outhouse in a green field
PHOTOGRAPH BY MICHAEL GREENE/EYEEM/GETTY

Is People Poop Good For Plants?

There may be some unexpected ecological benefits to using biosolids, otherwise known as human manure.

June 2, 2015

Using human excrement (also known as humanure, biosolids, or more traditionally, sewage sludge) on farm fields, home gardens, and even nonagricultural landscapes is a controversial topic to say the least.

On an industrial scale, biosolids are what’s left after sewage treatment plants remove the water from what gets flushed down toilets or is otherwise introduced into municipal sewer systems. Some commercial composters and municipalities market biosolids, once treated, as safe for farmers and home gardeners, boasting that they are rich in plant macronutrients like nitrogen, phosphorus, and potassium, as well as supplementary nutrients such as sulfur, magnesium, calcium, copper, and zinc.

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Those opposed to the land application of biosolids warn that they may contain heavy metals, pathogens, pharmaceuticals, pesticides, dioxins, and more. Proponents counter that since introduction of the Clean Water Act of 1977, followed by the Water Quality Act of 1987, such concerns have been addressed to the degree that biosolids are safe. As defined by the U.S. Environmental Protection Agency (EPA), Class A biosolids are those in which pathogens have been reduced below detectable levels. Class B biosolids have been processed to remove most pathogens but may still contain detectable levels, including infectious parasite eggs. There are greater restrictions on the use of Class B biosolids. Additionally, the EPA has set limits on the allowable levels of heavy metals.

University extension offices across the country offer varied opinions and advice about the use of biosolids on food crops.

From Colorado State University: “Class A biosolids are approved for use in production agriculture. However, it is advisable to avoid application to vegetable gardens due to the potential for heavy metals (such as cadmium and lead). Some cities sell or give away biosolids or compost made with biosolids. It is often extremely high in salts. Ask about the salt content. Use with caution.”

University of Washington associate professor Sally Brown, Ph.D., writes: “Because concentrations of dangerous organic chemicals in biosolids are so low, and the potential for them to be taken up by plants is so small, the risk to plants or consumers of plants associated with this category is minimal.”

The USDA’s National Organic Standards are unambiguous, prohibiting biosolids from use in organic agriculture. But even among those who adhere to organic principles, the debate rages on. Longtime Organic Gardening contributor and self-described “contrary farmer” Gene Logsdon admits he’s taken plenty of criticism over his relaxed stance on the use of biosolids. “I understand the problems with medicines and hormones,” he says, asserting that science has largely resolved these issues. Logsdon believes that the real challenge continues to be keeping environmental pollutants out of the organic waste stream in the first place. “We’ve got to figure out ways of putting human wastes back in soil where they belong.”

Logsdon’s recent book Holy Shit: Managing Manure to Save Mankind (Chelsea Green, 2010) reveals that each year, human excreta worldwide produces upwards of 50 million tons of nitrogen, phosphorous, and potassium--the three nutrients needed most for healthy plant growth. “We’re talking $50 billion a year in biosolids fertilizer that we are mostly throwing away, after spending incalculable amounts of money to do the throwing.” In 2010, the United Kingdom’s highly respected 67-year-old Soil Association produced a report titled “A Rock and a Hard Place: Peak Phosphorus and the Threat to Our Food Security,” strongly advocating for the use of biosolids in agriculture. “Worldwide 158 million tonnes of phosphate rock is mined every year, but the supply is finite. Recent analysis suggests that we may hit ‘peak’ phosphate as early as 2033, after which supplies will become increasingly scarce and more expensive...Necessary actions include...changing how we deal with human excreta: Globally only 10 percent of human waste is returned to agricultural soils.”

In 1995, author Joe Jenkins self-published a book advocating for at-home human waste composting,  "The Humanure Handbook: A Guide to Composting Human Manure" which became an unexpected best-seller, going on to sell over 60,000 copies to interested home gardeners and eco-minded individuals.

Still, major concerns exist regarding the use of biosolids on food crops. Consider the case of Georgia dairyman Andy McElmurray, who applied sludge to his fields between 1979 and the late 1990s, and lost half his herd to chronic diarrhea. He brought in his own experts after the EPA refused to test his fields. Tests revealed high levels of thallium, a toxic metal that’s the active ingredient in rat poisoning, linked to a nearby manufacturer of artificial sweeteners that was using the chemical as a catalyst.

For home gardeners, the jury is still out on the safe use of biosolids in food production. Bagged soil amendments and bulk commercial compost may contain ingredients you don’t want. While it is safe to assume that compost procured from a wastewater treatment plant contains biosolids, commercial composters are not required to use the term biosolids or sewage sludge on their labels and may even lawfully substitute the term organic materials. A good rule to follow if you want to avoid biosolids when buying compost is to always ask what’s in it.

Processors of biosolids point out that they are more heavily regulated than the rest of the composting industry, which allows for conventional food waste, municipal landscape waste, and manure from conventional farms—each waste stream bringing the potential for contamination. So know your composter, just as you know your farmer, and ask plenty of questions. Otherwise, make your own compost, or look for soil amendments certified by the Organic Materials Review Institute and bearing the “OMRI Listed” logo.