Sustainable Nutrient Management in Agriculture and Closing the Loop on Large-Scale Nutrient Flows

Bruce T. Bowman
Former Chair, CARC Expert Committee on Manure Management1
revised - April 3, 2003

From: Philip K. Barton and James W. Atwater. 2002. Nitrous Oxide Emissions and the Anthropogenic Nitrogen in Wastewater and Solid Waste. J. of Environ. Eng.128: 137 - 150.

"In the 20th century, human interference in the nitrogen cycle has caused a doubling of the global nitrogen fixation rate, thereby intensifying global nitrous oxide (N2O) production during microbial nitrification and denitrification. Nitrous oxide is a powerful greenhouse gas, important in climate change, and as well, is a stratospheric ozone-depleting substance. It is likely that much of the Earth’s population now relies on anthropogenic [human-generated] nitrogen in its food supplies, resulting in anthropogenic nitrogen contained in wastes requiring management."
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1 Ideas and opinions expressed in this document are those of the author and do not represent the official position or policies of any organization. This document is a work-in-progress and will be updated from time to time.

See also - Large-Scale Livestock Nutrient Recycling: A Key Part of a Sustainable Nutrient Management Strategy

The Issues:

  1. Sources of Mineral Fertilizers - In many parts of the world, major nutrients such as phosphorus (phosphates) and potassium (potash) are considered to be non-renewable resources because they are mined from finite natural deposits, and are used to supplement livestock-based nutrients in crop production. Some phosphate deposits have elevated levels of heavy metals, such as cadmium, which limits their safe use in crop production.

    The production of nitrogen-based fertilizers, through the Haber process of converting atmospheric nitrogen into ammonia or urea, is a rather energy-intensive process requiring large amounts of natural gas, which also has associated greenhouse gas emissions.

    Thus it makes both economic and environmental sense to conserve and recycle as much existing mineral fertilizers as is practicable.
  2.  Types of Major Large-scale Nutrient Flows:
  1. Nutrient flows through livestock operations (in this context, livestock includes poultry).
  2. Nutrient flows through human food systems.

In both types, the loop on nutrient flows (recycling) back to the source is frequently severed, resulting in large-scale regional nutrient accumulations in some areas while there are nutrient deficits in other areas.

Nutrient flows can be defined as:

  1. The large-scale anthropogenic [human-induced] movement/transportation of nutrients contained in feeds for animals or foods for humans from one geographic area to another, and/or,
  2. The movement/transport of nutrients in various chemical forms by water or air to locations distant from their original use. This type of flow is usually caused by natural processes, such as leaching, runoff or volatilization, which may be indirectly a result of anthropogenic activities.

This document focuses on the first part of the definition.

A.  Nutrient Flows through Livestock Operations

    Specialization and intensification in modern production agriculture has fostered the proliferation of farm operations focussing on either cereal production or on livestock/poultry production, resulting in fewer integrated farms with both crop and livestock production. There are several important consequences resulting from this trend:

  1. Increased amounts of energy (relative to integrated farming systems) are consumed in transporting feeds (nutrients) from crop production areas to livestock production areas (which may be in other countries). Increased amounts of air pollution and greenhouse gas emissions are secondary consequences of this nutrient transportation. With this practice, there is a greater tendency to export [nutrients in] cereals, rather than locallyfeed the cereals to animals and export value-added livestock products for human consumption. Consequently, large amounts of mineral fertilizers continue to be annually applied in cereal production to offset the export of nutrients in livestock feeds. Without livestock manure inputs, soil quality (organic matter content) and available nutrient levels in cereal production areas are more difficult to maintain.
  2. Intensive livestock operations frequently lack sufficient land base to properly utilize nutrients in the manures within their own operations. Currently there is no existing infrastructure to support the recycling of excess nutrients in these livestock wastes (manures, dead stock, etc) back to the original sources for subsequent cereal crop production. Thus, regional nutrient accumulations are developing in livestock production areas (see Figure 1), while in some cereal production areas, nutrient levels may be partially depleted.

Nutrient Flows Graphic

            Figure 1. Nutrient Flows in Agriculture


Strategies for Closing the Loop on Nutrient Flows through Animal Production Systems

There are extensive areas of farming operations where cereal production and livestock production are separated. There are at least two possible strategies to improve this situation:

1. Integrated Farm Approach

Encourage the establishment of integrated livestock-cereal production farms (new operations or conversion of separated operations) where recycling animal nutrients back to the land would be the normal practice. One approach might involve government intervention through policy changes, such as incentive programs, that would make integrated farms more sustainable (economically, environmentally, socially), relative to specialized cereal or intensive livestock operations.

This would have been a more viable option for pork and beef producers when there were many small abattoirs scattered throughout the countryside that could support a distributed livestock production system on integrated farms. However this option is now becoming much less feasible because very few, high capacity packing plants are operating close to existing high-density livestock operations. Corporate livestock operations, which are starting to dominate livestock production in various parts of North America, prefer to concentrate their livestock operations near these large packing plants to minimize shipping costs and reduce transportation stress on the animals.

This integrated farming approach may be more amenable for dairy farms, where fluid milk products can be shipped considerable distances for processing and distribution. Cereal-only farms remain the only viable option in some semi-arid regions, such as regions in the western prairies of North America, where there are insufficient water supplies (quantity or quality) to support livestock production.

2. Development of Consistent Organic Fertilizer Products with a Viable Market Value

Develop practices that can effectively recover [excess] nutrients from livestock operations and recycle them back to existing cereal production farms. This approach would capture and recycle excess nutrients from livestock manures by producing consistent, concentrated organic fertilizers, which could be economically transported back to cereal-based farms, and compete with the existing practices of using only non-renewable mineral fertilizers.

A key to this approach is the treatment of livestock wastes, de-watering the manures and converting them into granules or pellets that can be easily (economically) stored, transported considerable distances (same as for mineral fertilizers) and applied using standard fertilizer application equipment. These organic amendments, besides containing nutrients, will also improve soil quality, unlike their mineral fertilizer counterparts. It may be economically feasible to fortify these organic amendments with additional nutrients to guarantee a specific nutrient content that could be sold as organic fertilizers under the Fertilizer Act.

The manure treatment process, if using a closed vessel treatment system, can also solve most of the odour and pathogen problems currently plaguing the livestock industry, reduce greenhouse gas emissions in the storage and handling phases, and  provide the farmer with greater flexibility in managing excess nutrients in the livestock operation.

A second key element driving this option would be the development of the necessary business infrastructure to transport, store and distribute these organic fertilizers, much in the same fashion that agro-marts currently market mineral fertilizers. A necessary part of this process would be the production of consistent quality products with a clear market advantage over mineral fertilizers could be established.

B. Nutrient Flows through Human Foods

There is a second large-scale nutrient flow loop involving human food consumption that, for the most part, has also been severed. Nutrients in foods for human consumption are exported directly from cereal production areas, and indirectly to human use from animal production through milk, meat and poultry products (see Figure 1). According to Barton and Atwater (2002), 25% of agriculturally-applied nitrogen leaves the farm in food products. Nutrients from these sources tend to be disposed of through sewage systems, landfilling, incineration, and to some extent through biosolids, some of which may be recycled to agricultural production lands. Included in this group are considerable amounts of nutrient wastes from food terminals, the fast food industry, and other vegetable/food processing activities.

Since about 75% of Canada’s population resides in the 5 largest cities in Canada, recycling human waste and correcting that nutrient balance has become incredibly important. Efforts need to increase to capture these valuable sources of nutrients and organic matter and to recycle them to crop production areas. There are, however, several existing barriers that have limited the recycling of urban food-sourced wastes, including:

  1. the possible contamination of these wastes by toxic organics or heavy metals that may enter the urban waste stream from other sources. This risk can be more effectively minimized for bulk food wastes from processing plants, food terminals and the fast food industry, while the risk increases for the residential waste stream where it is more difficult and costly to assure and maintain source separation of food wastes to keep them free of contaminants.
  2. the lack of infrastructure between urban communities and agricultural systems to encourage the separation, collection, processing and transfer of the processed nutrient-containing wastes back to agricultural production areas. Currently, there are some biosolid utilization programs managed by provincial environment ministries which have a land application component. In some cases, however, there have been problems which have resulted in considerable rural resistance to land application of these urban-sourced biosolids. The perceived problems with these biosolids arises from the sewage or processed paper waste, where there are greater risks of contamination from heavy metals or other unidentified toxic organics.


In order for society to move towards more sustainable use of nutrients, there are two anthropogenic nutrient flow loops that need to be closed, one primarily through production agriculture, the other through human food production and consumption.

    1. Agriculture needs to close the loop on large-scale nutrient flows through livestock production systems, which are not currently being recycled back to crop production areas. Current fertilizer use practices place too much reliance on non-renewable mineral fertilizers for crop production. The continuing influxes of large annual amounts of "new" mineral nutrients from natural mineral deposits into crop production systems further increases global risks for water contamination by nutrients. This practice is unsustainable in the long term, making it virtually impossible to maintain current water quality standards. Increasing the use of recycled organic fertilizers will help stabilize global nutrient levels in the environment, while also having an important soil-building (soil quality) aspect that mineral fertilizers lack. In fact, extensive, continued use of mineral fertilizers will decrease overall soil quality.

    In the livestock production nutrient flow loop, two possible solutions include:

  1. Government incentives for increasing the proportion of integrated crop-livestock farms, which may be most feasible for dairy farms, which can be scattered through cereal production areas while the end products (fluid milk) can be readily transported over considerable distances to the processors and on to the consumer.
  2. Promoting manure treatment and nutrient scavenging systems that will facilitate the production of organic amendments/fertilizers suitable for transporting and competing with mineral fertilizers for crop production on cereal production farms. Developing an infrastructure to support this approach will be a key element to its success.

    2. The issue of closing the loop on nitrogen flows from agriculture to urban areas is a much broader societal problem requiring new partnerships and infrastructure to be developed between agriculture and municipal/urban entities. One key component to the success of closing this loop will be separating the heavy metals and other toxic contaminants from the human waste stream, so as to make recycling human wastes safe for land application in agricultural production areas.


Other related information


Supplementary Reading

Land Spreading of Animal Manures, Farm Wastes & Non-Agricultural Organic Wastes, Part 1: Manure (and Other Organic Wastes) Management Guidelines for Intensive Agricultural Enterprises; Owen T. Carton1 and William L. Magette2. 1Teagasc, Johnstown Castle Research Centre, Wexford; 2Dept. of Agricultural and Food Engineering, UCD, Earlsfort Terrace, Dublin 2. May 1999; ISBN 1 84170 23 8

Nitrous Oxide Emissions and the Anthropogenic Nitrogen in Wastewater and Solid Waste. 2002. Philip K. Barton and James W. Atwater. J. of Environ. Eng.128:137 - 150.




Bruce T. Bowman, Archivist
Last Updated: Saturday, February 02, 2019 12:36:54 PM