Manure contains valuable nutrients and organic matter, but it also has small amounts of heavy metals that don’t break down in soil. In a South Dakota State University Extension fact sheet, the authors explained that problems associated with heavy metals are gradual. Heavy metals disrupt soil microbial communities, slow nutrient cycling, inhibit root growth, and reduce crop vigor and yields.

Heavy metals enter soil though natural pathways, but they also build up because of feed additives, fertilizers, industrial emissions, and manure application. Some metals, like copper, zinc, iron, manganese, and selenium are required in small amounts and aid in animal and soil health. However, at high levels, they become toxic.

“Repeated manure applications can push essential metals past the point where they help and into the range where they harm soil microbes and crops,” the authors wrote.

Other metals, such as cadmium, lead, and mercury, have no biological benefits and are toxic even at low levels.

Copper and zinc are commonly added to swine feed to promote growth and animal health. Unfortunately, hogs excrete most of the metals they consume, the authors explained. This makes swine manure, both in liquid and solid forms, especially problematic. Further processing, such as composting or anaerobic digestion, does not break down heavy metals.

The authors provided a few suggestions to reduce heavy metals in swine manure. First, reduced application rates slow buildup in the soil. Regular soil testing and tracking trends helps monitor accumulation. Maintaining near neutral soil pH lowers metal solubility and toxicity. Deep tillage can dilute surface metal concentrations although it does not remove it from the system.

Reducing copper and zinc levels in feed lowers the amount of heavy metals in manure. This is one of the most effective long-term solutions, the authors noted, but it requires working with a nutritionist to maintain animal health and productivity.

This article appeared in the May 2026 issue of Journal of Nutrient Management on page 11.

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