soil testing – The Garden Professors™

This is the last part of our discussion on gardening in soils that contain heavy metals (you can catch up on part 1 and part 2 if you need to). Today we’ll focus on the strategies you can use in your gardens and landscapes to reduce your exposure to soil-borne heavy metals.

Raised beds can be an easy solution for gardeners with contaminated soils

Test your soil!

First – and this should really go without saying – you must test your soil to determine if it contains heavy metals of concern. The COVID19 pandemic provides the perfect comparison: you can’t assume you don’t have the virus just because you don’t have symptoms, and you can’t assume your soil doesn’t have toxic heavy metals just because you don’t think it does. The only way to know for sure, in either case, is through testing.

This eyesore did more than spoil the view.

Most soil tests routinely report aluminum, lead, zinc, and aluminum. Other metals, such as arsenic, cadmium, and chromium, may not be part of a basic soil test and you will need to request additional tests if these metals are likely to be present. Often, county health offices will provide free soil testing if you live in a region where there are known contaminants. For example, I live in the Tacoma area where large amounts of arsenic were deposited for decades downwind of an aluminum smelter. Residents of Pierce County can get free soil testing because of the potential danger.

The aluminum is higher than we would like to see, though everything else looks fine.

Even if you don’t live in an area where industrial or agricultural activity may have added toxic heavy metals to your soils, your soil may naturally contain high levels of some metal of concern. As I’ve mentioned in a previous post, our soils have high levels of aluminum. Because we are not downwind of the smelter site mentioned above, I would not have assumed we had any metals of concern, given the rural location of our land, but knowing this informs my choice of vegetables to plant.

The demolition of the Tacoma smelter. Finally.

Avoid adding more heavy metals

Fortunately, many of the consumer products that contained heavy metals are now gone and no longer will add to existing levels of soil metals. But there are still sources out there that gardeners are well-advised to avoid.

Older treated timbers. As mentioned in my first post, landscape timbers were once treated with a chemical preservative containing arsenic and chromium. Even though gardeners love reusing materials (we are a thrifty bunch!), these older timbers should be removed if they are still on your property. New timbers are treated with a copper-based solution, which is a more environmentally friendly preservative.
Kelp-based fertilizers and amendments. While these products are wildly popular with gardeners, they aren’t very effective fertilizers. Moreover, some kelp species accumulate heavy metals, like arsenic, in seawater and these metals will become a permanent part of your soils. Take a look at this fact sheet for more information.
Recycled rubber mulch. This product should be avoided for many reasons (you can read more about the problems in this fact sheet). As it disintegrates it releases high levels of zinc into the soil. And while zinc is an essential micronutrient in plants (and people!), high levels are toxic.
Unregulated composts and organic products. Certified composts and other organic products have been tested for pesticide residues and heavy metals: unregulated products have not. Unless you are making your own compost from materials you know to be free from contamination, your safest bet is to purchase certified products.

If you have materials like old timbers, you should never burn them or throw them away. They need to be disposed of as a hazardous waste, much like old cans of paint, mercury-containing thermometers, etc. Eventually, we may be able to use these hazardous discards for biofuel production through pyrolysis, or extract the heavy metals from them for reuse. For now, just dispose of them in a legal and environmentally responsible way.

Cedar is naturally decay-resistant and can be a good choice for rasied beds

Suggestions for safe gardening

If soil testing reveals high levels of metals of concern, there are work-arounds to allow you to still enjoy growing vegetables safely. If your soil tests reveal that your soil is safe for growing edibles, congratulations! You may still benefit from some of the suggestions below.

Cover exposed soil with ground covers and mulches (coarse organic or inorganic materials) to eliminate metal-laden dust.
Create raised beds for edibles using untreated wood or other metal-free materials. Line the bottom of the bed with an impermeable membrane to prevent movement of soil-borne metals into the beds.
If raised beds are not possible, use large containers to grow edibles.
Avoid using galvanized tubs, as they will leach zinc (and sometimes chromium) into the soil.
Fill beds and containers with clean (i.e., tested) soils or potting media.
Don’t plant vegetables near roadways, which are a source of airborne lead.

Have I mentioned how great arborist wood chips can be in gardens?

The popularity of home gardens is exploding as we wait out the COVID pandemic

So many of us are growing our own vegetables – either as experienced home gardeners or as COVID19-isolated novices. There is a lot of effort in figuring out garden beds, vegetable choices, and growing medium – but one of the issues rarely considered is whether there are heavy metals present in the local soil and/or growing medium. We can’t see heavy metals, or smell them, so we need to have a way of assessing their presence before we plant edibles.

In the next few months, I’ll tackle the complicated science behind this invisible threat. Today, let’s look at the heavy metals that are commonly found in garden soils and where they might come from.

What heavy metals do gardeners need to monitor in their soils?

Heavy metals are exactly that – they are dense elements that have certain chemical properties that define them as metals. In fact, most known elements are considered to be heavy metals. Fortunately, there are only a handful of heavy metals that are commonly found in residential soils. Some of these heavy metals are necessary for life – iron, manganese, and zinc, for example – but others have no known biological function. Arsenic and lead, for instance, can interfere with enzymatic activity and effectively poison biochemical pathways. There is no “safe” level of heavy metals that are not essential nutrients.

Here’s a table of the most common toxic heavy metals that might be found in your soil, and possible anthropogenic sources:

Heavy metal	Sources of contamination
Aluminum*	Smelting
Arsenic	Pesticides, smelting, treated timbers (old)
Cadmium	Paint
Chromium	Fly ash, metals industry, paint, leather tanning, treated timbers (old)
Lead	Gasoline (leaded), paint, pesticides, plumbing, smelting, solder
Nickel	Plumbing, smelting

*Aluminum is a light metal, not a heavy metal, but has similar biochemical poisoning activity as toxic heavy metals

Some of these sources of contamination are not relevant to where I live – why do I need to test my soil?

Gardeners may be tempted to look at the chart above and feel relieved, because pesticides and paint no longer contain heavy metals, they don’t use old treated timbers, and they know that leaded gasoline is a thing of the past. What many don’t consider, however, is that heavy metals are elemental – they don’t break down, though they may change their chemical form. They are a permanent part of soil chemistry unless they are removed by physical or biological means.

Old garden pesticides were very effective – this one contained both lead and arsenic

The underlying soil in housing developments built on old agricultural land often contains high levels of arsenic – because that was the active ingredient in pesticides many decades ago. If the topsoil was removed during construction, it may have been taken to a commercial soil facility where it would have been used to create landscape fill mixes for new landscapes elsewhere. The same is true for land near older roadways where lead from gasoline was released from vehicles over many decades. Not only are lead, arsenic, and other heavy metals in the soil, they also end up in the air when soil is disturbed by erosion or tilling.

Nearly all soils contain some level of some heavy metals. They are naturally occurring, after all, so their presence is not necessarily from anthropogenic activities. Regardless of the source, it’s important to know whether any of these harmful elements are in your garden soils, especially if you are growing edibles. A soil test is the only way to find out.

Here is a soil test of my own raised bed system. While my nutrient levels are optimum, and lead is very low, the aluminum level is quite high. What should I do?

Why aren’t there guidelines on heavy metal uptake in vegetable gardens?

It would be ideal if there was a list of “safe” and “dangerous” vegetables to plant when heavy metals are present. Unfortunately, real life rarely fits into lists and there are numerous sources of variability. Next month I’ll discuss the complications that arise when we consider plant species, heavy metals, and environmental variables.

Tag: soil testing

The complicated issue of heavy metals in residential soils. Part 3: How can we garden safely in the presence of heavy metals?

The complicated issue of heavy metals in residential soils, part 1: What are toxic heavy metals, and where do they come from?