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:
*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.
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.
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.
The Annual Meeting and Professional Improvement Conference of the National Association of County Extension Agents is that one time of year where extension agriculture professionals gather to share ideas, give talks, network, and let their hair down. The name of the organization is a bit outmoded: many states no longer call their extension personnel agents, but rather educators, experts, professionals, area specialists, and the like. Most aspects of agriculture are included: from the traditional cows and plows of animal science and agronomy to horticulture and sustainable agriculture (I’m the outgoing national chair of that committee). There’s also sharing on agriculture issues like seminars on engaging audiences about genetic engineering, teaching and technology like utilizing social media and interactive apps, and leadership skills.
It is the one time every year or so that Linda Chalker-Scott, grand founder of the Garden Professors, and I get to hang out. If we’re lucky we’ll meet up in some sessions, chat in the hallways, or grab a drink. But one of our favorite conference activities is taking a turn around the trade show floor. This is where companies and organizations are vying for the attention of extension educators to show them their newest equipment and products….we are, after all, the people that share growing and production information with a great number of potential clients across the country.
Since the organization runs on money, almost no company that comes calling with the money for a trade show spot is turned away. This means that the products may or may not stand up to the rigors of scientific accuracy. In years past we’ve found snake oil aplenty, like magical humic acid that is supposed to be this natural elixir of life for plant growth. The only problem is that humates don’t exist in nature and there’s little documentation of any effect on plant growth. The product that was supposed to be this magic potion was created from fossil fuels and no actual peer-reviewed research was offered by the company – hardly convincing. There were magic plastic rings that supposedly acted as protective mulch around mature trees and could slowly release water, except that mature trees don’t really need protective mulch and the amount of water would be negligible to a tree that size. So will we be smiling or scowling when we’ve made our way through the trade show.
Right off we set our sites on a company starting with “Bio”, which can be a good indicator of questionable rationale. That lit up the first indicator on our woo-ometer. Beneficial bacteria you apply to plants/soil: woo-ometer level two. So LCS and I engaged the representative. Asking about the product and what it does. We learned about their different products that could help increase the rate of decomposition of crop residues in farm fields, of turfgrass improvement, increased crop production, and treatment of manure pits on dairy and hog farms (which, if you’ve ever experienced one, you’d know would benefit from any help they can get in terms of smell).
Most of the products like this give vague descriptions of the beneficial bacteria it contains. They’re akin to compost teas that can have any number of good, bad, and downright ugly bacteria and fungi in them. Since you don’t know what’s in these products, any claims on soils or plants are suspect at best. However…our rep went on to tell us that the company created blends of bacteria from specific strains that had been researched for their effects on decomposition, soil nutrient availability, and plant growth. There was a brochure with the specific bacteria listed, along with studies the company had conducted.
We asked about peer-reviewed research, which is our standard for evidence here at the GP, and while he had no results to share he assured us that university-led research is currently in the works. And as we’ve stated in regards to applying of beneficial bacteria to soil – while there’s little evidence showing the effectiveness of applying non-specific bacteria to plants, using directed applications of specific bacteria which have been tested for specific functions are supported by research. So our woo-meter didn’t fully light up. We reset it and continued the hunt.
We scoured the rest of the trade show and found one other soil additive that lit up the first lights of our woo-meter, but the rep must have been out for lunch so without anyone to talk to we couldn’t confirm woo or no-woo.
However…..we did find something spectacular! The local employees of the USDA NRCS (Natural Resources Conservation Service) had an interactive demonstration of soil, specifically showing the benefits of reducing or eliminating tillage. The NRCS works with many farmers to incorporate conservation practices on farms, including no-till production, by providing technical assistance, farm plans, and even grants, cost-share, and easement programs. Many farmers have benefitted from their grant for season extension high tunnels (which are seen as a soil conservation technique, since they shelter soil). We were so enamored with the demonstration, we asked them to do it again…so we could record it. So, for your viewing pleasure check out the video below where you can see how well no-till soil holds its structure while tilled soil falls apart. This effect is from the exudates from all the beneficial microbes in the soil that act like glue to promote good soil structure. We’ll let the video speak for itself……
So not only does the trade show get a smile instead of a scowl from us, but also two thumbs up! Either there has been some weeding out of the trade show sponsors, maybe the snake oil salesmen didn’t get the traction they were hoping for at the conference, or hopefully some of these companies have failed to reach an audience with their pseudoscience.
Regular blog readers will remember that we moved to my childhood home a few years ago. With an acre or so of landscape I finally have enough room to put in a vegetable garden. My husband built a wonderful raised bed system, complete with critter fencing, and we’ve been enjoying the fresh greens and the first few tomatoes of the season.
We filled these raised beds with native soil. During a porch addition I asked the contractor to stockpile the topsoil near the raised beds. The house was built almost 100 years ago and at that time there were no “designed topsoils” (thank goodness) – soil was simply moved around during construction. Some of this soil had been covered by pavers and the rest had been covered with turf. [You can read more about designed topsoils in this publication under “choosing soil for raised beds.”] There had been no addition of nutrients for at least 7 years so I was confident that this was about as natural a soil as I could expect.
I’ve always advised gardeners to have a soil test done whenever they embark on a new garden or landscape project, so before I added anything to my raised beds I took samples and sent them to the soil testing lab at University of Massachusetts at Amherst (my go-to lab as there are no longer any university labs in Washington State for the public to use).
What I already knew about our soil was that it’s a glacial till (in other words it’s full of rocks left behind by a receding glacier). The area is full of native Garry oak (Quercus garryana), some of which are centuries old. The soil is excessively drained, meaning it’s probably a sandy loam. And that’s about all I knew until my results came back.
Because nothing has been added to this soil for several years, and because I had removed all of the turf grass before filling the beds, I assumed that the organic matter (OM) would be quite low. Most soils that support tree growth have around 3-7% OM. Hah! Ours was over 12%! All I can figure is that centuries of leaf litter has created a rich organic soil.
So here’s lesson number one: Don’t add OM just because you think you need it. Too much OM creates overly rich conditions that can reduce the natural protective chemicals in vegetation. This means pests and diseases are more likely to be problems.
I was pleased to see our P level was low. First soil test I’ve ever seen in my area where P was below the desirable range! Does that mean I’m adding P? No – because there is no evidence of a P deficiency anywhere in the landscape. And my garden plants are growing just fine without it.
Lesson number two: Just because a nutrient is reportedly deficient, look for evidence of that deficiency before you add it. It’s a lot easier to add something than it is to remove it.
Likewise, our other nutrient values are just fine, and I was pleased to see that lead levels were low. Given that this is an older house that had lead paint at one time, and given the fact that the soil being tested was adjacent to the house, I was prepared for lead problems.
However – we do have high aluminum in the soil. Exactly why…I don’t know. Perhaps the soil is naturally high in aluminum? There’s no evidence that aluminum sulfate or another amendment was ever used. In any case, that was an unexpected result that does give us some concern for root crops. I’ll be doing some research to see what vegetables accumulate aluminum.
Finally, note our pH – 4.9! This is completely normal for our area, which is naturally acidic. In addition, the tannic acid accumulation from centuries of oak leaves has undoubtedly pushed the pH even lower. Are we going to adjust it? Again, no. There is no evidence of any plant problems, and even our lawn is green. Why would we adjust the pH if there is no visual evidence to support that?
Which leads to lesson number three: Don’t adjust your soil pH just because you think you should. If your plants are growing well, the pH is fine. Plants and their associated root microbes are pretty well adapted to obtaining the necessary nutrients. If you have problems, don’t assume it’s a pH issue. Correlation does not equal causation! You’ll need to eliminate all other possibilities before attempting to change your soil chemistry. And remember it is impossible to permanently change soil pH over the short term. Permanent pH changes require decades, if not centuries of annual inputs (like our oak leaves).
Will I test my soil again? Probably not. I have the baseline report and since I don’t plan to add anything I don’t expect it to change much. If I had a nutrient toxicity I would retest until the level of that nutrient had decreased to normal levels. But with everything growing well, from lawn to vegetables to shrubs and trees, there really is nothing to be concerned about.
Lesson number four: Unless you have something in your soil to worry about, don’t.
You see a bright shiny package at the garden center saying that it can help you have the most bountiful garden ever, the greenest lawn in the neighborhood, your plants will have miraculous growth, or it will supply every element on earth to make sure that your plants are living their best life. It’s got what plants crave….It’s got electrolytes! You reach out to grab that package and ……. Woah! Pump the brakes! Do you know if your plants even need to be fertilized? Are you just falling for that shiny marketing, or do your plants really need added fertility to grow?
It turns out that many gardeners add fertilizer out of habit or because a shiny package or advertisement told them they needed to do it. The fact is, though, that you may or may not need to add fertilizer to get your plants to grow healthy. It is actually more likely than not that the level of nutrients in soil is perfectly adequate for healthy plant growth. And guess what, there really is a way to know what plants crave…or at least are lacking: A soil test.
We here at the Garden Professors (and those of us who work in extension) often get questions or hear comments about gardeners adding fertilizer or random household chemicals and items to their plants and soils with no idea what they do or even supply. They’ll throw on the high powered 10-10-10, the water soluble fertilizer, rusty nails, or even (shudder) the oft mentioned Epsom salts because it is just what they’ve been told to do.
A few months ago, my GP colleague Jim Downer talked about why to amend soil– focusing mainly on organic material and a little bit of fertility. In this article, I’m going to share some how and what: what plants need in terms of nutrients, how to determine what nutrients you need to add, what you can use for increasing fertility (conventional and organic), and how to calculate how much fertilizer to add.
What plants really need
Plants have a number of essential plant nutrients that they need from the environment in order to properly grow and function. Hydrogen, carbon and oxygen are all important, but are not something that gardeners have to supply since they are taken in by the plant in the form of water and carbon dioxide (unless you forget to water your plants, like I sometimes do — but death will occur from dehydration well before lack of hydrogen).
There are six soil macronutrients, which means that they are used in larger amounts by the plants. These include nitrogen, phosphorous, and potassium, which form the basis of most common fertilizers that have those magic three numbers on them (example: 10-10-10). Those three numbers indicate that the fertilizer contains that percentage of the elemental nutrient in it. For this example, the fertilizer contains 10 percent nitrogen, 10 percent phosphorous, and 10 percent potassium. The other three soil macronutrients are magnesium, sulfur, and calcium. Depending on your location, your soil may be abundant or deficient in these nutrients, especially magnesium and calcium. Sulfur is commonly released during decomposition of organic matter, so it is usually present in sufficient amounts when soil is amended with (or naturally contains) organic matter.
If a soil is deficient in a nutrient that a plant requires it is usually a macronutrient since plants use them at higher levels. However, deficiency is still unlikely in most soils unless there is a high volume of growth and removal, such as in vegetable gardens and annual beds (or if you’re growing acres of field crops like they do here in Nebraska). These are also the nutrients that are most common on soil tests, since they are the ones that are used the most by plants.
Soil micronutrients are needed in much smaller amounts. Those nutrients are boron, copper, chlorine, manganese, molybdenum, and zinc (remember the periodic table?). These are also usually supplied from organic matter or from the parent soil material so deficiency is even less likely than for macronutrients. Tests for these aren’t usually part of a basic soil test, so if you suspect you might have a deficiency you might have to get a specialized test. There are some basic physiological signs of deficiency that plants might exhibit in response to specific deficiencies, but their similarity to other conditions make it an imprecise tool for diagnosing a deficiency.
Compost is a good source of nutrients, especially micronutrients (as we’ll read later). Using compost alone may be sufficient for many gardens, such as perennial beds. However, higher turnover and higher need areas like vegetable gardens may need supplemental fertilization beyond compost. That’s where the soil test comes in.
What’s on the menu….interpreting soil test results
If you’ve had your soil tested by a lab (which is recommended, since it is much more precise than those DIY test kits), you’ll get results back that give you the level of nutrients in your soil and usually recommendations for how much of each nutrient you need to add to the soil for basic plant health. This is a general recommendation that is common for most plants, which is generally sufficient for average growth. If the test says that the nutrient levels are normal, you don’t have to add anything….I repeat….YOU DON’T HAVE TO ADD ANYTHING. If it says you need one nutrient of the other you’ll need to add it to your garden or around the plant. As we’ve said before, disturbing the soil as little as possible is best, so if you’re using a fertilizer product aim for one that you can broadcast on top of the soil or is water soluble. This goes for compost as well – try to apply it to the top of the soil and it will incorporate over time.
Your soil test results will usually tell you to add nutrients in pounds per a certain square footage. In the example pictured, there’s a recommendation of 3.44 lbs of Nitrogen per 1000 square feet. That number is for the actual nitrogen, and since different nutrient sources have different amounts of nitrogen you’re going to have to do some math to figure out how much fertilizer you need per 1000 square feet and then multiply that by how many thousands of square feet you have.
I’ll note here that soil labs do not usually test for nitrogen due to the variable nature of nitrogen in the soil and the lack of affordable or reliable tests. Nitrogen fluctuates widely over a short period of time and is not as persistent in the soil as other elements due to plant take-up, microbial action, and weather conditions. Nitrogen recommendations are usually made based on the crop indicated for the test and may be informed by the levels of other nutrients.
Let’s say that I’m using an organic fertilizer product I purchased at the garden center and the nutrient analysis is 4-3-3 (these numbers are standard for organic nutrient sources, which have lower nutrient levels than conventional fertilizers). That means that for every 100 lbs of that product, 4lbs are nitrogen, 3 are phosphorous, and 3 are potassium. My (hypothetical) garden is 10ft by 20ft, which is 200 square feet.
So we divide 200 by 1000 to get .2, which represents that my area is 20% of the area listed on the recommendation. If my garden were 3500 square feet, then that number would be 3.5.
Next, multiply the Nitrogen recommendation of 3.44 lbs by .2. This give me 0.688. This tells me that I need .688 lbs of nitrogen to amend my 200 square feet.
So I just need to weigh out .688 lbs of the fertilizer, right? Nope – we have to account for the fact that my fertilizer is only 4% nitrogen- only 4 lbs out the 100 lb bag. We can estimate amounts by figuring out how much nitrogen is in smaller amounts of the fertilizer. Since we know that 100lbs has 4lbs of N, then 50lbs has 2lbs of N, and 25lbs has 1lb of N. If I want to get a more precise amount of fertilizer poundage to get my .688 lbs of N, then we divide the pounds of N needed by the decimal percentage of N in the fertilizer. So that would be .688 / .04, which gives us 17.2 lbs of fertilizer.
Now, considering that the bagged product that I bought is $25 for 8lbs, I may want to reconsider using it for this application…unless I enjoy throwing my pearls before swine or I’m fertilizing my money tree.
If you do the math, you’ll note that this fertilizer will add more than the recommended amount of phosphorus and potassium. You’ll either need to decide if that is acceptable or if you need to find another source of nutrients.
If you’re not using a prepared fertilizer product but rather an organic source of nutrients, you can still calculate how much to add to get to the recommended amount. The following are some good lists of nutrient ranges of organic materials:
Another thing your soil test will tell you is the pH of the soil. In general, plants prefer a soil pH just on the acidic side of neutral (between 6.0 and 7.0). There are plants that prefer different pH levels – such as blueberries and azaleas and their need for a more acidic soil between 4.5 and 5.2. Changes in pH affect the availability of nutrients to plant by affecting ionic bonds of the elements. For the most part, the nutrients are more available at that neutral pH. You’ll note that iron is more available at lower pH levels, which is why those acid-loving plants grow better at lower pHs – they’re heavier iron feeders.
If your pH is extreme in one way or the other, you’ll either need to find plants that thrive at that level or adjust the pH if that isn’t possible. To raise pH in acidic soils the most common method is application of lime. To lower pH, you’ll need something high in sulfur. For more information, visit https://articles.extension.org/pages/13064/soil-ph-modification .
Along with the trends of buying local food, buying organic, etc., there seems to be an increasing interest in the ultimate local food source – a garden. This includes in urban areas. Urban gardening is a great way to save money on food, a great source for fresh vegetables – especially in “food deserts”, and an easy way to introduce kids to where the food on their plate comes from. However, there are a couple potential obstacles you should consider first before starting your urban garden.
First, in urban environments the possibility that soil could have been contaminated with heavy metals, petrochemicals, etc. is pretty high, especially in older neighborhoods. Lead, which was once a common additive to gasoline and paint, is a common contaminant in urban soils. and can be absorbed by the roots of the vegetables you grow. Because of this, that lead can eventually end up in the food on your plate. Most lead poisoning comes from ingesting lead (like eating lead paint chips…), so it’s important to know that the soil you’re using for your garden is safe. You should take some soil samples and send them to a lab in your state that can test for heavy metals like lead. Usually the Land Grant university in your state (if you’re in the US) will have a soil testing lab where these tests can be performed for a nominal cost. Other forms of contamination are possible as well, such as chemicals from cars, asphalt , laundry-mats, etc. These chemicals are more difficult to test for, so your best bet is to find out the history of your garden plot. These records should be available from your local city government, perhaps even online. Read more about contamination in this post.
Second, urban soils are often compacted from foot, car, or perhaps machinery traffic. Compacted soils make it difficult for plants to grow, mainly because the plant roots are not strong enough to penetrate the compacted soil, and thus cannot gather enough water or nutrients for the plant to survive, let alone grow and produce vegetables. Compacted soils are especially common in newer housing developments where entire blocks of houses were built around the same time. The construction companies often remove all of the topsoil prior to building the houses. The soils are then driven over by construction machinery and compacted. Then sod is laid directly on top of the subsoil. This makes for soils with very poor growing conditions for both lawns and gardens.
A good alternative for areas with either contaminated or compacted soils is to use a raised garden bed with soil that was brought in from a reliable source. You can buy bags of potting soil from a local home and garden supply store, but a more economic alternative is to have a trailer full of topsoil trucked to your raised bed. When you build your raised garden, be sure to use untreated wood. Some of the chemicals used to for pressure treated lumber are designed to kill fungi that break down wood. These chemicals, some of which contain arsenic, can leach out of the wood and into the soil used for your veggies! However, untreated wood, though it might not last as long, will still last for decades and is probably cheaper anyway. There are lots of great designs and how-to sites that show you how to build a raised garden bed. Here’s an extension bulletin from Washington State University on raised bed gardening. The raised beds shown below are from when I first installed them in my community garden plot in Manhattan, Kansas. One is now a strawberry patch (the border helps contain the strawberries to a defined area), and the other is used for mostly cold season crops.
Space is also another consideration. If you don’t have the space for a garden or a raised garden, then perhaps you need to think outside the box (raised garden pun intended) and consider container gardening. Container gardening is exactly what its called – growing ornamental or vegetable plants in containers. Containers can be traditional plant pots, buckets, plastic totes, or any other container with an open top.
The advantages of container gardening include:
Containers can be arranged to optimally use the space available, or rearranged if you like to mix things up sometimes
Potting soil can be used, and can be trusted to be lead/chemical-free
Work can be performed on a bench, thus avoiding working on your knees
Containers can be arranged to provide decoration for your outdoor space
Many objects found around the house can be cheaply converted into decent containers
Vertical gardening is a version of container gardening that uses your available space efficiently. Much like using shelves to save space inside your home, vertical gardens use shelves, stairs, racks, etc. to make use of vertical space. The options for vertical gardens are only limited by your imagination. Here are a few extension bulletins on vertical gardening from Tennessee State University and the University of Nebraska.
In summary, the biggest obstacles to urban gardening are soil contamination, soil compaction, and space limitations. I’ve given you a few good alternatives to overcome those issues. Also, be sure to fertilize appropriately, lime as needed, and make sure the plants that you pick are appropriate for the sunlight that’s available. Your local garden supply store or extension agent can help you with suggestions on those issues.
If you know of an urban gardening obstacle that I didn’t address, please leave a comment and I’ll see if I can help out.