There is not enough manure (or compost) to sustain agriculture

October 18, 2017
By Andrew McGuire

There is not enough manure. Not enough to supply nutrients to our crops, not enough to maintain our soils. Those were the conclusions in my last two posts, but before we see what this means for agriculture, let’s look to other organic amendments. Is there enough of any of them?

What about other organic amendments?

Organic amendments come from living organisms with by far the largest amounts produced by plants. These are organic in the sense that they are produced by organisms, not in the sense that they are amendments approved for certified organic farming (some of which are not organic, like rock phosphate). It is plants’ primary production – combining sunlight with elements to produce biomass – that ends up as the bulk in organic amendments. So when we take a survey of what organic amendments (see Table 4 of this publication) might be available in greater quantities than manure, we need to look at the plants growing on our land. This is the basic limitation on the quantity of organic amendments available.

Table 1. US Land Use, % of total (2012, USDA ERS Major Land Uses)

Grassland, pasture, and range 29%
Forest 28%
Cropland 17%
Roads, parks, industrial, military, rural homesteads 14%
Miscellaneous 9%
Urban areas 3%

 

Grasslands, pasture and rangeland do not produce significant waste products. Some organic amendments, such as bone and blood meal, come from grazed livestock, but the amounts are much less than manure.

There are forests, and sawdust and wood chips from lumber production are valuable soil amendments, but they are not available to many farms. Increasingly, these are used in fabricated wood products and so not available at all.

Cropland is where we could hope to find more organic amendments. Corn, soybeans, and hay make up about 70% of cropland with most of this going to feed livestock. This is our source of manure, which we have found insufficient. The rest of cropland goes to food for humans. In the future, we will have to figure out how to better recycle our own waste back to cropland. However, human waste is not now available in similarly large quantities as manure. Food waste can be available in significant amounts in some locations, but again, the amounts are much less than manure, based on land areas alone.

Then there are the parks, yards, and “miscellaneous” land areas, many covered by plants producing biomass. Much of what is harvested here (mowed and bagged mainly) is now composted to keep it out of landfills, but the quantities, due to the much smaller land area and because parks and lawns are not managed for high yields, are much smaller than manure.

Is compost any better?

A lot of compost is produced using manure and so is part of the manure stream. The rest must be traced back to one of the land uses we reviewed above, and so the quantities will be necessarily less than those of manure produced from 70% of our cropland.

Furthermore, composting manure is not the answer for nutrient recycling because much of the nitrogen is lost during the composing process (Chromec and Magdoff 1984). However, for building soils, composted manure is slightly better than raw manure because the overall losses of organic material are less for composted manure (Bernal et al. 1998). The significant cost of composting must be weighed against the advantages such as reduction of weight and volume, pathogens, and weed seeds (CSANR compost website).

Can we use crops as amendments?

Another option is using the crops themselves as amendments, not the crop’s residues – stems, leaves, chaff-  which I will cover below, but the actual grain or soybean. For example, I have seen organic fertilizers that are made from processed soybeans. This practice, in effect, increases the land area needed to raise a crop by the amount of soybean acreage it takes to produce the fertilizer. It is also a transfer of nutrients that, like imported manure, makes the receiving field look more sustainable at the expense of the exporting field. While this “fertilizer” may make economic sense because of the high cost of organic nitrogen fertilizers, it makes no sense whatsoever for sustainability.

So, we can conclude that manure is the top organic, “natural,” fertilizer/amendment in terms of quantity, and we don’t have enough of it1. What are the implications of this?

1The exceptions are when soil organic matter levels are 1% or less or when losses are very low, see previous post.

Use manure in combination with other practices

Even in the best-case scenario where a field produces a feed crop and the manure produced by livestock eating that feed crop is returned to that field, there is not enough manure to either supply nutrients for another feed crop nor to maintain the field’s soil organic matter level (by itself, and in most, not all soils). However, manure in combination with synthetic fertilizers can provide sufficient soil nutrients, and manure in combination with other practices can maintain soil organic matter levels. The other practices either reduce losses of soil organic matter, or add organic matter to the soil, or do both:

Table 2. Practices that help maintain soil organic matter (SOM) levels. (Modified from Magdoff and Van Es 2009)

Practice or effect of management Reduces SOM loss rate Adds to SOM
Reduce erosion

X

Perennial crops in rotation

X

X

Cover cropping

X

X

High residue crops

X

Conserve crop residues

X

Reduce tillage intensity

X

Application of organic amendments

X

 

Overall, evidence shows that the long-term level of soil organic matter is directly related to the amount of plant (shoots and especially roots) and plant-derived materials (manure, compost) added to the soil. Therefore, the main sustainable source of C inputs to the soil will always be those crop residues produced on the soil we are trying to sustain.

Strategies from research results:

  • “Long-term, low-rate annual amendment might be a more economically, agronomically, and environmentally desirable alternative to single-year high-rate applications.” (Stone et al. 2004).
  • “It is possible to have a high-quality soil even with a moderate level of SOM as long as sufficient quantities of a variety of residues are routinely present…” (Magdoff and Weil 2004).

Importing fertility and soil health

This will always work: Take the organic waste off a large land area and apply it to a much smaller land area, and it will improve the soil, plant growth, etc. It does not matter whether it’s done in a garden, on a farm, even a tropical forest. It’s the same whether we are talking orange peels, manure, or compost. However, it is not sustainable. It’s an illusion we see when we ignore the source of the organic amendments. Applying manure or other organic amendments in agriculture is a zero-sum game, one field’s gain is another field’s loss. If we remember to look at the source of the material being added, it will rarely be a win-win result.

Beware of farming system comparisons that involve manure and compost.

I was all prepared to do an in-depth analysis of the many comparisons of organically farmed soils to conventionally farmed soils. I even had a review of what amounts to a researcher brawl with back-and-forth paper punches over many years, but decided that these comparisons are missing the point. If there is not enough manure or other organic amendments, then it does not matter whether those amendments are used on organic or conventional fields, on kale or corn producing fields. If the application rates used supply all the nutrients or increase soil organic matter levels, it will almost always be at the expense of other fields.

If more manure is used on organic farms, there will be less available to conventional farms. In the wide view, the same area of soil will be affected. The more we concentrate it to improve one field, the more other fields do not get it nor its benefits. Overall, for the soil, it does not make a difference who uses it.

This is often overlooked in research projects comparing farming systems where the conclusion is often that one system can maintain or build soil organic matter levels better than another. Manure imports can give the appearance of sustainability, and can even overcome the detrimental effects of tillage and production of low residue crops. But now we can see that this comes at a cost.

To resolve this in future research, Kirchmann et al. (2016) concluded that comparisons of farming systems should be based on organic amendment application rates that are in line with the system’s productivity. In other words, a sustainable rate of manure application is equal to the manure that could be produced by the crops produced on the land receiving it over a full crop rotation. If a system is only able to maintain its SOM levels by importing manure from off-farm, then it is no longer the system differences that are being measured, but each farm’s ability to import organic amendments. In coming to this conclusion, they note that, in the big picture, SOM gains from manure are not a system characteristic because most manure is recycled to the soil anyway; soils will be improved by manure application no matter what farming system is being used. It is important not to confuse practices with farming systems.

Using this guideline, imported organic amendments cannot be used to hide detrimental practices such as growing only low-residue vegetables, intensive tillage, or erosion.

1 – In 2012 (USDA 2017)
2 – Using 3% soil organic matter, 3% loss rate, from Table 2 of previous post.
3 – 2016 USDA survey data, percent using manure is uncertain.

Climate change and manure

The problems noted above with systems comparisons also apply to studies that draw conclusions about manure use for climate change mitigation. Yes, applying manure can help store carbon in the soil. No, it cannot do this (with the exceptions found in the previous post) without also degrading the soils of fields not receiving manure. In his paper, Carbon sequestration in soils: Some Cautions Amidst Optimism, Schlesinger (2000) points out a now familiar fact, “greater levels of SOM in manured fields can be expected to be associated with lower inputs of plant residues on a proportionally larger area of off-site lands. SOM will decline on those lands, because the return of crop residues to the soil is important to the maintenance of SOM in agricultural systems.” Because of this “myth of manure” Schlesinger states “Applications of manure are often assumed to increase C sequestration in soils, but manure is not likely to yield a net sink for C in soils.”  Again, sustainability for a field does not mean sustainability for agriculture.

For livestock production

Finally, I can ask all these questions only because we have large concentrations of manure. This is a consequence of how we raise livestock. So we can ask, should manure be concentrated in the first place? Grazing livestock do not produce manure that we can collect and apply to other fields. Perhaps by viewing manure as a solution for the soil, we have ignored the bigger question of why we have manure in the first place? On one side are graziers like Joel Salatin who I have heard say, “If you can smell it, it is being mismanaged.” On the other side are those who work for better solutions to problems with concentrated manure. The WSU CSANR has done lots of work with how to better handle manure to both generate energy and conserve nutrients. Indeed, recent analysis (Swain et al. 2018) shows that intensification of meat production can actually reduce environmental impacts. Nevertheless, it won’t matter if new plant-based and cultured meats reduce livestock production by any means. Then manure will cease to be a problem, but will also cease be a significant source of nutrients and organic matter.

This is not about how we manage manure, but how we think about manure’s role in sustaining agriculture. These results should change our view of manure. It is a scarce resource, one that only a small portion of crop producing fields can benefit from. It is not a substitute for fertilizers in supplying nutrients. Manure should no longer be thought of as a fertilizer. The nutrients in manure are already part of the agriculture, and by using manure we recycle those nutrients, but overall manure does not replace nutrients lost to leaching or to our eating of food. Nor is it a substitute for good soil health practices. If it is used by itself to supply nutrients or to maintain or increase soil organic matter, it is only at the expense of other fields not receiving manure.

Once you start asking questions, innocence is gone. -Mary Astor

So long as you have food in your mouth, you have solved all questions for the time being. -Franz Kafka

References

Chromec, F. W., and Fred Magdoff. 1984. “Alternative Methods for Using Organic Materials Composting vs. Adding Directly to Soil.” Journal of Environmental Science and Health . Part A: Environmental Science and Engineering 19 (6): 697–711. doi:10.1080/10934528409375188.

Kirchmann, H., T. Kätterer, L. Bergström, G. Börjesson, and M. A. Bolinder. 2016. “Flaws and Criteria for Design and Evaluation of Comparative Organic and Conventional Cropping Systems.” Field Crops Research 186 (February): 99–106. doi:10.1016/j.fcr.2015.11.006.

Magdoff, F., and R. Weil. 2004. “Soil Organic Matter Management Strategies.” In Soil Organic Matter in Sustainable Agriculture. Advances in Agroecology. CRC Press. doi:10.1201/9780203496374.ch2.

Schlesinger, William H. 2000. “Carbon Sequestration in Soils: Some Cautions amidst Optimism.” Agriculture, Ecosystems & Environment 82 (1–3): 121–27. doi:10.1016/S0167-8809(00)00221-8.

Stone, A. G., Scheuerell, S. J., Darby, H. M., Magdoff, F., & Ray, R. (2004). Suppression of soilborne diseases in field agricultural systems: organic matter management, cover cropping, and other cultural practices. Soil organic matter in sustainable agriculture, 9, 131-177.

US EPA. 2007. Estimated Animal Agriculture Nitrogen and Phosphorus from Manure. https://www.epa.gov/nutrient-policy-data/estimated-animal-agriculture-nitrogen-and-phosphorus-manure

USDA-ERS. Major Land Uses. 2017. 2012 statistics, https://www.ers.usda.gov/data-products/major-land-uses/

Filed under Sustainability

4 comments on “There is not enough manure (or compost) to sustain agriculture”

  1. Jim Ossman said on October 20, 2017:

    Excellent. I learn something every time you do a post Andrew.

  2. Edward Wheeler said on October 25, 2017:

    While I assume you have good intentions with this article, many of your statements are conjecture not supported buy fact and may lead some readers to the wrong conclusion and certainly does not explore a holistic approach to sustainability which is necessary. For example, you state that “A lot of compost is produced using manure and so is part of the manure stream.” This is not true in Washington (nor I believe the rest of the county). The majority of compost (as defined by state rule) is produced from biomass sources other than manure. Another example is your statement “The significant cost of composting must be weighed against…” Composting is one of the least expensive methods to develop usable soil amendments that may add to the sustainability of a system. Finally, your use of the concept “sustainability” is missing the mark. Whether or not a system is sustainable is highly dependent upon the scope of your project. Sustainability needs to encompass ALL aspects of an ecosystem. I think that many readers will be mislead by statement made in this article and you should contemplate removal or inclusion of caveats. Thank you.

    • Andrew McGuire said on October 25, 2017:

      Edward, thanks for the comments. While “A lot of compost…” is not precise, it is accurate, and does not mean “a majority.” Although I could not find exact estimates, in 2016 WA compost facilities that use manure represented about 25% of the total compost production for the state. Not the majority, but still “a lot.”

      Regarding the cost of composting, it is significant compared to not composting, as can be done with manure application on a feed crop.

      Regarding sustainability, while it may make sense to limit the scope for some issues, when it comes to importing organic amendments such as compost or manure, I think we need to widen the scope to include the sources of those amendments.

      I am not saying that applying manure or compost is a bad practice, only that we cannot generalize the benefits of these amendments to agriculture overall because they are not available in sufficient quantities.

      • Edward Wheeler said on October 25, 2017:

        Andrew, thanks for your response. Unfortunately the details are too complex to discuss via an email. My main point is that many of the statements on your website could be misleading to folks who are don’t have enough industry/science background and knowledge. This can lead to misconceptions about the information. In fact that is what prompted me to respond in the first place.

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