(Note: The following article is based on the new K-State Research and Extension publication MF3166, Gardening on Lead-Contaminated Soils. See the full publication at: www.ksre.ksu.edu/bookstore/pubs/MF3166.pdf)
Urban soils are often used for gardening and food production. It may be a good idea to have these soils tested for contaminants, such as lead.
Lead affects everyone. Lead accumulates in humans, and the body releases that lead slowly. Children between the ages of 6 months and 5 years have been found to be the most vulnerable to lead toxicity. The Centers for Disease Control and Prevention (CDC) currently considers 5 μg dL-1 as the threshold for “elevated” blood lead, while pointing out that no safe blood lead level in children has been identified.
Mean lead concentrations in uncontaminated surface 6 inches of soils in the U.S. are 22 parts per million (ppm). Where lead levels are unusually high in urban soils, human activities are the main cause (Figure 1).
Soil dusts from former mining sites and airborne particles from smelting metals may have deposited lead on surface soils. Before being outlawed in the United States in 1986, automobile emissions from gasoline-powered engines led to significant deposition of the organic compound tetraethyl lead on the surrounding soils. Urban soils in city centers exposed to heavy automobile traffic have higher lead concentrations than suburban or rural areas with reduced traffic volumes.
Another potential source of contamination is paint. Most houses built before 1978 were painted with lead-based paint. The deterioration of these homes over several decades due to poor maintenance or harsh environmental conditions can result in paint chips or deposits near the sides of these homes.
Figure 1. Lead exposure in the home environment.
Studies show lead levels in urban soils may range from 50 to about 3,000 ppm. The demolition of old homes; wastes generated from former paint factories; and shops that either fabricated or recycled metals, often within a small geographic area, account for the wide variation in soil lead distribution in urban environments.
Some residential properties now are sited on land previously used for industrial activities that contributed to high levels of lead in the soil. Preventing housing growth on contaminated soils could be assisted by soil testing before construction. However, soil tests are often overlooked or even ignored due to the pressure for housing. Moreover, movement of lead through soil is slow; hence, even though the use of these lead contaminants was banned over many decades, they can still pose health problems to residents.
Currently, there are no set regulations for gardening on urban soils. The Office of Solid Waste and Emergency Response residential soil screening level for lead, also used as the upper limit for child play areas by the U.S. Environmental Protection Agency (EPA), is 400 ppm. However, scientific studies have also expressed some concerns about gardening on soils with lower lead concentrations. Figure 2 summarizes suggested actions that should be followed if soil test results report lead concentrations in the specified range provided.
Figure 2. Soil lead level limits for growing food in garden.
Exposure to lead in soils primarily happens in two ways: direct exposure to lead-contaminated soil or exposure to plants that grew in lead-contaminated soil.
This mainly involves direct exposure either by ingesting the soil or breathing contaminated dust. Incidences include:
Plants grown on contaminated soils may accumulate lead in their root and shoot systems; however, research has shown that most plants do not absorb high amounts of lead into their systems. Some crops absorb more lead than others. Root crops such as carrots and beets are more prone to lead absorption than leafy vegetables. If grown in highly contaminated soils (lead concentrations greater than 1,000 ppm) and poor soil conditions (low pH and organic matter), eating the edible portions of leafy vegetables may become a concern. If in doubt, take a soil sample to your local K-State Research and Extension office.
Factors to consider when growing on urban soils
Nutrient level and soil pH
Urban soils often have low levels of soil nutrients. They suffer from heavy compaction and erosion losses. The alteration of the natural soil profile has been chiefly responsible for the degradation in soil structure and texture (major factors affecting the movement of water in soils). Hence, fertilizer and/or organic manure additions are required to improve soil fertility levels and improve the soil structure. Recent studies have shown that organic matter inputs on moderately (100 – 400 ppm) contaminated lead soils reduce lead uptake in vegetables.
Sources of this organic matter include kitchen/local composts, animal manure, and treated biosolids. Suitable recommended mix ratios of the manures can vary between 30 to 50 percent. Compost addition helps dilute the total lead concentration in soils, making it less bioavailable (the extent to which it can be used by the body). If soil pH tests are low (less than 6.5), levels can be increased with lime applications. Lime additions are usually done to the soil to achieve a desired soil pH in the range of 6.8 to 7.5.
Choosing your crop
If you have tested your soil and found the lead levels to be greater than 400 ppm, then you should mainly grow leafy and fruiting vegetables. Fruiting vegetables such as eggplant, tomatoes, and peppers are recommended on mildly elevated soils. The outer leaves of the vegetables should be discarded and the vegetable should be thoroughly washed in prepared solutions (1 tablespoon of vinegar or liquid detergent dissolved in about a gallon of water).
With root crops (e.g., carrots, beets, radishes, or potatoes) grown on soils where lead concentrations are below 400 ppm, it is advisable that the washing procedure be conducted before peeling. Soil particles bound tightly to the root surfaces may be accidentally ingested when eating raw carrots. Research has shown that more than 80 percent of lead in the soil is bound to fine clay particles. Additionally, it is well established that surface contamination can cause much more damage than what is absorbed by plants.
Other practices to follow when gardening in lead-contaminated soil
DeAnn Presley, Soil Management Specialist
Ganga M. Hettiarachchi, Soil and Environmental Chemistry
Phillip Defoe, former Agronomy Graduate Student