Those in the custom manure hauling and application business know to never refer to manure as a “waste” product. As a valuable and natural source of plant nutrients, manure advocates know the product is anything but a waste.
The problem is, most of the world is not in the custom manure hauling and application business.
New analysis out of Cornell University, published April 15 in Nature Sustainability, reiterates what many manure proponents have been saying for years: animal manure, as well as human waste, could easily meet most of the U.S.’s fertilizer needs (102 percent of its nitrogen and 50 percent of its phosphorus needs) and save the industry an estimated $5.7 billion annually – in theory.
However, while proponents have often identified geographic mismatch and the difficulty moving manure as the main hurdle – along with public acceptance – the Cornell analysis found there may be some welcome nuances within that conversation.
“We did key analysis, spatially… when you consider some of the realities, the challenges, the logistics, a substantial amount [of manure] can feasibly be used as fertilizer for crops,” explains Chuan Liao, assistant professor in the Cornell CALS Ashley School in the College of Agriculture and Life Sciences. In his conversation with Manure Manager, Liao said despite the perception of frequent mismatches between the location of the manure – often in areas densely populated with people or livestock – and agricultural regions with the highest nutrient needs, mapping and analysis, mostly from publicly available data, revealed that large percentages of recoverable nutrients – 37 percent of nitrogen and 46 percent of phosphorus – could be used locally, and more than half of the surplus nutrients could theoretically be redistributed to nearby regions with low economic and environmental costs.
It’s been well-known for years that despite all the studies that show that manure usually lives up to – or exceeds – its potential in terms of the yields it helps achieve, it’s still being underutilized by farmers. A 2022 study from the USDA’s Economic Research Service found for the year 2021, manure was applied to only about eight percent of the 240.9 million acres of U.S. cropland that hosts the seven major field crops in the U.S. – leaving potential for much more activity, especially considering the rising demand for organic food in the U.S.
Liao and the research team looked at data from existing sources such as the USDA, Food and Agricultural Innovation and other sources, mostly to compare supply and demand. Areas considered to be “demand” zones were those where the 15 major crops in the continental U.S. were located. In aligning them with areas where high sources of manure could be supplied, Liao said “very substantial amounts” could be easily transported. One of the “low-hanging fruits” is identifying operations with the essential infrastructure to process manure on-farm, before transporting.
Transporting manure is complicated – it can only move so far. Liao explains it usually comes down to the weight of unprocessed manure. “Because it is so heavy, the nutrient density is not as high as synthetic fertilizer – in fact, it’s far, far under that.” Indeed, liquid manure can consist of up to – or over, according to some sources – 95 percent water. Solid manure also contains high moisture, meaning transport and fuel costs, now even more precarious, simply don’t align with the value of the nutrients.
There are also concerns about safety and regulations. “Some people worry about pathogens and potential pollutants,” Liao added. But with the right infrastructure, that problem can become a minor concern at best – the problem is not a “resource problem,” but simply a “coordination problem.”
Key processes
Liao has studied the issue more from a social science perspective, but through his CALS colleagues, has learned about processing such as pyrolysis and anaerobic digestion, which can separate solids or condense nutrients into lighter, purer forms that are transported and applied more easily.
“I would say it’s usually a combination of different technologies to make and process [the nutrient products]. What we want to do is minimize the transportation costs, building on-site facilities,” Liao explains. For example, while it might not be feasible to distribute human waste from New York City to the cornbelt in Iowa, says Liao, one could identify “hotspot” areas such as pig farms in Iowa that are in close geographic relation to the state’s large corn farms, which are more likely to have – or invest in – the infrastructure needed to process the manure.
That processing could do double duty, also allowing producers to diversify revenue streams. According to the 2022 study, 371 U.S. counties have been identified as having more manure-supplied nutrients than their crop needs, which can lower the value of manure. As such, every repurposing of manure and its byproducts – and every extra mile it’s transported – is crucial.
Manure and justice
The team found nutrient inequality mirrored social inequality in some senses. For example, areas of very high or low nutrient supply often overlapped with poorer areas – where people are more vulnerable to either food insecurity or worse health outcomes (or both). In surplus regions, more waste also washes into bodies of water, and in areas of low supply, farmers rely more on synthetic fertilizer, which also have negative environmental effects.
Addressing the inequality could potentially bring a sense of environmental justice, says Liao. That’s where the idea of a “decentralized system,” like the theoretical Iowa pig farm redistributing its nutrients to the nearby corn farms, comes in.
Next step: Acceptance
The research is not complete yet – the team wants to look at multiple sources of waste, such as tree biomass, and also look at redistribution on a more global scale.
“Countries in Africa cannot afford to buy commercial fertilizers,” says Liao. “[Conflicts in] Russia and Ukraine, Iran, that adds more factors to affordability. Those kinds of decentralized systems could help these countries reclaim their nutrient sovereignty.”
Another hurdle the team will look at for the next phase is public acceptance – an issue in which Liao, with his social science perspective, is particularly interested.
“A small minority of people just don’t want that,” he says. “There are people who are inherently resistant to the idea, no matter what kind of sound, scientific evidence you present – they just don’t like it. They want it to be hauled away to a landfill that’s over 500 miles away.”
Helping them understand the nutrient sovereignty concept might be the key, he says.
