Planning for New Vegetable Gardens: Common Causes of Soil Toxicity

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Planning for New Vegetable Gardens: Part 5: Common Causes of Soil Toxicity

Have you been telling people you have a black thumb and kill all plants, or that nothing survives in your yard? I’ve heard these statements hundreds of times from frustrated gardeners and new clients. Soil toxicities may be to blame. For purposes of this article, soil toxicity is anything in soil that is harmful or could be harmful to plants, people, pets, or wildlife.

This is the 5th in a series of articles offering advice and solutions for common challenges when starting a garden in California. See below for links to other articles in the series.

Home gardening is one of the safest and most-rewarding activities during the COVID-19 pandemic. For other helpful articles, see our blog page Dig In.

What Types of Properties are Likely to Have Toxic Soil?

• Properties with buildings predating 1970. The likelihood of high levels of lead in soil increases the older the buildings are on a given property. In my experience, most properties with buildings predating 1950 have problematic levels of lead. Older neighborhoods throughout Los Angeles and Southern California where houses date to the 1930s and older usually have high levels of soil lead and other metals. Leaded paint was used inside and outside of buildings during the 20th Century and back to the Colonial Era throughout the United States until this was functionally outlawed in 1978.
• Properties located near freeways or busy streets accumulate copper and zinc as a result of brake dust and tire dust.
• Properties where dogs or other pets have been kept for years or decades when allowed to do their business in the yard. This typically causes accumulations of soil copper that are problematic or toxic for plants. These may occur in specific or limited areas. Parkways and public-access areas along sidewalks may also accumulate soil copper over years and decades as a result of dogs being walked in the area regularly.
• Properties in or near industrial areas, or wherever soil was exposed to metal, such as where cars where stored in yards or parked on soil for extended periods of time. Properties with metal features such as roofs and fencing, especially when metals are buried in soil or come into contact with soil.
• Parking lots and large areas of hardscape laid before the 1980s may have been sprayed with arsenic (purportedly to control weeds), which may persist indefinitely in soil. For reasons that are difficult to explain or understand decades later, arsenic was commonly applied as an herbicide until the late 20th Century in residential yards and before parking lots and other hardscaping were laid.
• In my experience, when more than one of the conditions above occur at a property, the likelihood of soil toxicity increases.

What Metals Are Commonly Problematic or Toxic in California Soils?

• Moderate levels of lead can be toxic to plants. Lead in any amount is potentially harmful to humans, but is particularly dangerous to young children and pregnant women. People take in lead from contaminated garden soil primarily by breathing soil dust, but also by eating fruit and vegetable crops without sufficient washing to remove contaminated dust, and by eating plants, especially root and leaf crops, that have absorbed lead while growing in contaminated soil.
• Other toxic metals such as arsenic are occasionally problematic in California soils. In some cases when asphalt or parking hardscape is removed now, such as to install a school garden, soils contain sufficient arsenic to make gardening difficult and dangerous to humans. Arsenic can’t easily be removed from soil and may persist for hundreds to thousands of years or longer. While arsenic poisoning in humans most often occurs from drinking contaminated groundwater, research has shown that some plant species uptake arsenic when grown in arsenic-contaminated soil. Arsenic is toxic to plants, and humans should not eat plants grown in soils contaminated with arsenic.
• Metals that normally aren’t harmful to people, such as copper, zinc, and iron, can be highly toxic to plants when they occur in soils at more than tiny amounts of several parts per million to a few dozen parts per million depending on the plant species and specific metal. Any of these metals can occur naturally in soils, but are more often associated with human pollution.
• Copper enters urban soils from brake dust, and also from pet urine. It’s common in urban soils, especially near busy streets and freeways and on properties where dogs or other pets have been kept for years or decades.
• Zinc is associated with tire dust, and as a result as is often very high in soils near busy streets. Iron and zinc enter soil from metals used for roofing, fencing, metal irrigation pipes or other piping, fasteners such as nails or screws, and metal landscape features, among other sources.
• A combination of metals on a given property may have unpredictable effects on plants that are more severe than having a toxic level of a single metal. For example, a 1940s house near a busy street may have levels of lead that are toxic to both people and plants, and also levels of zinc and copper that are harmful to plants.
• For more extensive information on what substances are toxic to humans and in what quantities, see the Center for Disease Control, Agency for Toxic Substances and Disease Control.

Other Harmful Pollutants

• Boron and sodium are among the most common soil pollutants in Southern California that are harmful to plants.
• Boron is a common ingredient in detergent and is harmful to plants at more than tiny amounts. When hand laundry is done outdoors and wastewater containing boron is dumped into the soil, boron will quickly build up in the soil to the point that it becomes toxic to plants. The use of graywater in landscapes requires nontoxic and biodegradable detergents and soaps to avoid long-term toxification of soils. Other sources of boron may include pesticides such as ant and roach baits.
• Sodium may occur naturally in soils and can enter soil from numerous sources. It’s often problematic in coastal and coastal-influence areas due to leaching from salty groundwater and progressive surface deposit of sodium over years and decades from ocean air. Sodium is an ingredient in some pesticides, and may be a known or unknown component in some mineral fertilizers. Perhaps the most common source of sodium and salts is municipal water used for irrigation, which leaves minerals behind in soil as the water evaporates. Excess salts are generally flushed to a lower soil profile by rain, but often build up and become problematic for plants in California’s dry Mediterranean climate where we sometimes go years with low rainfall. Sodium inhibits uptake of other nutrients. Sufficient salinity from sodium and other soil salts causes dead/brown leaf tips and edges, and can stunt or kill plants.
• Almost any element or substance, including nitrogen and even water, can be harmful to plants if sufficiently persistent or present in large quantities. High levels of some nutrients, such as phosphorous and calcium, generally aren’t directly harmful to plants.

Best Practices for Managing Urban Soil Toxicity

• GardenZeus recommends testing your soil for toxicities before starting a garden, especially on properties with pre-1970 housing, properties near busy stress or freeways, and properties where dogs or other pets were kept for years or decades.
• Avoid breathing soil dust or eating leaves, roots, and stems/shoots of vegetables from your garden unless you know that your soil doesn’t contain lead. Wash all produce and fresh food thoroughly to remove dust and soil, including all purchased produce and all organic produce.
• Use quality soil tests from reputable laboratory testing services. You often get what you pay for with lab testing. I know of no off-the-shelf product or soil-test kit that provides reliable results for soil metals. After side-by-side comparisons of bargain testing versus a professional lab for the same soil samples, I’ve found that many inexpensive lab tests for soil metals and nutrients may be grossly inaccurate. Because of the complexity involved with understanding and responding effectively to soil-test data, I’m not providing a lab recommendation here. Contact me with a description of your situation and needs.
• For soils found to be high in lead, the only reasonable management options may be to discontinue using the soil, to protect human health by adding barriers such as thick mulch, and/or replacing the soil. According to the prevailing scientific research, lead generally doesn’t enter reproductive structures in plants (flowers and fruits), so growing fruit trees and vegetables where the vegetable fruit is eaten (such as tomatoes and squash) in soil high in lead may be a reasonable option provided that the gardener avoids exposing himself or herself to the soil.
• Don’t import soil to your yard without first determining if it is free of toxins. Hiring a landscaper to import soil as part of a gardening project? Insist that the soil be tested and look at the lab results before you agree to allow it into your landscape. Thinking of picking up free soil advertised online or in the neighborhood? Always test it first. In my experience, the majority of soil given away for free contains toxic metals.
• Is bagged garden soil free of toxins? I’ve looked into this carefully, and concluded that it varies. I consider bagged amendments that are mostly organic matter, such as potting soil and bagged compost, to be reasonably safe. I believe that bagged soils such as topsoils and topsoil blends from reputable sources are much less likely to contain toxins than local free soil. Consider contacting the manufacturer to check on their policies for testing soil that they sell and/or check on their sources for the soil. If you receive a generic answer, such as “locally sourced” when the soils aren’t routinely tested, I personally would avoid purchasing or using the soil.
• Consider growing vegetables in raised beds with barriers to minimize the upward migration of toxic soil. And remember, either purchase the soil used to fill the raised beds from a reliable vendor, or test the soil before purchasing it.

• Obtain professional gardening help to understand lab test results and construct a plan to mitigate or respond to any issues. Labs may provide basic advice or instructions that are more understandable than soil-test data but this advice might not apply to your specific needs or objectives.

And Remember

• Humans have caused significant pollution and environmental problems in cities and almost everywhere on Earth, but most soil toxicity problem are solvable. GardenZeus recommends determining your soil toxicity through a lab test before putting in countless hours of work and ongoing expenses for gardens or landscapes that might never thrive because of unknown toxicities. Because of the complexity involved with understanding and responding effectively to soil-test data, I’m not providing a lab recommendation here. Contact me with a description of your situation and needs.

Other articles offering advice and solutions for common challenges when starting a garden in California:

Part 1: Siting and Sunlight

Part 2: Seasonality

Part 3: Timing

Part 4: Introduction to Soil Toxicity

Part 6: Introduction to Soil Compaction

Part 7: Decompact Soil Quickly

Part 8: Decompact Soil Slowly

Future installments include:

Part 9: Introduction to Soil Alkalinity

Part 10: Managing Soil Alkalinity

Part 11: introduction to Soil Fertility

Part 12: Watering

Part 13: Pest and Pet Exclusion

Part 14: Minimizing Pest Insects

GardenZeus has plenty of information to help you get started growing fruits and vegetables. To receive customized growing information for your area, click here.

Articles of interest for novice gardeners include:

How to Choose the Best Seeds for California Gardens

The GardenZeus Guide to Starting Tomato Seeds

Container Gardening: Cucumber, Corn, Snap Peas and Eggplant