Animal and human effluent is loaded with agricultural gold: tons upon tons of phosphorus, potassium and nitrogen, critical nutrients for the world’s crops. In fact the abundant waste is itself so abundant with nutrients, a new study finds, that it could replace existing fertilizer use on farms.
Huge amounts of nutrients in manure from cattle, chickens, ducks, goats, pigs and sheep are lost to run-off and leaching each year, or simply never reach agricultural fields because livestock waste can be expensive to transport. Meanwhile, in many countries, human waste is improperly managed, polluting waterways and relinquishing its nutrients. If it does go through waste management systems, often these aren’t set up to harvest the abundant nitrogen, phosphorus and potassium within.
This makes the fact even more striking that many parts of the world lack sufficient fertilizer to feed their crops: “It doesn’t make any sense to pollute our environment, especially our waters and soils, and then have not enough fertilizer for agriculture,” one of the study’s authors said in a press release.
To explore this discrepancy and determine what could be done about it, the team of researchers on the new Nature Sustainability study gathered information from several databases on fertilizer use in important perennial crops and grasslands for up to 146 countries. For each country they looked not only at current fertilizer use—only part of the picture, as some regions have no or limited access to these nutrients—but also the recommended amount of fertilizer that crops worldwide actually need to thrive. They also brought in global figures on human and livestock effluent and estimates of the nutrients these contain.
These estimates revealed that if we gathered up all the fertilizing nutrients from human and livestock waste, it would amount to more nutrients than are currently applied in global agriculture. In some cases, the excess is huge: for example, there would be a 506% surplus of potassium compared to demand. With this abundant waste, “we could theoretically satisfy all our fertilizer use at present,” the authors emphasize.
However, the global need for fertilizer is much greater than current use. So the researchers also compared the available nutrients with the amount of fertilizer nutrients that would be needed to reach maximum cropland yields. This showed that recycled animal and human effluent could still make a significant dent—supplying 8%, 14% and 16% of phosphorus, potassium, and global nitrogen needs respectively.
When the researchers explored the data at national scales, the benefits of nutrient recycling came into even sharper view. Of the world’s top three fertilizer-using nations, the available nutrients in underutilized human and livestock waste could substitute 27% of current fertilizer use in China, 26% in the United States, and 47%—almost half—in India.
The study turned up some other intriguing discoveries. In 21 countries including Namibia, Afghanistan, and Gabon where fertilizer use rates are relatively low, there are more nutrients available in human waste, alone, than are currently applied to crops. This means that the available waste could totally offset the need for the synthetic fertilizers that are currently in use. In some of those countries, the availability of nutrients in human effluent is double what’s currently used on the land, opening up the possibility that farmers could increase nutrient applications to their crops from this supply, and potentially boost yields.
In other countries, making use of the waste would replace a significant chunk of the fertilizers that are imported each year. In fact, in over 60 countries that are net importers of fertilizer currently, mining nutrients from local sewage and farm waste could completely substitute nitrogen and potassium imports, while replacing more than half the imported phosphorus.
Mining this agricultural gold would require some big global changes: better waste management systems, improved nutrient recovery, and measures like subsidies to encourage farmers to recycle livestock waste. That would be a lot to orchestrate—but the prize would be helping to break the global reliance on fossil fuel-intensive synthetic fertilizers. That could help to curb the environmental impacts of phosphorus mining and of nitrogen fertilizer production—the latter responsible for 1.4% of global CO2 emissions each year.
In any case, it’s time for recycling and circularity to become a serious part of the conversation in agriculture, the researchers believe. “We need to close the loop from poorly utilized nutrients, wherever they come from.”
Devault et. al. “Nutrient recycling potential of excreta for global crop and grassland production.” Nature Sustainability. 2024.
Image: ©Anthropocene Magazine
