Manure vs Fertilizer
Manure adds a lot more than crop nutrients to your soil. Just ask J.Mark Powell. Dr. Powell, whose title is “Agroecologist” at the USDA Agricultural Research Service U.S. Dairy Forage Research Center at Madison, Wis., offers a list of the benefits from manure that you can’t get from chemical fertilizers. “You’re applying organic matter, which enhances the physical properties of the soil structure,” Powell says, explaining that the manure lightens the texture of the soil, enhancing rainfall infiltration. He goes on, “It increases the cation exchange capacity of the soil, or the ability of the soil to hold on to nutrients and pesticides and make them more effective than if you were just applying fertilizer.”
There’s more. Manure applications leave the microbial community in the soil more active, which improves soil quality. The extra organic matter also helps soil hold onto water a little better. “If a farmer is land applying manure,” says Powell, “and just surface applying it—which we don’t recommend, because a lot of the ammonia nitrogen is lost that way; it goes up into the atmosphere, but a lot of farmers do that in no-till operations—that organic matter that stays on the soil surface actually impedes runoff.”
Although 70% of the nitrogen applied to Upper Midwestern crops is of the commercial fertilizer variety, Powell says manure and fertilizer “complement each other, sure enough.” But there’s more to the nutrients in manure than just nitrogen. “The rations that we feed our dairy cows have a huge impact on the chemistry of the manure,” he says. “If the animals are being fed corn silage versus alfalfa silage; if they’re being fed mineral supplements—only a small part of that goes into milk.” Just 25-30%, he says, adding with a laugh, “Cows are kind of manure machines. They produce milk, but they produce a lot more manure than they do milk.”
Powell’s colleague, Dr. Michael Russelle, is both a researcher at the Dairy Forage Research Center and an adjunct professor at University of Minnesota; he says most of the 21 nutrients needed by grain crops, including trace nutrients like sulfur, zinc, boron and molybdenum, can be gleaned from manure. “They are recycled more or less efficiently, depending on where the manure’s deposited,” says Russelle, “so in a confinement herd, almost 100% of what’s excreted can be recycled onto the land.”
But the key to it all is the nitrogen, and that can be lost as the manure sits on the barn floor or in a storage lagoon. Russelle says there are ways to reduce those losses; one of them is to cover the manure storage. “That’s been adopted in Europe under mandate in the Netherlands, and under voluntary approaches in other countries,” he says. “Because nitrogen fertilizer, like all fertilizers, has gone up in price, there’s been an interest in doing it, and there are cover systems here. They can be impermeable covers made of different kinds of rubber, for example; there are also straw covers that you can put on lagoon storages, and those will greatly reduce ammonia losses.”
Whether a producer needs to supplement the nitrogen in manure with commercial fertilizer depends on the ratio of land to animals. Not all of the nitrogen applied to the land will come back in the form of manure; Russelle says if a cow’s ration is properly balanced between energy and protein, up to 30% of the nitrogen she consumes will leave her body as milk. Then, “with the losses that can occur in storage and during application, that’s not a complete recycling by any means,” he says. “So there needs to be some supplementation either through purchased fertilizer, or through legume crops that can fix a fair amount of nitrogen and leave it in the soil.”
Russelle is conducting a research project right now measuring the nitrogen credit of alfalfa to the first year of corn. “There were questions,” he says, “because most of the dairy states in the Midwest recommend that you don’t apply any nitrogen, except perhaps starter, in the first year of corn after alfalfa. But farmers and their advisors were asking, “With our much higher yields of corn—when we’re talking, not about 130 or 150 or 180 bushels per acre, but about 200 or 250 bushels per acre—is it still true that alfalfa can provide all the nitrogen?” So far, the research shows that if you have an alfalfa stand of at least four plants per square foot, there is plenty of nitrogen for the next corn crop, even with yields exceeding 200 bls. However, Russelle cautions the work is not yet completed; the project is being conducted on farm at 17 sites, and ten have yet to be measured.
The other key nutrients required by crops—potassium and phosphorus—can also be recycled effectively through manure. “A lot of potassium is taken up by crops used for feed in the herbage,” says Russelle. “Not so much in grain crops. But if you’re cutting a corn crop for silage, there’ll be a lot of potassium in there.” Phosphorus applications, though, need to be managed carefully, using the recommendations developed by each state. “We’re facing situations where neighboring freshwater streams or lakes have Total Maximum Daily Load regulations for phosphorus,” he says. “It’s really important that manure generators…don’t exacerbate the problem so that the soils are overloaded in phosphorus, because that will end up restricting manure application on those fields.”
Producers whose enterprises are designated Concentrated Animal Feeding Operations (CAFOs) have to have a manure management plan that spells out how the manure will be applied, at what rates, and on what land. If they have more manure than land to which to apply it, they have to enter into contractual agreements to apply the manure on neighbors’ land. But that can present a marketing opportunity; as Russelle says, “In today’s economy, manure can be worth more per thousand gallons that you have to move, than the milk is worth.” The key is to convince the neighbor of its value, something that can be done by testing the manure, and by delivering and applying it in such a way that those values are maintained.
If manure is kept in liquid form and applied properly—that is, by injection or by incorporation within a couple of hours of application—the amount of nitrogen it will provide can be predicted fairly precisely. If it’s not quickly soil incorporated, Russelle says, “a lot of that nitrogen will be lost as ammonia, and then it’s just about anybody’s guess how much nitrogen will be available for the next crop.” There are two other points in the process where variability of nutrient application can occur; those are where manure supplies of different consistency are being mixed in a pit storage or lagoon prior to application, and through use of the applicator itself. “Our solid manure applicators are notoriously variable in the ability to distribute manure evenly,” Russelle says. “Liquid applicators, if they are well maintained—and especially the modern ones—are extremely uniform in application rates.”
Manure is not a panacea, of course. If a producer has Johne’s disease or another ailment running through his herd, he has to be sure to incorporate manure applications; otherwise, the organisms may survive on the harvested forage crop and infect the rest of his animals—or, if he’s marketing his manure, someone else’s animals. Russelle says even when exposed to sunlight and drying conditions on plant surfaces, many pathogens can live from six months to a year.
Also, manure is bulky and much less concentrated than commercial fertilizer, so the transportation cost is an issue. Says Mark Powell, “It has to go out in manure haulers, so farmers have to go back and forth from the barn quite a bit, whereas a bag of fertilizer can spread a pretty large area.” In addition, he says current research shows that manure also has pathogens and antibiotics that may be harmful to aquatic systems and soil systems.
So there are potential negative tradeoffs, but as Powell points out, many generations of farmers have depended upon manure. In a research paper, Contributions to Society: Manure-Fertilizer/Fuel, Developed Countries, he talks about how it may become more important to conserve the nitrogen in manure if energy costs continue to rise, and will also reduce CO² greenhouse gas generation by displacing manufactured fertilizer. And the energy trapped in the manure itself can be harvested either through combustion, gasification or pyrolysis.
“It was only after World War II,” Powell says, “when the bomb factories were converted to nitrogen fertilizer factories, that manure became less important. Prior to 1950, manure was the only thing farmers had, and there are many parts of the world where manure is the only thing farmers have; they don’t have access to chemical fertilizer. So manure is really important in many, many societies.”