Lead is a toxic heavy metal that can be very harmful to your health. It has been used in a lot of products including paint, gasoline and cans of food. Lead levels in soil found along roads is higher than in soil located farther from the road and some people won’t grow vegetables in a front yard for this reason.
One reason people use raised beds is to grow food in soil that is not contaminated with lead. Is normal garden soil really a problem? Does the purchased soil that is used to fill the raised bed have lower levels of lead?
Should lead levels in produce be a concern and does organic food have less lead?
Why Worry About Lead?
Lead is a metal and as a solid it is not really a big concern. However, when lead oxidizes it forms lead ions that are soluble in water. In this form it distributes throughout the soil layer and if plants absorb the lead, it will enter our food.
One of the problems with lead is that it is a neurotoxin that accumulates in soft tissues and bones where it causes neurological disorders ranging from behavioral problems to brain damage, and it affects cardiovascular, and renal systems.
How Does Lead Get Into Soil
Lead gets into soil in two ways. The first is through pollution or direct contamination. Cars driving down the road release lead in their exhaust which floats through the air until it lands on soil. Car tires also contain lead and as you drive the rubber containing lead is slowly abraded off, landing in the soil along the road. Years ago lead plumbing was common and soil in contact with it gets contaminated. Older paint contained lead and the peeling paint on older homes causes the soil around them to have high levels.
Many industrial processes, especially foundries, use material that contains lead and it ends up in the exhaust air and water from such facilities, resulting in contaminated soil around the facilities.
The other source of lead in soil is due to natural levels. Soil is made from degraded rocks and many rocks contain lead. All soil has a natural level of lead.
Natural Levels of Lead in Soil
Lead is naturally found in almost all surface soil around the globe. It sticks tightly to soil particles, is not very volatile nor is it mobile. That means it stays in the upper layers of soil and is permanent unless removed through mediation.
Actual Levels of Lead in Soil
The actual level of lead in soil depends very much on the location. Levels in cities tend to be higher than rural areas. Older homes have higher levels than newer homes, partly due to the previous use of lead paint. Homes near industrial areas are also higher.
Urban soils have lead levels ranging from 150 mg/kg to as high as 10,000 mg/kg. A study in Canada reported lead values between 2.8 to 4800 mg/kg for parks and urban areas.
The EPA has established a safe level of lead in soil as 400 mg/kg in play areas and 1200 mg/kg for other areas.
Susan Carpenter used a commercial lab to test various brands of bagged soil (mostly soilless mixes) and found levels between 1 and 8 mg/kg.
Soil Amendments can also contain lead.
- Bone meal – 3.1 mg/kg
- Meat and bone meal – 3.0 mg/kg
- Meat and bone meal ash – 13.1 mg/kg
- Rock Phosphate – 10 mg/kg
Lead is the most common contaminant in urban soil. In a Toledo, OH study that tested 81 soil samples, 8.6% had total lead equal to or greater than the US EPA level of concern (400 mg/kg).
Lead Contamination Beside Roads
Traffic along roads is a major source of lead contamination. Both the volume of traffic and the distance from a road are important factors to consider. Most of the lead accumulates in soil within 33 m (108 ft) of the road.
Lead and pH
Soil pH affects the availability of lead. At neutral or high pH (>6.5), lead is tightly bound to soil particles and is mostly unavailable to plants. At low pH (<5), lead separates from soil and is more soluble, thereby becoming available to plants.
Do Plants Absorb Lead?
Plants absorb lead mainly through their roots but also via dust entering stomata in leaves. The amount they absorb depends on the type of plant, the amount of lead in soil, soil pH, soil particle size, soil CEC, surface root area and the amount of root exudation.
At higher pH, plants absorb very little lead because it is tightly held by soil. Soils with a higher CEC (more clay, more organic matter) also hold on to the lead and makes it less available to plants.
Plant condition the pH of soil right around their roots (the rhizosphere) and generally make it more acidic. This means that roots absorb more lead than you might guess based on your soil pH.
Absorbed lead does move throughout the plant, but most of it is retained in the roots. For example, in the edible fern Athyrium esculentum, (now called Diplazium esculentum), lead accumulation in the leaf, stem and root are 0.26, 0.33 and 3.79 mg/Kg, respectively.
Does Organic Food Contain Less Lead?
Consumer Report analyzed 50 nationally distributed packaged baby foods and found similar levels of heavy metals, including lead, in both conventional and organic products. Organic food does not have lower levels of lead.
Is Hydroponics Safer?
It may not be safer since most vegetables grown in soil have low levels of lead, but most hydroponically grown vegetables should have lower levels of lead.
Lead in hydroponics can come from various sources including the support media (ex. coir is a plant), the fertilizer and even the water source. Plants grown in closed rooms will have less soil and dust contamination than hydroponics done on the roof of building, or in a back yard. The plastics used can contain lead (ex. PVC)
Plants grown hydroponically will absorb lead and should not be considered to be lead free.
Will Plants Grow in Lead Contaminated Soil?
Moderate concentrations of lead (< 500 mg/kg) have no significant effect on plant growth. As levels increase, plants do start having trouble growing because lead affects enzymes which control just about every chemical reaction in a plant. Lead toxicity shows up as stunted growth, chlorosis and blackened roots.
Is it Safe to Eat Plants Grown in Lead Contaminated Soil?
Since all soil contains lead and we have been eating plants grown it for a long time, it is safe to say we can eat such plants.
The question needs to be reframed to: At what soil lead level are plants safe to eat?
As a general statement, plants absorb relatively small amounts of lead and much of that is accumulated in the roots. Eating the above ground part of plants is normally safe. Fruits such as tomatoes, peppers, melons, okra, apples, and oranges and seeds such as corn, peas, and beans generally have the lowest lead concentrations and are the safest portions of the respective plants to eat.
Laboratories can measure the lead in your soil, but that does not tell you how much will be absorbed by the plants you grow. As explained above many variables determine how much lead will end up in the plant. You really don’t know how much lead is in your food unless you test the food.
Testing Soil for Lead
Most soil testing labs can perform a lead test. It is important that you understand that there are two different tests. One measures the total amount of lead and all of the numbers presented in this post are “total lead” amounts.
Labs can also measure “reactive” or “plant available” lead. The soil sample for this test is treated with a weak acid (e. g. Modified Morgan Extraction) and only the extracted lead is measured. UMass testing data indicates that a 22 mg/kg “plant available” lead value is approximately equal to 300 mg/kg total lead. Many of the above mentioned variables, like pH, affect this correlation. Make sure you know which test is used for your sample and check with the lab to see if they can use the result to estimate the other value.
Is Lead a Concern for Gardeners?
The FDA estimated that 16% of the total lead in a 2 yr old’s diet comes from food, 75% from dust and 1%from soil. For women of child-bearing age, this percentage shifts to 43% from food, 31% from dust, and an insignificant quantity from soil.
Direct contact with soil is not an issue because lead is not absorbed through the skin. Ingestion is a minor problem that is mostly eliminated by washing both hands and food.
Eating food grown in the garden is also not a concern provided that lead levels are below the 400 mg/kg level. Even when food is grown in soil above this level, the top part of plants (leaves, stems and fruit) accumulate very little lead and are safe to eat provided they are washed well. Root crops such as carrots, turnips, radishes and beets may be an issue when grown in soil with high levels of lead.
A lot of people are under the misguided opinion that garden soil is very toxic and should not be used. One way they solve this problem is with raised beds. The reality is that most garden soil is quite safe for growing food and raised beds are not a total solution either. Over time lead from the air settles on the raised bed increasing its lead level. Before you decide to use raised beds because “your soil is toxic” – have it tested. Don’t act on fear.
The reality is that lead exposure in developed countries has decreased significantly in the last 30 years due to three main changes; reduction of lead in gasoline, elimination of lead in paint and elimination of lead in food cans.
How to Reduce Lead Levels in the Garden?
So you have high levels of lead in your soil, or at least you think you have high levels, how do you lower the lead level?
There are two options. One is to actually lower the total lead level. The other is to reduce the amount of plant available lead.
There are a couple of ways to reduce the total lead level. One is to remove the soil completely and replace it. This only makes sense in cases of very high levels. Remember that the replacement soil will also have lead in it. The other option is to grow plants in the soil and remove the root system from the area. This works, but is a slow process.
A more realistic approach is to reduce the plant available lead and a good way to do this is to add compost. Compost has a high CEC value which reduces available lead. If you have acidic soil, raise the pH. As the pH rises, more of the lead is tied up and kept away from plants. If you already have neutral or alkaline soil you are unlikely to have a lead issue. Phosphate combines with lead and precipitates, thereby making it plant un-available. But adding too much phosphate causes other problems so only add it if your phosphate level is low. Mulching reduces the amount of soil dust entering your home and landing on plants.
The bottom line is that much of the current concern about lead in gardens is exaggerated. In most cases it is not an issue. If you happen to live in an old neighborhood or one which is or was very industrialized, have the soil tested. Don’t try to fix a problem you don’t have.