Lead in soil needn't mean no vegetables

Lead-contaminated soils need no longer prevent the home gardener from producing food crops in his own backyard. The soils can be treated with organic soil amendments that effectively lock up the lead, excluding it from the plants.

Nina L. Bassuk, a plant physiologist and program leader at Cornell University's Urban Horticultural Institute, has reached this conclusion after 2 1/2 years of intensive trials at the university's Ithaca, N.Y., campus, using high-lead soils taken from the heart of New York City and other soils deliberately polluted for experimental purposes.

What she has discovered makes good news in the urban home, where high traffic volumes and the longtime use of lead-based paints have resulted in a buildup of the metal in garden soils.

Vegetables can, in fact, be safely grown in lead-poisoned soils, provided certain precautionary steps are taken.

In brief, the contaminated soils must be heavily buffered with decomposed organic matter and the pH level of the soil maintained at between 6 and 7, or very close to neutral. This soil treatment, automatic among those gardeners who follow the principles of organic gardening, does not eliminate lead from the soils but simply holds it in a nonsoluble state. In other words, the lead remains ''locked up'' and unavailable to plant roots.

In certain soil situations, nutrients are also locked up out of reach of plants. But when organic matter is added and the soil is brought close to the neutral range, these nutrients are made available. Thus, the very soil treatment that releases nutrients to the plants is the same one that locks up the lead contaminants.

In acid soils, where lead is in a highly soluble state, lead is taken up by the plant and fixed in its roots, stems, and leaves. Little or no lead finds its way into the fruits. So while eating tomatoes, peppers, eggplant, and the like which have been grown on contaminated soil presents no problem, eating other vegetables - lettuce, carrots, and potatoes, among them - would mean ingesting some lead.

In her experiments, Ms. Bassuk found that lettuce grown in soils containing low levels of organic matter readily absorbed lead. But ''in soils containing large volumes of organic matter (40 to 50 percent by volume) the lead uptake was zero even though lead concentrations were as high as 3,000 parts per million.''

Ms. Bassuk's conclusions are that where lead is a problem, vegetable soils should be made up of at least 25 percent, by volume, of organic matter. She points out that the organic matter should be moderately decomposed, as is the case with compost or manures. Simply to add peat moss or shredded leaves to the soil would be less effective, because they have not yet decomposed, ''but by the following year they would do a lot to lock up the lead in the soil.''

The plant physiologist recommends that leaves and garden waste be composted before incorporating them in the top 12 inches of soil. Manures, which have begun decomposing in the digestive tracts of animals, are immediately effective in controlling lead. Muck soils, which are built up almost entirely of organic matter, would also be safe for vegetable production, Ms. Bassuk adds.

A high organic content in soil tends to keep it near the neutral level. A simple pH test should also be taken, however. Inquire from your local extension agent where soil samples should be sent for testing. The agent will be able to recommend the rate of application of lime or wood ashes to raise the pH to the required level. Garden centers also sell soil-test kits. There is even a pH meter that tests the acidity or alkalinity of soil.

Soils above neutral (7 on the pH scale) will also effectively lock up any lead, but few vegetables are capable of growing well under alkaline conditions.

Another problem of lead contamination comes from vegetables that have been exposed to auto exhausts. Lead blown onto plants in this way is not absorbed by them, Ms. Bassuk found, but it does stick to the leaves in significant quantities.

Simply washing the leaves removes very little of the lead, but ''washing the leaves in water containing a little vinegar (1 percent) removes the lead effectively,'' she concludes.

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