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 might be available in greater quantities than manure (see Table 4 of this publication), 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%|
|Roads, parks, industrial, military, rural homesteads||14%|
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 (see Cogger et al. 2013). 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|
|Perennial crops in rotation||
|High residue crops||
|Conserve crop residues||
|Reduce tillage intensity||
|Application of organic amendments||
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. The amount of these inputs need not be large: “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), nor the organic matter levels high: “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. 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 prepared to do an in-depth analysis of the many comparisons of organically farmed soils to conventionally farmed soils. I even wrote 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 broad 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, use a sustainable rate of manure application that 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.
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 concentrated 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 eating of food, or to leaching. 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
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/