Can Manure Supply Nitrogen and Phosphorus to Agriculture?
September 7, 2017
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 farming, regenerative 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.
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.
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 http://nmplanner.missouri.edu/tools/pan_calculator.asp, 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.