2. Can Manure Sustain Soils?


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Agronomist Andrew McGuireFAFDL CONTRIBUTOR: Andrew McGuire | Agronomist | Washington State University Center for Sustaining Agriculture and Natural Resources | @agronomistag

Andrew McGuire is an agronomist working in the Columbia Basin’s irrigated cropping systems. His current focus is on helping farmers build soils, save money, and maintain yields through high residue farming systems and cover cropping.

This piece previously appeared on CSANR blog. It appears here by permission of the author. It is the second in a three part series that can be found here.


The Role of Manure in Sustainable Agriculture – Part Two: Can Manure Sustain Soil Organic Matter?

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

My first question about manure, “Can Manure Supply Nitrogen and Phosphorus to Agriculture?” was answered here. But manure is more than nutrients. The bulk of manure is organic material, the carbon that the primary-producer feed crop took from the air and built into organic molecules (hence the name “organic”). When added to the soil, some of this manure bulk ends up as soil organic matter.

Organic matter is a small but crucial portion of soil. If we can maintain a soil’s organic matter levels, we have gone a long way in maintaining soil health and function. Can manure do this? Can manure sustain soils?

To answer this, we must calculate the rate of manure needed to maintain a soil’s organic matter (SOM), assuming that higher rates would then increase SOM levels. We will use the previously calculated rates of manure production, but first we first need to know how much organic matter is lost from the soil each year.

Manure lagoon

How much soil organic matter is lost each year?

Organic matter breaks down over time through biological activity and chemical weathering. Some of it breaks down quickly, other parts of it break down very slowly. The primary factors in determining the breakdown rates are temperature, moisture and oxygen levels. Warm soils, if wet and aerated, can have high rates of soil organic matter loss. Extremes, very hot or cold, dry or wet, will slow decomposition. Soil texture affects these. Sandy, well-drained soils lose a greater percentage of SOM each year, while tight clay soils lose less. Management also makes a difference. Tillage, by adding oxygen to the soil, promotes organic matter breakdown. Assuming that erosion is not a problem, annual organic matter losses range from 1-5% of total SOM (Magdoff and Weil, 2004). A tilled clay soil might lose 2% of its organic matter each year while a sandy loam soil could lose 4%. The actual losses will depend on the soil’s level of organic matter. Here are the annual losses for a range of loss rates and SOM levels1.

Table 1. Annual loss of soil organic matter by level and loss rate, lb. dry matter per acre1.

  Soil organic matter level, %
SOM loss rate, % 0.52 1 2 3 4 5 6
1 100 200 400 600 800 1000 1200
2 200 400 800 1200 1600 2000 2400
3 300 600 1200 1800 2400 3000 3600
4 400 800 1600 2400 3200 4000 4800
5 500 1000 2000 3000 4000 5000 6000

1 Top 6” of soil, 2,000,000 lb. per acre

2 SOM levels in sands and some soils in the arid West are this low.

How much of the applied manure ends up as soil organic matter?

The table above shows us what needs to be replaced every year to maintain SOM levels. However, when we apply an organic amendment (biomass) such as manure to the soil, not all of it ends up as soil organic matter. Much of the biomass is decomposed, eaten by soil microbes. This feeding is a vital function of adding biomass to the soil, but for our purposes here of maintaining SOM levels, it is a loss. Research shows that this loss depends on the soil and the material involved. In PNW soils near Pendleton OR, Wuest and Reardon (2016) estimated losses ranging from 50-100% for various organic materials. Manure averaged 79% loss. In other research, losses from manure ranged from 80-92% (Gregorich et al. 1996, Triberti et al. 2008). So, for maintaining SOM, only 8-21% of the applied manure becomes soil organic matter.

How much manure does it take to maintain soil organic matter levels?

Now we can combine all this to calculate the manure needed to maintain SOM levels. We will assume, optimistically, that 21% of our applied manure ends up as soil organic matter (Wuest and Reardon, 2016)

We need enough manure so that the (manure rate) x 0.21 = SOM loss

Or Manure rate = (SOM loss) ÷ 0.21

Applying this to the SOM loss numbers from Table 1 above gives us these manure application rates in tons dry matter per acre.

Table 2. Manure application rates required to replace annual losses from soil organic matter, tons dry matter per acre.

  Soil organic matter level, %
SOM loss rate, % 0.5 1 2 3 4 5 6
1 0.2 0.5 1.0 1.4 1.9 2.4 2.9
2 0.5 1.0 1.9 2.9 3.8 4.8 5.7
3 0.7 1.4 2.9 4.3 5.7 7.1 8.6
4 1.0 1.9 3.8 5.7 7.6 9.5 11.4
5 1.2 2.4 4.8 7.1 9.5 11.9 14.3

These are dry matter numbers. With added moisture, 33%, typical manure from a dirt cattle feedlot, here are the numbers.

Table 3. Manure application rates required to replace annual losses from soil organic matter, tons per acre at 33% moisture.

  Soil organic matter level, %
SOM loss rate, % 0.5 1 2 3 4 5 6
1 0.4 0.7 1.4 2.1 2.8 3.6 4.3
2 0.7 1.4 2.8 4.3 5.7 7.1 8.5
3 1.1 2.1 4.3 6.4 8.5 10.7 12.8
4 1.4 2.8 5.7 8.5 11.4 14.2 17.1
5 1.8 3.6 7.1 10.7 14.2 17.8 21.3

This how much manure we must apply each year to maintain soil organic matter, the soil-demand for manure. More is required to increase it. As you can see, a lot of manure is required, especially for soils with high SOM levels.

These numbers will be close for both cattle (earth lot) and broiler manure. Multiply by 2.2 for dairy manure (at 72% moisture) or 2.8 for swine (slurry at 91% moisture).

Now let’s compare these rates with manure production rates, or manure supply.

How many acres of crop production is needed to produce the manure needed to maintain soil organic matter?

If we divide the tons of manure needed to maintain one acre of SOM by the tons of manure produced by an acre of feed crop (using the average of best and worst manure production rates from previous post, 3.6 tons per acre, manure at 33% moisture), we get the number of acres of manure (through feed) production it takes to maintain one acre of soil organic matter.

Table 4. Acres of crop production required to produce the manure needed to maintain soil organic matter levels of one acre. Red text shows where more than one acre of manure production is required to maintain one acre of organic matter loss.

  Soil organic matter level, %
SOM loss rate, % 0.5 1 2 3 4 5 6
1 0.1 0.2 0.4 0.6 0.8 1.0 1.2
2 0.2 0.4 0.8 1.2 1.6 2.0 2.4
3 0.3 0.6 1.2 1.8 2.4 3.0 3.6
4 0.4 0.8 1.6 2.4 3.2 3.9 4.7
5 0.5 1.0 2.0 3.0 3.9 4.9 5.9

Are these numbers reasonable? I found a few research papers that had done similar calculations. Schlesinger (2000), using long-term data from the Sanborn plots in Missouri, estimated that 3 acres of cropland were required to provide the manure for one acre of SOM maintenance. Magdoff and Amadon (1980) estimated that an annual application of 19.6 wet tons of manure per acre was required to maintain 5.2% SOM in their conditions. Working backward, they estimated that this amount of manure would be produced by feeding the corn silage from 2.5 acres. So our numbers are similar.

Can we now say whether manure can sustain soils? Not yet. First we must decide which soils we want to sustain with the manure.

Can manure sustain the soils that produced it?

The ideal situation is where the manure is recycled back to the field that produced it, at the rate that it is produced, similar to what happens in a grazing system, but with the additional losses we have discussed. Here, we can consider the manure as a transformed crop residue. Crop residues – the stems, straw, leaves, chaff, cobs, whatever is not harvested – are the main source of organic material for the soil. Because soil organic matter levels reflect the balance of what we add to the soil and what is lost (Buyanovsky and Wagner 1998), we want to conserve as much of this material as possible, including manure.

Cycling manure back to the cropland it came from.
Graphic: A. McGuire, 2017.

For this situation, the table above shows us that manure, by itself, cannot sustain soils except in low SOM soils, 1-2% or less, or with higher SOM levels but with low 1% SOM loss rates. So manure can sustain soils that many consider having sub-adequate levels of soil organic matter (Loveland and Webb 2003) or soils that are managed to have very low loss rates. Because tillage is the major factor in loss rates, systems where tillage is eliminated such as with perennial crops or no-till annual crops would work here.

For combinations of SOM levels and loss rates in the red region of Table 4, manure cannot sustain the soil. Manure is a renewable resource; by growing crops and feeding livestock, we can produce it indefinitely. However, renewable does not mean unlimited. When we use manure in excess of its production rate – the amount of manure produced from a feed crop growing on the same soil – it is only at the expense of other fields.

Here is an example:
We have a field with 3% organic matter and a 3% loss rate, and we have enough manure available to apply the 6.4 tons of manure per acre each year (Table 3) to maintain organic matter levels at 3%. It is all sustainable at the field level; soil organic matter is maintained along with all those soil functions related to SOM. However, at the regional level, we know that it takes the production of almost 2 acres (Table 4) to produce the manure for this one acre of SOM maintenance. This means that for every acre where manure sustains soil organic matter, there is another acre that will not get manure and where soil organic matter will decrease.

For soils with higher SOM levels or loss rates greater than 1%, manure, by itself, will not sustain the soil. But manure is not the only source of SOM; crop residues can also be a significant source of organic material that ends up as SOM. High residue crops like corn (for grain) and wheat contribute the most while low residue crops like most vegetables, or crops where all the residue is removed at harvest, like corn or other silage, and hay crops, have little or no effect on SOM. Therefore, combining manure with a high residue crop and minimal tillage will expand the range of soils where SOM levels are maintained, pushing the red area of Table 4 to the right.

This discussion has been based on manure production rates, supply and demand, but now let’s shift to the topic of manure distribution and its effect on soils.

Can imported manure sustain soils?

The ideal situation above is not the norm. Because feed (dry grain and soybeans) is easily transported but wet and heavy manure is not, our current production system tends to leave a concentration of manure far from the fields that produced it.

Manure moves biomass from distant crops fields and concentrates on a few local fields.
Graphic: A. McGuire, 2017.

This apparent abundance of manure is the main reason manure is viewed as a sustainable input.

First, let me be clear that manure is beneficial. Soils receiving manure can have higher nutrient and soil organic matter levels than soils without manure. Even better for those that can do it, applying manure can overcome the detrimental effects of tillage and production of low residue crops. However, we can now see that sustaining SOM levels in most situations, requires manure rates that are higher than production rates. The concentration of manure in one location does not change this. Even in locations with abundant manure, where we can apply enough to sustain soils, this imported manure is a loss to those distant fields that produced it. Relying on imported manure sustains some fields at the expense of others; we rob Peter to pay Paul.

Those researchers that have taken a critical look at the use of imported organic amendments have concluded the same, and not just for manure, but for compost too. Magdoff and Weil (2004) state “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.” This concentration of manure from many fields onto one field is not sustainable.

Manure Is a Scarce Resource

When faced with piles of manure and limited area to spread it on, it is easy to forget that manure is connected to crop production. Like other products that are used far from their source (e.g. tropical hardwoods, seafood), manure’s real scarcity or abundance is hidden.

Some farmers are in the enviable position of having access to lots of manure, or of being able to pay for it to be transported to their farm. Their unique situation does not change the fact that there is not enough manure to go around, nor does it make them more sustainable. As we have found, manure rates that can supply a crop with nutrients or increase soil organic matter levels are often only possible because of the degradation of the fertility or SOM of other fields. It is not possible for all farmers to use manure to maintain their soil’s fertility or organic matter level.

What does this finding tell us about farming system comparisons, carbon storage in soils for climate change mitigation, and livestock production systems? Those are questions for my next post.

Three Part Series: The Role of Manure in Sustainable Agriculture

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Buyanovsky, G.A., and G.H. Wagner. 1998. Carbon cycling in cultivated land and its global significance. Global Change Biology 4: 131–141.

Gregorich, E. G., B. C. Liang, B. H. Ellert, and C. F. Drury. 1996. “Fertilization Effects on Soil Organic Matter Turnover and Corn Residue C Storage.” Soil Science Society of America Journal 60 (2): 472–76. doi:10.2136/sssaj1996.03615995006000020019x.

Loveland, P., and J. Webb. 2003. “Is There a Critical Level of Organic Matter in the Agricultural Soils of Temperate Regions: A Review.” Soil & Tillage Research 70 (1): 1–18.

Magdoff, F. R., and J. F. Amadon. 1980. “Yield Trends and Soil Chemical Changes Resulting from N and Manure Application to Continuous Corn.” Agronomy Journal 72 (1): 161–64. doi:10.2134/agronj1980.00021962007200010031x.

Magdoff, F., and R. Weil. 2004. “Soil Organic Matter Management Strategies.” In Soil Organic Matter in Sustainable Agriculture. Advances in Agroecology. CRC Press.

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.

Triberti, L., A. Nastri, G. Giordani, F. Comellini, G. Baldoni, and G. Toderi. 2008. Can mineral and organic fertilization help sequestrate carbon dioxide in cropland? Eur. J. Agron. 29:13–20. doi:10.1016/j.eja.2008.01.009

Wuest, Stewart B., and Catherine L. Reardon. 2016. “Surface and Root Inputs Produce Different Carbon/Phosphorus Ratios in Soil.” Soil Science Society of America Journal 80 (2): 463. doi:10.2136/sssaj2015.09.0334.


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