Since not all plants take up metals, and since some soils even require the addition of some metals, why do metals pose such a threat?
It is well known that the difference between the necessary and the toxic is very small in the case of plant uptake of metals. Metals that are taken up by plants are removed from the soil when the crop is harvested. But since most crops become animal feed, most of the nutrients and metals end up in animal manure [and other harvested “bi-products” — like straw] This manure in turn is put back on the land, predominantly to make more animal feed.However, when the manure is not returned to the animal feed-producing soils, some metals ( e.g., copper and zinc) are at risk of being depleted and need to be added to those soils. But these same metals readily become toxic to plants and animals in concentrations just a very small amount greater than what the plants need for healthy growth. We know that sludge adds metals in higher than ambient amounts and that damage to crops and livestock does indeed occur. (Such damage goes either unreported, or if reported is routinely relegated to the categor
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