There are many soil health benefits of applying high rates of organic amendments. For example, a recent meta-analysis found adding organic amendments increased soil organic matter across multiple studies by an average of 29% in croplands and 34% in grasslands (Figure 1 below, Beillouin et al., 2023). This is the power of winning a pyramid scheme. What the studies don’t consider, however, is what happens to the losers of a pyramid scheme. What do I mean by all this? With organic amendments, all of it had to grow somewhere.
All of it Grew Somewhere.
This is an often ignored, but important fact about organic amendments like compost, manure, and biochar (and even bale grazing, feed in chicken tractors, and orange peels). Before these organic materials were composted, turned into manure through livestock, or pyrolyzed into biochar, they were plants fixing carbon through photosynthesis on a piece of land. And when we apply an amendment to a soil where it was not originally grown, the soil where it was grown is deprived of the carbon and other nutrients that were in that biomass. As a result, the losing soil may have reduced soil organic matter, less nutrients, and decreased biological activity, while the winning soil gains the opposite. One field’s gain is another’s loss. Further, at the rates typically used, the area of land with degrading soil will often be larger than the improved land area, and so these rates are neither sustainable nor scalable.
“Application of organic amendments that originated elsewhere can be expected to involve degradation of the soil from which the C and nutrients were originally harvested, making the practice questionable from an overall sustainability viewpoint.” Magdoff and Weil (2004)
Don’t get me wrong here; diverting waste streams to composting and using compost to improve soils are beneficial and recommended practices. This analysis takes a broader view of these practices, looking at how far they could be scaled by including the land where the biomass in organic amendments originally grew.
I’ve written previously about this sustainability issue with manure. For this post, I want to demonstrate this same sustainability issue with compost, and include a way to evaluate the sustainability/scalability of specific application rates. To do this, let’s consider three values:
- Compost application rates
- Compost productivity rates
- The ratio between these two values
The first is how much compost is being imported and applied. The second is the potential amount of compost that could be produced from the land’s annual biomass production. This represents the rate of sustainable, scalable compost production. The last gives us a way to compare the two values. This ratio represents the amount of land required to generate the amount of compost being applied.
How much compost is being applied?
Reported compost application rates vary widely in both cropland and rangeland studies (Table 1).
| Study | Site Type | Location | Rate (reported units) | Rate (dry tons/ac) |
|---|---|---|---|---|
| WSU project in dryland wheat | Cropland | WSU Wilke Farm, Odessa, WA | 50,000 kg/ha* | 18 |
| Sullivan et al. (2003) | Cropland | Puyallup, WA | 155 Mg/ha† | 36 |
| Tian et al. (2023) | Cropland | Fulton County, IL | 165 Mg/ha | 74 |
| Leger et al. (2022) | Rangeland | Pima County, AZ | 16 kg/m2‡ 33.6 kg/m2‡ | 71 150 |
| Ryals and Silver (2013) | Rangeland | Browns Valley, CA Nicasio, CA | 7 kg/m2 | 31 |
| Withers (2023) | Rangeland | San Antonito, NM | 3.2 kg/m2 6.4 kg/m2 12.8 kg/m2 | 14 29 57 |
| *20% moisture content assumed for calculations. †48% moisture content used for calculations. ‡These compost rates were in addition to adding a 15-20cm depth of mulch made from mesquite branches. |
These compost application rates often provide remarkable benefits to the soil. Because of this, it’s tempting to want to see those benefits scaled up to a regional or even global scale. But we must remember that this compost, whether made from grass cuttings, food waste, or manure, all ultimately came from plants that had to be grown somewhere. If we think of compost as a resource that we want to apply to the land, how much land does it take to create the amount of compost these studies are applying? This brings us to our next question.
How much compost can the land produce?
Obviously, the answer to this question varies widely with soil and climatic conditions, but we can do a rough calculation here to demonstrate that the amounts of compost being applied far exceed the compost production potential of the receiving land.
To estimate how much compost the land can produce, we need to know how much aboveground biomass the land can produce.
Compost Production Potential of Cropland
Let’s use a range of corn and wheat yields for our cropland calculations. If we know the grain yield of the crop, we can estimate how much total aboveground biomass the crop produced using the harvest index, which is the ratio of the grain yield to the total aboveground biomass.
Harvest Index = Grain Yield/Total Aboveground Biomass
Rearranging this equation gives us:
Total Aboveground Biomass = Grain Yield/Harvest Index
Harvest index values vary by crop and variety, but for this estimation, a harvest index of 0.53 was used for corn and 0.39 for wheat (Prince et al., 2001).
Subtracting the grain yield, which is exported from the system, from the total aboveground biomass leaves us with the amount of potentially compostable biomass:
Compostable Biomass = Total Aboveground Biomass – Grain Yield
When biomass is composted, there is a loss that ranges from about 25-75% (Eklind and Kirchmann, 2000; Tiquia et al. 2002). For this estimation, let’s assume a 50% biomass loss during composting.
Putting it all together we can estimate the amount of compost that can be produced from both corn and wheat crops of varying levels of productivity (Table 2).
| Crop | Grain Yield (bu/ac) | Total Aboveground Biomass (ton/ac) | Compostable Biomass (ton/ac) | Composted Biomass (ton/ac) |
|---|---|---|---|---|
| Corn | 75 | 4.0 | 1.9 | 0.9 |
| (HI = 0.53) | 100 | 5.3 | 2.5 | 1.2 |
| 125 | 6.6 | 3.1 | 1.6 | |
| 150 | 7.9 | 3.7 | 1.9 | |
| 175 | 9.2 | 4.3 | 2.2 | |
| 200 | 10.6 | 5.0 | 2.5 | |
| 225 | 11.9 | 5.6 | 2.8 | |
| 250 | 13.2 | 6.2 | 3.1 | |
| Wheat | 25 | 1.9 | 1.2 | 0.6 |
| (HI = 0.39) | 50 | 3.8 | 2.4 | 1.2 |
| 75 | 5.8 | 3.7 | 1.8 | |
| 100 | 7.7 | 4.9 | 2.4 |
Average corn and wheat yields in the US are about 175 bu/ac and 50 bu/ac, respectively. Full aboveground residue removal for composting in these average production scenarios would yield only about 2.2 tons/ac of composted corn residue and 1.2 tons/ac of composted wheat residue. Contrast that with the 18 to 74 tons/ac of compost being applied in our selected cropland studies (Table 1).
Compost Production Potential of Rangeland
There is increasing interest in the effects of compost application on rangeland—primarily its effects on soil health and carbon sequestration (Fig. 2, Graveur et al., 2019; Kutos et al. 2023).
In the case of rangeland, we can make a rough estimate of the total aboveground biomass productivity of an area using annual precipitation values.
Total Aboveground Biomass =35.69 x (1-e(-0.0012 x Precipitation))
This equation was adapted from Del Grosso et. al (2008) to US standard units. Precipitation is entered in inches per year and total aboveground biomass is calculated in dry tons/ac.
As with the crop residue example, we can expect about half of the biomass to be lost through the process of composting. Table 3 shows the results for a range of precipitation levels.
| Annual Precipitation (in) | Total Aboveground Biomass (ton/ac) | Composted Biomass (ton/ac) |
|---|---|---|
| 5 | 0.2 | 0.1 |
| 10 | 0.4 | 0.2 |
| 15 | 0.6 | 0.3 |
| 20 | 0.9 | 0.4 |
| 25 | 1.1 | 0.5 |
| 30 | 1.3 | 0.6 |
| 40 | 1.7 | 0.8 |
| 50 | 2.1 | 1.0 |
| 75 | 3.1 | 1.5 |
It’s no surprise that the estimated quantity of composted biomass that rangelands can produce is lower here than that produced by cropland. Annual crops like corn and wheat produce much more aboveground biomass compared to the perennial species that characterize rangelands. Furthermore, annual crops are also bred and managed for high yields and provided the nutrients they need to optimize yields.
Next, we compare the applied rates of compost to the receiving land’s compost production potential.
How many acres of land does it take to supply the compost for an acre of land?
The purpose here is not to suggest that all compost must be produced by the land that it is applied to, but rather to compare the compost rate to the potential compost production of that land. By dividing the compost application rates by the compost production potentials given above, we can get a sense of how much land would be required to supply the amount of compost that is being applied in these studies (Table 4).
| Study | Site Type | Compost Application Rate (tons/ac) | Compost Production Potential (tons/ac) | Land required to supply compost for one acre (ac)* |
|---|---|---|---|---|
| WSU project in dryland wheat | Cropland | 18 | 1.8 | 10 |
| Tian et al. (2023) | Cropland | 59 | 1.6 | 47 |
| Sullivan et al. (2003) | Cropland | 36 | 2.1 | 17 |
| Leger et al. (2022)* | Rangeland | 71 150 | 0.36 | 197 419 |
| Ryals and Silver (2013) | Rangeland | 31 | 0.61 | 51 |
| Withers (2023) | Rangeland | 14 29 57 | 0.36 | 39 81 158 |
| *An alternative way of looking at this is as a frequency of application at the sustainable rate. Instead of 10 acres being required to produce the compost, that rate is applied once every 10 years. This does not account for the problem of “saving” up biomass production for 10 years to produce that one application rate. |
The Pyramid Scheme of High Organic Amendment Rates
Our calculations reveal that high, and even moderate compost application rates (those with ratios >1) become pyramid schemes when viewed according to potential compost production rates. Is it any surprise that we see a positive benefit of applying the compost generated (and nutrients harvested!) from 10, 50, or even 419 acres to one acre? But these rates are not sustainable nor scalable. Not every acre can be a winner in the pyramid scheme of organic amendments. The benefits of adding such massive amounts of organic material to the soil have simply been concentrated and relocated, not generated (Fig. 3).
Adding organic amendments like compost is promoted as a sustainable practice because the source of these materials, and the effect of their export from their source, is ignored. Even experts like David Montgomery overlook this. In his book Growing a Revolution: Bringing Our Soil Back to Life, he confuses what he did in his garden through mulching and composting with imported organic materials with what farmers can do in large fields. He is not alone.
“Adding organic materials such as crop residues or animal manure to soil, whilst increasing SOC, generally does not constitute an additional transfer of C from the atmosphere to land, depending on the alternative fate of the residue”
Powlson et al (2011)
Why it Matters
At high, even moderate rates, compost and other organic amendments can provide remarkable benefits. Many see these results and call for more use of these materials to regenerate the soil, replace synthetic fertilizers, and sequester soil carbon. However, we should temper our enthusiasm by remembering the pyramid scheme behind the rates needed to achieve these benefits. These rates just don’t scale, and therefore have limited application. There is no avoiding the winners and losers. Sites close to compost production will be winners, but a larger area, the source of the compost, will be losers. We must view the winners as a lucky anomaly rather than demonstrating something that can be implemented widely. Compost and manure are valuable because they are a concentrated import of photosynthetic production. However, similar to fossil fuels, they are not renewable at many of the rates used. The benefits of high organic amendment rates are similar to the benefits of being at the top of a pyramid scheme; the wealth and benefits concentrated there can only be a reality for a select few.
Edited by Angela Florence.
References
Beillouin, D., M. Corbeels, J. Demenois, D. Berre, A. Boyer, et al. 2023. A global meta-analysis of soil organic carbon in the Anthropocene. Nat Commun 14(1): 3700. doi: 10.1038/s41467-023-39338-z.
Corden, C. 2019. A Mathematical Deconstruction of Pyramid Schemes. Leicester Undergraduate Mathematical Journal 1(0). https://journals.le.ac.uk/ojs1/index.php/lumj/article/view/3437 (accessed 22 March 2024).
Del Grosso, S., W. Parton, T. Stohlgren, D. Zheng, D. Bachelet, et al. 2008. Global Potential Net Primary Production Predicted from Vegetation Class, Precipitation, and Temperature. Ecology 89(8): 2117–2126. doi: 10.1890/07-0850.1.
Eklind, Y., and H. Kirchmann. 2000. Composting and storage of organic household waste with different litter amendments. I: carbon turnover. Bioresource Technology 74(2): 115–124. doi: 10.1016/S0960-8524(00)00004-3.
Gravuer, K., S. Gennet, and H.L. Throop. 2019. Organic amendment additions to rangelands: A meta-analysis of multiple ecosystem outcomes. Global Change Biology 25(3): 1152–1170. doi: 10.1111/gcb.14535.
Kutos, S., E. Stricker, A. Cooper, R. Ryals, J. Creque, et al. 2023. Compost amendment to enhance carbon sequestration in rangelands. Journal of Soil and Water Conservation 78(2): 163–177. doi: 10.2489/jswc.2023.00072.
Leger, A.M., K.R. Ball, S.J. Rathke, and J.C. Blankinship. 2022. Mulch more so than compost improves soil health to reestablish vegetation in a semiarid rangeland. Restoration Ecology 30(6): e13698. doi: 10.1111/rec.13698.
Magdoff, F., and R. Weil. 2004. Soil Organic Matter Management Strategies. Soil Organic Matter in Sustainable Agriculture. CRC Press
Montgomery, D.R. 2017. Growing a Revolution: Bringing Our Soil Back to Life. W. W. Norton & Company. Pg 242.
Powlson, D.S., A.P. Whitmore, and K.W.T. Goulding. 2011. Soil carbon sequestration to mitigate climate change: a critical re-examination to identify the true and the false. European Journal of Soil Science 62(1): 42–55. doi: 10.1111/j.1365-2389.2010.01342.x.
Prince, S.D., J. Haskett, M. Steininger, H. Strand, and R. Wright. 2001. Net Primary Production of U.S. Midwest Croplands from Agricultural Harvest Yield Data. Ecological Applications 11(4): 1194–1205. doi: 10.1890/1051-0761(2001)011[1194:NPPOUS]2.0.CO;2.
Ryals, R., and W.L. Silver. 2013. Effects of organic matter amendments on net primary productivity and greenhouse gas emissions in annual grasslands. Ecological Applications 23(1): 46–59. doi: 10.1890/12-0620.1.
Sullivan, D.M., A.I. Bary, T.J. Nartea, E.A. Myrhe, C.G. Cogger, et al. 2003. Nitrogen Availability Seven Years After a High-Rate Food Waste Compost Application. Compost Science & Utilization 11(3): 265–275. doi: 10.1080/1065657X.2003.10702133.
Tian, G., C.-Y. Chiu, O. Oladeji, T. Johnston, B. Morgan, et al. 2023. JumpStart of soil organic matter with highly stabilized organic amendment: Implication for climate-smart agriculture. Environmental Challenges 12: 100726. doi: 10.1016/j.envc.2023.100726.
Tiquia, S.M., T.L. Richard, and M.S. Honeyman. 2002. Carbon, nutrient, and mass loss during composting. Nutrient Cycling in Agroecosystems 62(1): 15–24. doi: 10.1023/A:1015137922816.
Withers, Z. 2023. Final report for FW20-363 – SARE Grant Management System. https://projects.sare.org/project-reports/fw20-363/ (accessed 23 October 2023)
Comments
Thanks. I have been asking this question, too. Compost added comes at the removal of biomass elsewhere. So nice to see the analysis
In general, I agree with the premise. But, in my opinion, the compost should be for stimulating the biology of the degraded soil, not applied at rates to build the soil immediately. The desire for instant results is more “costly” to the compost source.
I also think it needs to involve a comparison between the originating soil and the receiving soil. I don’t agree with this statement “Application of organic amendments that originated elsewhere can be expected to involve degradation of the soil from which the C and nutrients were originally harvested.” I think that is true only if repeatedly harvested. But a highly functional soil should be able to handle occasional compost harvesting, if monitored.
The real question is if the gain on the receiving land is greater than the short-term set-back of the contributing land.
Paul, thank you for the comments. I agree that the details could change my characterization of the effects on the soil providing the biomass for the compost, but we don’t often know those details, and scaling up composting would make my characterizations more likely. I think you idea about treating degraded soil rather than improving functional soil is solid.
Harvest by definition is removing carbon from the soil. Doesn’t all agriculture do this? How it’s replaced is the key to degradation, sustainability or regeneration.
Maurice, you are right about harvest. In my examples, it is the harvest of all the biomass produced (not just grain) for multiple years, up to 400+, that is incorporated in the high rates of compost found in some studies, and in some actual operations. This is what is not scalable, nor sustainable. In the long-term, the soil’s SOM level will adjust to whatever C inputs it is receiving, so it is really only the idea that we should use compost to IMPROVE soil health, which takes high rates, that I am commenting on.
Most compost is made from material that would otherwise end up in landfills. Food waste, yard waste are two primary ingredients. Municipal biosolids is another. Very rarely are compost feedstocks grown to be compost feedstocks. Agricultural wastes, fish waste food processing wastes- these are compost feedstocks
Sally, thanks for the comments. As I stated in the post, “diverting waste streams to composting and using compost to improve soils are beneficial and recommended practices. This analysis takes a broader view of these practices, looking at how far they could be scaled by including the land where the biomass in organic amendments originally grew.” The point is not that the land actually produces the material to compost, but asking to what extent does the rate of compost rely on other land to provide it?
There seems to be floors to any farming system baling straw to burn at power stations puts a value on the straw which is maybe to high for livestock producers.
Making ethanol out of wheat encourages wheat production on land that could maybe put to better use growing food?
There’s lots wrong with farming that’s not caused by farmers I think finding the floors in compost if its a waste product and safe to use at sensible rates isn’t something to pick holes in?
Thanks for the comments. As I stated in the post, “diverting waste streams to composting and using compost to improve soils are beneficial and recommended practices. This analysis takes a broader view of these practices, looking at how far they could be scaled by including the land where the biomass in organic amendments originally grew.” There is nothing wrong with being a winner in this pyramid scheme, but those winners are the fortunate few because it works like a pyramid scheme, requiring a greater area of land to provide the compost than the area of land that benefits from it.