields and protecting the environment will include applying manure:

• At or below crop N requirements (plus some supplemental commercial fertilizer N often including a starter fertilizer at planting) and
• To the same field only after soil P levels return to levels required for optimum crop production.

Yes, manure substitution for fertilizer can benefit our environment.