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A Few Thoughts on Manure Management

By Frank Larney

The use of manure is as old as agriculture itself and was an integral part of farming prior to the arrival of chemical fertilizers. However, in recent years,manure is often discussed as a problem, or a waste for disposal. Should it be?

compost windrows
Compost windrows at AAFC-Lethbridge Photo: Frank Larney

There’s nothing wrong with manure per se. It’s a really good soil amendment. One of the main issues with manure concentration. Some geographic areas have too much manure, others do not have enough. Pardoning the pun, manure needs to spread around more. Manure undervalued = manure over-applied, and that is when we run into problems like nitrate leaching to groundwater or phosphorus (P) runoff to surface waters.

Manure impacts soil health across (1) chemical, (2) biological and (3) physical environments; which are often inter-related. As an example, manure supplies (1) a slow-release organic nitrogen (N) form, rather than a readily available inorganic N form (nitrate-N), like chemical fertilizer that may leach or runoff. (2) High organic carbon (C) content that provides an instant energy source and boosts soil microbial activity. And (3) organic matter that helps mitigate poor soil physical conditions, such as slow water infiltration or compaction.

compost piles aerial
Aerial view of manure management research trials at AAFC-Lethbridge, showing manure stockpiles and compost windrows. Photo: Frank Larney

Compost is a much broader term than manure. It refers to organic materials (e.g. animal manure, leaf and yard waste, food waste, biosolids, even livestock mortalities) put through a composting process, i.e. an aerobic biological treatment where microorganisms convert organic matter into a stable humus-like product. The microorganisms consume oxygen while feeding on the organic matter and, in the process, generate carbon dioxide, heat and water vapour. Composting is essentially the application of controlled conditions (e.g. optimum mix of organic materials, aeration, turning/mixing, time) to speed up what is fundamentally a natural decomposition process. Therefore, compost should be defined by its starting feedstock(s); which can be a single source or a mix of several different ones (co-composting). Another distinguishing feature is that manure is often denoted fresh or raw whereas compost is said to be mature, stable or finished.

The public perception of compost is much more positive than manure. The content is similar: basically organic matter and nutrients, but minus the undesirable features of manure such as pathogens, odor, or excess water. Composting offers some major advantages such as reduced mass, volume and water content compared to fresh manure. This in turn reduces transportation requirements. If you want to get manure nutrients from Point A to Point B, then turn manure into compost. Research at AAFC-Lethbridge showed that on an equivalent ‘as-is’ basis, composting allowed haulage of 56% more N, 84% more P, 91% more zinc, and 76% more copper than fresh manure. This is advantageous in terms of moving nutrients and trace elements from high to low loading areas, i.e. getting the nutrients to soils where they are needed. Ancillary benefits of composting include elimination of pathogens, parasites, weed seeds, veterinary antibiotics, pesticide residues, and malodours on land application.

Stop by any garden centre and there are lots of products on display that urge you to feed your soil. In this context, feeding a soil takes on an almost human-like connotation. When we think of manure or compost, we often just focus on N or P or maybe a trace element. But one of main advantages of manure and compost amendments, over inorganic fertilizers, is that they contain carbon, which is a source of food for soil microorganisms. A lot of our dryland under annual cropping relies on crop residue and roots to put carbon back into soil. If we run into drought, yields are low and therefore carbon returns to soil are also low. What better way to boost soil carbon and hence organic matter, than applying manure or compost? Other advantages of manure and compost over inorganic fertilizers is that they can have an antagonistic effect on pathogens, which may be useful for disease suppression.

Addition of organic amendments, such as manure or compost, to agricultural soils is governed by the philosophy of best management practices that aim to match application rates to potential plant uptake in order to minimize nutrient losses. This improves the economic value of manure as well as maintaining environmental quality. In an idealized scenario, manure is produced by animals, applied to soils, and used by crops which are then fed back to animals, hence closing the nutrient loop and maintaining soil health. As such, manure is an almost unique agricultural commodity, in that it integrates livestock production with crop production, by means of soil management. Recycling manure back to the land is hardly new, yet we need a new way of thinking to facilitate this mature technology.