Mo in Black Shale Soil and Maize: What Farmers Should Know

Soils above black shale can carry a lot of molybdenum, a metal that isn’t always harmless. In a study from southwestern China, researchers measured the amounts of Mo in rocks, soils and maize plants grown there. The raw shale had 6 to 53 mg of Mo per kilogram, and after weathering the soil on top held between about 1 and 50 mg/kg. The metal tends to rise as the rock breaks down, but its jump from weathered rock into soil can vary. When scientists looked deeper at how Mo is bound in the soil, they found that more than 80% of it sits in a stubborn “residual” part that plants can’t easily take up. About 15% is tied to organic material, and less than 2% is in the other, more mobile fractions. The amount that can be extracted with a simple ammonium‑oxalate solution was only about 8% of the total, confirming that most Mo is not bioavailable.

Maize grains averaged 0. 55 mg of Mo per kilogram, a figure that rises when the soil has more total Mo. The metal distribution in the plant follows husk > grain > cob, meaning the outer parts hold more Mo than the edible core. This uneven distribution also skews the copper‑to‑molybdenum ratio in the husk and cob, raising concerns that animals eating this maize could suffer copper toxicity. For people, however, the risk from eating the grain is tiny because the Mo levels are low. The study highlights that while black shale areas naturally enrich soils in molybdenum, the element does not move much into crops. The findings help shape safety plans for regions with high background Mo.

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