DENVER, Colo. — Millions of tons of chemicals dumped on plants to kill pests and germs contain heavy metals that are permanently destroying the productivity of the land, a federal agricultural scientist warned Monday.

The threat from the heavy metals has not been recognized before because little has been known about plant mineral nutrition, said Dr. John C. Brown, a soil scientist at the United States Department of Agriculture’s plant stress laboratory in Beltsville, Md.

New research has disclosed that heavy metals such as copper, zinc, molybdenum and boron block a plant’s ability to absorb iron from the soil, he reported at the annual meeting of the American Association for the Advancement of Science.

Iron is the single most important nutrient for plants. It is essential for the formation of chlorophyll. “Based on our knowledge, we view this as a serious threat. The whole of agriculture is threatened,” Brown said. “If we don’t stop the use of heavy metals, in 30 to 40 years from now we will destroy some of our most productive farmland,” he added.

The danger from heavy metals already is showing up, he said. Citrus trees in Florida are suffering growth problems because the soil in many areas has been saturated with a fungicide containing copper sulfate.

In many areas of South Carolina, farmers are having difficulty growing cotton because of the widespread use of a bacteriacide containing zinc, he said.

Michigan farmers are having trouble with soybean because of high phosphate levels in the soil and in the Pacific Northwest chemicals containing arsenic added to the soil to kill pests are hampering plant growth, Brown said.

Once in the soil the heavy metals last indefinitely, permanently destroying the productivity of the land. Without adequate amounts of iron, the fruits of the plants are nutritionally deficient and the plants eventually die.

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“The thing that bothers me is that we are still adding things to the soil that contain the heavy metals,” he said. “If we don’t stop it, we will have nothing left.”

Most of the compounds are added to the soils without any basic understanding of how they affect the mineral nutrition of plants, which scientists only now are beginning to understand, Brown said.

We are not set up in agriculture to know what we are doing. We need to know what we are adding to the soil and how the soil is affected,” he said.

Millions of dollars are being spent to develop methods of placing treated sewage on farmlands to increase their yield, said Brown. But the sewage contains high levels of heavy metals which eventually would make that farmland unproductive, he explained.

The Department of Agriculture needs more money so that it can establish regional laboratories that can analyze soils, plants and compounds intended to be added to the soil to avoid the danger of making the soils poisonous from heavy metals, he said.

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