Can Manure Supply Nitrogen and Phosphorus to Agriculture?

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

Manure, whether fresh, old, or composted, is often declared a key component of sustainable agriculture. From countless trials, researchers have come to similar conclusions (Haynes and Naidu 1998). Manure use is promoted as a solution in discussions of sustainable agriculture topics including: soil fertility, soil health, organic farmingregenerative farming, carbon sequestration, and renewable resources. However, I have questions. Not about the actual spreading of manure, or calculating application rates, but about manure’s role in sustaining agriculture. Is manure a sustainable source of nutrients? Is manure a sustainable organic soil amendment, able to build soil organic matter, store carbon in the soil, and so assist in reducing greenhouse gases? When is manure application a sustainable practice?

In my next few posts, I will answer these questions with the hope of putting manure in its proper role in sustaining agriculture. First, let’s look at the nutrient-supplying potential of manure. It all starts with figuring out where manure comes from.


How do you say manure: two or three syllables? muh-NOO er, or as I remember it from Northeast Nebraska, muh-NER. Photo: I. Barbour via Flickr cc

Where does manure come from?

Manure comes from cattle and sheep and swine and poultry, right? Well yes, but livestock are not the source of the materials in manure. Those materials come from “primary producers” as scientists call those organisms that produce “biomass” from inorganic compounds. Biomass is the stuff that living organisms are made of. Our biomass is flesh and bones and blood, but we, like livestock, are not primary producers. We and they put together our biomass from other biomass, our food. Livestock eat various biomass feeds and what’s left is manure. So then, manure is processed biomass from plants. Plants are primary producers because they don’t live on the biomass of other organisms. They get energy from sunlight and from it produce biomass.

Manure, then, comes from plants, specifically from crops grown on fields. Although the feed (crop biomass) is changed as it goes through livestock – bacteria and other organisms are added, some sloughing of livestock flesh occurs – everything in manure is derived from the feed. This is important because it means that manure production is tied directly to crop production.

How much manure can we produce?

Here’s the process. Sunshine falls on plants, which transform light energy into valuable feed, which farmers harvest and transport to livestock who eat it and produce manure. To calculate the manure produced per acre, we need to know one thing about this process. What are the losses? The biggest loss between crop in field and manure on ground is to the livestock themselves. Some loss goes to producing steaks, chicken breasts, or hotdogs, the rest to providing the basic energy needs of living animals. There are also losses at harvest, in transport, and losses in recovering the manure (collection and transport).

Here are the losses from field to livestock and back to field (percent of initial dry solids lost):

  • 57-81% between feed and fresh, excreted manure (ASABE, 2005)
  • 4-8% lost in collection and transport (10-40% of excreted manure, NRCS 1995)

It is not just organic matter that is lost, nutrients are also lost, but we’ll get to that.

From a crop in a field to the manure being applied to a field, the total loss of dry matter is 61-89%, along with the associated nutrients. Now we can start with a feed crop yield, apply the losses above, and find the amount of manure produced per acre.

Crop to applying manure2

Graphic: A. McGuire, 2017.

In a feedlot, chicken or swine house, or in a confined dairy, livestock rations are mixtures of grains, legumes (mainly soybeans), and forage (hay and silage). They all end up as manure, and that manure is applied somewhere, so we can simplify this by looking at just one crop. Corn (and corn silage, see best case scenario below) is a good one because it is grown in many parts of the country, mainly for feed, and makes up a large part of many livestock rations.

Here are the resulting numbers:

Low scenario1: 1.6 tons manure per acre

High scenario2: 5.7 tons manure per acre

Surprised? I was. This is so little that it would be near impossible to spread evenly over an acre. And if you did, you would barely be able to see it. But what does it mean? To answer that we need to look at common manure application rates.

How much manure is needed to provide nitrogen and phosphorus for a corn crop?

Since feed crops produce manure, let’s look at what it takes to produce the corn crop that produced the 1-6 tons of manure per acre. There are many Extension publications on calculating manure application rates for supplying nutrients. First, let’s focus on nitrogen as it is needed in the largest amounts. To supply nitrogen to our 174 bu. per acre corn crop (average corn yield in US, used in low scenario above) would require 22.2 tons of manure per acre (1.2 lb. N per bu. of corn, 209 lb. N per acre needed, 9.4 lb. available N per ton manure3). We found above that a 174 bu. per acre corn crop fed to cattle (finishing) will produce 1.6 ton of manure per acre. 1.6 ton produced vs. 22 tons needed. That’s the effect of the losses. Even with an irrigated corn yield of 250 bu. per acre we couldn’t produce enough manure. Only if we applied this 22 tons per acre once every 14 years (22.2÷1.6), could it be considered a sustainable source of nutrients (and then there is the matter of where the nutrients come from).

Is it any better for suppling phosphorus? Our average 174 bu. per acre corn crop will remove about 61 lbs. of phosphate (P2O5) while our cattle manure will supply about 15.9 lbs. per ton. This means we need 3.8 tons of manure per acre to supply phosphorus (61÷15.9). This is much better than with nitrogen, but still above our low manure production rate of 1.6 tons per acre.

We can look at this another way. To supply nitrogen to one acre of our corn crop, we would need the manure produced from 13.8 acres of corn (low scenario). For phosphorus, one acre of manure-supplied phosphorus requires the manure from 2.4 acres of corn production (low scenario). Using the high scenario, we still need 3.9 acres of corn for every acre of manure supplied nitrogen. Only for phosphorus will the higher rate provide enough nutrients.

How about the overall supply of manure in the U.S., how much of N and P demand will manure supply? These NRCS maps show that for most of the country, manure cannot supply more than 50% of the nitrogen and phosphorus needed by crops. And given our calculations above, much of the area in the “50% or less” category is probably much less than 50%.




Look at it this way. Even if we managed a crop for perfect uptake of nutrients. Even if we eliminate all leaching and movement of nutrients out of all farm fields. Even if we had no losses between harvest and feeding the crop to livestock, and no losses in the recovery and transport of manure, we could recycle all the manure to the field that produced it and still be lacking nutrients. Why? Because we export nutrients in the meat, milk, and eggs. Those nutrients must be replaced.

Moreover, when we look at agriculture in a region or country, we see that the nutrients supplied in manure are not a new supply. Applying manure recycles a portion of the nutrients originally used to produce crops, which is good, but it does not replace nutrients removed when crops and meat/eggs/milk are exported to cities because the nutrients in manure were already in the ag system. Manure is not a primary source of nutrients; it is a secondary source.

This means that farms that are close to manure sources, that can apply enough manure to supply nutrients to their crops, can do this only because there are other fields that are producing the manure (through feed crops) but that are not getting any manure in return. These fields, the majority, will have to rely on other sources of nutrients such as fertilizer, or in the case of nitrogen, legumes, to replace the exported nutrients.

The other option is returning the exported nutrients, but it would require large scale recycling of the nutrients from cities back to distant fields. This is not now feasible. As Magdoff et al. (1997) conclude in their excellent review of nutrients in sustainable agriculture, “…  promoting long-term sustainable nutrient management will ultimately require radical changes in the way agriculture and society are organized.”

So why does manure seem like a sustainable source of nutrients? I think it is because, where manure is readily available, it is often available in large quantities, quantities that can supply all or most N or P for nearby fields. Often, the quantities are so great that nearby fields are overloaded with nutrients and manure becomes a waste problem rather than a valuable resource. This is a side effect of our current livestock production systems, where easily transported feed is fed to concentrated livestock populations, which then produce large quantities of heavy, expensive-to-transport manure. This creates apparent an abundance of manure, but it is not widespread. As we have seen above, there is not enough to go around.

So, to answer the question we started with; no, manure cannot supply nitrogen and phosphorus to agriculture in the amounts we need. In fact, manure can only provide a small portion of the nutrients needed in agriculture.

But manure provides more than nutrients; my next post will answer this question: Can manure sustain our soils?


1 Beef finishing, 70% loss to livestock, 15% loss in manure recovery, and average US corn yield of 174 bu./ac, manure at 33% moisture.

2 Dairy production, milking cows, 57% loss to livestock, 20% loss in manure recovery, 30 ton/ac corn silage yield at 65% moisture, manure at 33% moisture.

3 Using calculator at, and ASABE data.



American Society of Agricultural and Biological Engineers. (2014). Manure Production and Characteristics (Standard No. ASAE D384.2).

Haynes, R. J., and R. Naidu. 1998. “Influence of Lime, Fertilizer and Manure Applications on Soil Organic Matter Content and Soil Physical Conditions: A Review.” Nutrient Cycling in Agroecosystems 51 (2): 123–37. doi:10.1023/A:1009738307837.

Magdoff, F., L. Lanyon, and B. Liebhardt. 1997. Nutrient Cycling, Transformations, and Flows: Implications for A More Sustainable Agriculture. Advances in Agronomy 60 (January): 1–73. doi:10.1016/S0065-2113(08)60600-8.

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Cover crop best bet is monoculture, not mixture

Can you see 17 species in this cover crop mix? Photo: A. McGuire.

Can you see 17 species in this cover crop mix? Photo: A. McGuire.

Cover crops are great. If I thought I could get away with it, I would just grow cover crops in my garden. They protect the soil, feed microbes, build soil structure, add root channels, and support beneficial insects. I think they look cool too. When cover crop mixtures got popular a few years ago, I got excited and grew a 17 species mix. It looked really cool, I mean, diverse, with all sorts of seeds that became all sorts of plants.  I took pictures, showed my kids, and even had a neighborhood open garden event! (Well, maybe not that last one) Then I grew some vegetables after the cover crop. They did OK. Just OK. I wanted it to be the best tomato/squash/cucumber/lettuce crop ever, but I could not tell the difference between these vegetables and those I had grown after many previous un-biodiverse cover crops. Recent research results may explain this. Continue reading

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Comparing effects of herbicides, fertilizers, and tillage on the soil

Is this better than an herbicide for the soil? Photo: United Soybean Board.

Is this better than an herbicide for the soil? Photo: United Soybean Board.

In a past post, I argued for the use of an herbicide instead of tillage to kill a soil-building cover crop. My post was mostly observation of the damage of tillage on the soil as compared to the lack of damage, at least visually, from the herbicide. But others suggested that herbicides may not be as benign in the soil as I portrayed them. Here is the latest science on the topic.

A series of reviews have been published on the effects of herbicides on the soil, starting with Bunemann et al. in 2006. They concluded, “The herbicides generally had no major effects on soil organisms.” More recently, a review by Rose et al. (2016) found, “Overall, the majority of papers reported negligible impacts of herbicides on soil microbial communities and beneficial soil functions when applied at recommended field-application rates.” Continue reading

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Crop rotation: In praise of deliberate, sequenced disruption of natural systems

For years, researchers have been looking to polycultures, biodiversity in space, as a way to improve agriculture (Trenbath 1974; Tilman et al. 1997; Cardinale et al. 2011; Finney and Kaye 2016). Behind this research is the idea that nature is the best model for agriculture. Because we find that nature is generally a polyculture, we should mimic this biodiversity on the farm. Natural is now viewed as the best option. Today, however, I want to commend a most unnatural practice, crop rotation.

The unnatural, disruptive transition of wheat monoculture to bean monoculture – good for agriculture

The unnatural, disruptive transition of wheat monoculture to bean monoculture – good for agriculture

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The New Urban Indoor Industrial Agriculture… but Why?

Vertical farm crop wall demonstration project. Where is the soil? Photo: State Dept. via Flickr cc

Vertical farm crop wall demonstration project. Where is the soil?
Photo: State Dept. via Flickr cc

There is a new style of urban agriculture appearing around the world. The efforts differ in details, but they all use buildings or structures not originally designed to grow plants – no greenhouses. Carried out in old shipping containers, warehouses, and high-rises, perhaps even in an old factory or two, these “farms” bring agriculture fully indoors. Without sunshine, these farms rely on artificial lights shining on plants 24 hours a day in some cases. Without soil, plants sit in plastic pipes, or float on polystyrene rafts, stacked in tiers.  Without rain, nutrient enhanced water is cycled to the plant roots through piping, pumps and filters. Without wind, fans provide ventilation, ducts and vents deliver heated or cooled air for year-round production.

All this requires energy. These farms are plugged in, reliant on outside power. Outdoor farm fields are off the grid, at least for the production portion of the food chain. Even a continuous corn crop, the scorned example of “industrial” agriculture, is not affected by a blackout. While an outdoor “industrial” crop is still subject to the biological realities of crop growth cycles and seasons, crop production in these indoor farms can be sped up and streamlined. All it takes is lots pipes and tanks, cables and lights. Continue reading

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The Fallout of October Rains in the Desert

Photo: C. Chene via Flickr cc.

Photo: C. Chene via Flickr cc.

Here in the Columbia Basin, something extraordinary has happened; it rained a lot in October. Although not technically a desert, we are normally desert-like from June-October. Not this year. How much rain did we get? Well, in Ephrata where I live, we have seen over 2.5 inches of rain. I know, not much, even by Inland Northwest standards. But 2.5” is record rainfall for us – never have we seen so much rain in October – and it has had some consequences.

They don’t often admit it, perhaps out of respect for dryland farmers to the East, but farmers in the Columbia Basin prefer to get their water out the end of a sprinkler. They like to control how much and when the water falls on their fields.  When it comes out of the sky, it messes things up. The rains have delayed harvest of late potatoes, onions, dry beans and other crops. Although I expect all these crops will be harvested, the wet ground and crops probably caused some yield losses, and equipment traffic on wet soils likely compacted soils which will require additional tillage to fix.

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Communication intercept reveals 21st century cities were alien food project

Intercepted communication of Earth Concentration Project leader, 2016, between Outpost Dq12 and exoplanet HD 40307g. Translated to English, NSA technical bulletin 358G.

“Our concentration program is progressing well sir. In fact, their own collective has observed that in 35 years, two-thirds of them will be in CAFOs [closest term we have for this word]. In one of their political entities, the USA, we have over 70% of the human population in our CAFOs”

“Are there any signs of rebellion?”

“Not really. In fact, instead of resisting, they continue to work on how to mitigate the problems of concentration rather than fighting the process.”

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The Ideological Threat of Organic Farming

At the core of organic farming is an ideology which bans the use of synthetic fertilizers and pesticides. Over the past couple decades, this ideology has been incrementally embraced by the scientific community. This mingling of science and ideology in organic farming research and education has serious consequences for both science and society.

This video was first shown at the symposium titled ‘Sustainability Challenges in Organic Agriculture’ that was organized by the Organic Management Systems Community of the American Society of Agronomy at the Annual meeting in Phoenix, Arizona in November 2016.

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Sustainability Problems with ‘Repackaged’ Synthetic Nitrogen in Organic Agriculture

Here is a video by USDA-ARS researcher Eric Brennan, shown at the same session as my video.

ORGANIC vegetable growers often use nitrogen from CONVENTIONAL animal manure & slaughter by-products. These ‘repackaged’ synthetic N fertilizers cause SUSTAINABILITY problems in organic SOIL FERTILITY management. IMPORTANT QUESTIONS: Should organic farmers be ALLOWED TO USE PURE synthetic nitrogen fertilizers rather than just repacked synthetic N? Can’t LEGUMES produce the agricultural nitrogen needs for our current world population? Can nitrogen fertilizers be produced with RENEWABLE ENERGY? Are synthetic nitrogen fertilizers toxic? What’s ‘SPORGANIC agriculture’ ?

This video by USDA-ARS scientist Dr. Eric Brennan was first presented at a symposium ‘SUSTAINABILITY CHALLENGES IN ORGANIC AGRICULTURE” at the the American Society of Agronomy annual meeting in Phoenix, Arizona in November, 2016.…


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Defending the Proper Use of Monoculture

Monoculture of dry edible beans. Photo: A. McGuire.

Monoculture of dry edible beans. Photo: A. McGuire.

Many bloggers have it wrong, Wikipedia had it wrong, and when I found that Agronomy Journal got it wrong, I was compelled to write on the topic once again. Monoculture is not the year-after-year production of the same crop in the same field. That is mono-cropping or continuous cropping, where the better alternative is crop rotation. Monoculture is “when only one crop species is grown in a field at a time” (Loomis and Connor, 1992), and the hard-to-manage alternative is polyculture or intercropping. You can take a picture of monoculture, but not of mono-cropping.

Just where this widespread misuse of “monoculture” started, I am not sure. It probably precedes the internet, and may have something to do with the similarity of monoculture and mono-cropping. More recently, Wikipedia played a part. For years it had a definition that combined the meanings of monoculture and mono-cropping. I suspect that this incorrect definition, and the fact that many people without agricultural backgrounds write about agriculture, has led to the widespread misuse we see today. Continue reading

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