Growing leafy greens and other edibles in toxic soil can make you very sick. In some cases, it can kill you. Toxic soil contains heavy metals and other poisons. Often found under landfills, junkyards, and factories, toxic soil is increasingly found in urban areas.
What makes soil toxic?
Healthy soil contains a balance of organic matter, air, water, and minerals that plants use as food. Some of those helpful minerals, such as boron or molybdenum, can reach toxic levels in the soil. Heavy metals can also make soil toxic. So can organic pollutants, such as creosote, excessive fertilizer, herbicides, industrial solvents, pesticides, explosives, and petroleum products. In some cases, radioactive materials, such as radon and certain forms of plutonium, can be in your soil. It ends up that fill dirt used to be brought in from questionable locations when building homes. [Hopefully, that doesn’t happen any more.] The problem is, without soil testing, you don’t know what you have.
Soil is the earth’s filtering system. Like our kidneys, it can only handle so much. Heavy metals and other toxins in the soil often leach into groundwater. They can also become part of the dust that you inhale and the foods you eat. Toxins can be absorbed through your skin and may even coat produce you grow or buy at the store. [Always rinse off your leafy greens and root vegetables, and wash your hands frequently, just in case.]
Is your soil toxic?
The first step to learning whether or not you have toxic soil is a soil test. Not those cheap plastic things. A real, lab-based soil test. They are inexpensive and extremely valuable. Especially if your soil is toxic.
If your soil test results indicate heavy metals, such as lead contamination, or other toxins, there are steps you can take to remove those dangerous materials. Traditionally, that meant simply digging up the toxic soil and burying it somewhere else. Today, many researchers are looking to plants for a solution.
Put plants to work!
As plants absorb water and nutrients, they also take up nonessential elements, such as cadmium, lead, and mercury, which can contaminate soil. Using plants to remove toxins from soil is called phytoremediation. Phytoremediation uses plants to contain, remove, or render toxic contaminants harmless.
Phytoremediation plants can be classified as accumulators or hyperaccumulators. Accumulators (A) are plants that pull toxins out of the soil and up into their aboveground tissues. Beets, for example, will absorb and accumulate radioactive particles found in the soil. Hyperaccumulators (H) collect toxins particularly well, absorbing up to 100 times the toxins of accumulator plants. Sorghum is a hyperaccumulator of arsenic.
How does phytoremediation work?
Accumulators and hyperaccumulators can reduce toxins in the soil through several different processes:
There are advantages to using plants to clean toxins from soil: it’s inexpensive; it doesn’t harm the environment; and it preserves valuable topsoil. The disadvantage is that this is a slow process. It can take years.
Several studies have demonstrated that specific varieties of certain plants are very good at dealing with toxic soil. While I understand that Latin plant names can feel tedious at times, different cultivars behave differently, so getting the proper plant makes a big difference. For example, not all willow species are useful at cleaning soil. Studies have shown that Salix matsudana and S. x reichardtii are far more effective than other willow species.
Many trees, including American sweet gum, larch, red maple, spruce, Ponderosa pine, and tulip trees are able to accumulate radioactive particles (radionuclides), such as radon and plutonium.
Which plants remove which toxins?
I created the chart below from information provided by several studies on toxic soil and phytoremediation.
You can email me if you would like a larger version of this chart,
Keep in mind that, just because a plant will absorb toxins, does not mean it is something suited to your garden or your region. Some nasty invasives have become firmly established that way.
Did you know that some companies extract these toxic and sometimes valuable minerals from plants? This is called phytomining.
Now you know.
For every acre of garden, there is the equivalent of two mature cows, by weight, of soil bacteria living there. Ponder that a moment.
Your average cow weighs about one ton. Two cows weigh about the same as a car. That’s a lot of soil bacteria! For a different view, you could fit 15 trillion bacteria into a single tablespoon, if nothing else was there.
What are all those one-celled creatures doing in your soil?
Truth be told, much of your garden soil is made up of dead bacteria. Affectionately known as ‘bio bags of fertilizer’, soil bacteria are important players in nutrient cycling and decomposition. While still alive, their excretions improve soil structure by binding particles together into aggregates. This improved soil structure results in better water infiltration rates and it increases your soil’s water holding capacity. As bacteria breath, they release carbon dioxide into the soil. Plants love carbon dioxide.
Soil bacteria are most commonly found in the film of water that coats soil particles. Bacteria can’t move very far on their own. They generally move with water, though they sometimes hitchhike on passing worms, spiders, and insects. This is called phoresy.
Under ideal conditions, a single bacteria can produce 16 million copies of itself every 24 hours, doubling its population every 15-30 minutes. Conditions are rarely ideal, so bacteria reproduce as much as they can, whenever they can.
There are four basic groups of soil bacteria: decomposers, mutualists, lithotrophs, and pathogens. Most soil bacteria are beneficial. Pathogens are the troublemakers.
The majority of soil bacteria are decomposers that break down plant and animal debris into simple compounds which plants and other living things then use as food. This makes soil bacteria an important part the soil food web. Some decomposers can break down pesticides and pollutants. Decomposers also store a lot of nutrients in their bodies. When they die, those nutrients become available to your tomato plants. [Soil bacteria are 10-30% nitrogen.]
Mutualists have working arrangements with plants that benefit both sides of the equation. The most commonly known mutualists are the rhizobia bacteria which convert atmospheric nitrogen into a form useable by plants. Very often, these mutualists live on or in the roots of legumes, such as peas and beans. Other mutualistic soil bacteria are able to convert atmospheric nitrogen without the help of plants, but the plants still benefit.
You don’t hear much about lithotrophs, but this group is unique in that they don’t eat carbon compounds, the way other bacteria do. Instead, they manufacture their own carbohydrates, without photosynthesis, and feed on chemicals, such as hydrogen, iron, nitrogen, and sulfur. This group is also known as chemoautotrophs. These soil bacteria help break down pollutants and are an important part of nutrient cycling.
These are the disease-causing bacteria. They can cause fireblight, bacterial wilts, cankers, galls, and soft rot. The beneficial soil bacteria are always at war with these germs, competing for food, space, air, and moisture.
Killing bacteria is difficult. Most often, if conditions become difficult, a bacteria will simply enter a dormant stage. This is why many Quick Fix treatments don’t work. They don’t kill the bacteria, they just send it on a temporary hiatus. There are some soil bacteria (Streptomycetes) that actively protect plants from bad bacteria.
Why do soil bacteria matter to gardeners?
Most soil bacteria are valuable members of your team. They provide a huge benefit to your soil and plants. And you need to know what the bad bacteria look like when they start to set up housekeeping. The earlier you break those disease triangles, the faster your can return to harvesting your delicious crops.
Most bacteria are aerobic, which means they need oxygen. This is why turning your compost pile makes everything decompose faster. You are providing the decomposer bacteria with the air they need. If you don’t, the anaerobic, non-air breathing bacteria take over. Those are the ones associated with rot and purification. [Ew!]
Did you know that soil bacteria will consume more water than they can hold, causing their bodies to burst? Yet another argument against over-watering...
Sand slips through your fingers. Clay clods defy your shovel. And somewhere in-between is the sweet spot with bits and pieces of soil just the right size for plant roots. Whatever the size, these chunks are called soil aggregates.
To learn about soil aggregates, you will need a scoop of dry soil from your garden. Put the soil in a bowl. Are there a lot of different sized pieces or are they mostly the same? If you look closely at the photo below, you can see a clear line between the old clay layer and all the decomposing mulch and compost that I have been putting on top. Over time, those organic materials work their way down, into the clay, reducing compaction and improving drainage. These improvements will occur because of soil aggregates.
Take another look at your soil. Stir it around a bit. Pick some of it up and roll it around in your hand. Rub it with your fingers. Does it feel gritty? Or powdery? Do the clumps mostly hold together? Do they crumble completely or do they feel like rocks? Soil aggregates, also known as ‘peds’, are the clumps that tend to stay together when you work the soil.
Why do soil aggregates matter?
Healthy soil has a variety of aggregate sizes, with plenty of large spaces (macropores) between the aggregates and tiny spaces (micropores) inside the aggregates. These spaces are used by roots and gases to move through the soil. These spaces are also what allow water to soak in, increasing your soil’s water holding capacity. And plant nutrients stick to these clumps.
In some cases, aggregates are not as important. Sand, for example, has no aggregates, but there are so many spaces between grains of sand that plant roots, water, and gases have no trouble moving around. [Hanging on to water and nutrients is something else altogether!] Soils with low bulk density are another case where aggregates don’t matter as much. For the rest of us, the soil aggregates in our gardens have a huge impact on plant health, especially tender seedlings. If your soil’s aggregates are unstable, seedlings can suffocate.
Aggregates are described according to their stability. If your soil crumbles into dust, you probably have a lot of clay or silt and that can mean your soil has low aggregate stability. Low aggregate stability increases problems with erosion, gas exchanges, root development, and permeability. More immediately, as rain, irrigation, or sprinkler water strikes the soil surface, flimsy aggregates can be broken. Those tiny broken bits clog the spaces in the soil, making life difficult for plant roots, worms, and soil microorganisms. It also causes crusting which can kill seedlings before they get a chance to grow.
How do soil aggregates form?
Healthy soil aggregates are held together by clay, organic matter, and glomalin. Glomalin is a protective fungal excretion that helps the fungi feed your plants and binds soil aggregates together. Bacteria have similar excretions which are not as effective.
Organic materials in the soil usually mean decomposition is taking place. Decomposition means fungi, worms, bacteria, and microorganisms are present. Those life forms excrete coatings and other materials that help soil aggregates form and stabilize. Finally, as clay particles become moist, they act as a cement, holding molecules and particles together into aggregates.
Test your soil for aggregates
Returning to your soil sample, select a few particularly large clods and gently set them aside to dry completely. Once they are really dry, dip them into a glass of water. If they break up quickly it means your soil has low aggregate stability. If the clods retain their shape for 30 minutes or more, your soil’s stability is high. Because my soil contains so much clay, it pretty much dissolves immediately. As more organic material is incorporated, my soil will breath better, hold its shape better, and provide plenty of pores for roots, water, and microorganisms.
How can you improve the aggregates in your soil?
Start by taking a look at your soil test. If your soil contains a lot of calcium or iron, it probably already has good aggregates. If your soil holds too much salt, aggregates are harder to come by. The biggest indicator of good soil aggregates is the amount of organic matter found in the soil. By mulching and top dressing your soil with manure and aged compost, you are encouraging all the life forms that help soil build healthy aggregates. This is why no-dig gardening has become so important. We learned that excessive digging, plowing, and rototilling disrupt the soil dwelling populations that create and maintain good soil aggregates.
If your soil aggregates are unsatisfactory, use these tips to encourage better soil structure in your garden and landscape:
How did your dip test turn out? Let us know in the Comments!
Why is beach sand mostly white and tan while rich farmland is practically black? What does soil color tell you about your soil?
Soil occurs in many different colors. Iron-rich soil tends to be reddish orange or green, while peat can be practically purple!
Go outside and collect a handful of your soil and put it in a clear container. Shake it around a little bit. Is it wet or dry? What color is it? Brown? Maybe. But I’ll bet it’s not that simple.
What does soil color tell us?
Each layer of your garden soil has a unique color. The deeper you dig, the lighter those colors tend to get. Soil color tells us which minerals are present and the level of decaying organic material. It can also tell you how old a soil is, which processes are occurring, and about local water behavior.
We are not going to explore soil age or the chemical processes that take place in soil, but you can use soil color to make better decisions about irrigating and fertilizing your garden.
Soil moisture levels
We all know that soil looks darker when it is wet. But soil color can tell you how long the soil stays wet. Soil that does not drain well and stays wet for much of the year tends to be a dull yellow or grey. Wet soil contains less oxygen than dry soil. Oxygen causes some minerals to oxidize, or rust. Iron-rich soil that contains a lot of moisture most of the time will turn grey or greenish, while drier soils expose iron to more oxygen, turning the soil red or yellow.
Soils that stay wet often have more complex color patterns, while arid soils are more uniform. If your soil colors are uniform, you know that the water table is lower and you will probably have to irrigate more often. If your soil is reddish, you will probably never need to amend with iron. Remember, the minerals found in soil are plant food.
Minerals make a difference
Other minerals in your soil can also affect its color. Knowing what these colors mean can help you select the best soil amendments and irrigation schedules.
What color is your soil?
Take a closer look at your soil sample. What do you see? Is it yellowish-brown or dark brown? Or something else entirely? When we first moved to our San Jose, California house, the soil was a pale, tan color and as hard as concrete.
For many of us, identifying a specific color can be tricky. Brown is brown, right? But soil can be all sorts of shades of brown, along with a bunch of other colors. To help you get really specific information about the color of your soil, you may want to go to the library and check out their copy of the Munsell soil color book.
Munsell’s color book
Soil color is so important that a system of soil color classification has been developed. This classification method is called the Munsell soil color system. A Munsell book is the gardener’s equivalent of a paint chip book, containing 199 color chips. Its pages are heavy card stock and they are organized by color. Underneath each color chip is a hole in the card stock that lets you hold a soil sample underneath for comparison. On the opposite page tells you the universally accepted name for that color. This is a coding system used around the world by soil scientists, farmers, and gardeners like you!
You artists out there know a lot more about this than I do, but let me give it a shot. According to my Munsell book, colors are described using hue, value, and chroma. Hue is the wavelength we see as color. Munsell’s book gives codes for red (R), yellow (Y), green (G), blue (B), and purple (P). Those wavelengths are measured in graduations of purity, ranging from 2.5, 5. 7.5 and 10. A pure hue is rated at 5. Numbers above 5 indicate the presence of other hues. Value indicates lightness or darkness. A value of zero indicates pure black, while 10 is white. Finally, chroma refers to a color’s strength or intensity, ranging from greyed out (/0) to full intensity (/14).
A Munsell soil color rating is written with the hue letter first, followed by a space and then the number value, a forward slash (or virgule), and then the chroma number. Decimals can be used to provide greater clarity.
Looking at a photo of my soil when we bought our place in 2012, I see that the color most closely matches 5YR 7/1. According to Munsell, that soil would be called 'light grey'. As noted earlier, this indicates high calcium carbonate, gypsum, magnesium, sand, and/or salt levels. It can also indicate too much moisture. Funny thing, the previous owner loved to apply fertilizer and overwater the property. According to my 2015 soil test, soil organic matter was at 3.5% and all the nutrients, except iron, were through the roof! Iron was extremely low.
Seven and a half years later, after adding lots of mulch and compost, a little nitrogen, appropriate watering and nothing else, my soil has been transformed to 2.5YR 3/0, with 7.6% organic matter and nutrient levels (slowly) dropping to where they should be. [These changes never happen overnight. When they do, beware! Something is very wrong.]
The new color is 'very dark grey' which goes along with all the chicken bedding, wood chips, and other organic materials I've been adding. And my iron levels are still way too low, which is why the chroma numbers have stayed low.
So, take another look at your soil sample. Does it tell you more than it did? If you live nearby, feel free to bring a soil sample by so we can take a look in my Munsell book together. If not, head to the library.
Did you know that carpet manufacturers use the Munsell soil color system to match local soil colors with carpet dyes so that their carpets will look cleaner longer?
Now you know.
When your house was built, the soil was significantly altered. Construction soil can be severely compacted and rocky. This problem persists for many years, long after the bulldozers have moved on.
What can you do to transform construction soil into friable garden soil?
What is construction soil?
When a house is built, no one wants it to fall down. Around 500 B.C., a man named Pythagoras figured out the correct angle for walls to be built to reduce the likelihood of collapse. Well, the soil under those walls is equally important for building stability.
Building sites are scraped flat, removing much of the nutrient-rich topsoil, and then mechanically compacted. This is great for your house and terrible for the soil. And if the local soil isn’t stable enough for building, nutrient-poor fill dirt is brought in, mixed in and compacted, until builders have the surface they need. After construction is complete, sod is installed, a few trees and shrubs popped into place, and a cosmetic planting of annual flowers makes everything look lovely. But it’s a lie.
The soil under new construction is reeling in shock. Heavy equipment, trucks, materials, and foot traffic have been crushing the soil, plant roots, microorganisms, insects, and worms for weeks or months of building. Simply adding an attractive top dressing of plants does not correct the problems.
What can you do about construction soil?
Of course, over time, most plants and lawns manage to push roots into the soil and grow. But they could be far healthier and easier to care for if the construction soil they are trying to grow in was transformed into something loose, nutrient-rich, and populated with helpful microorganisms.
You can make that happen with these tips:
If you do not currently compost kitchen and yard waste, you can easily start a compost pile wherever your least healthy soil is. Simply drop equal parts brown and green materials into a pile, water it and flip it every few days, and within a few weeks (depending on the season) you will have a nice batch of aged compost and that spot will be super-charged with nutrients, microorganisms, worms, and other soil beneficials. If you have a few chickens, adding their bedding and manure to the pile makes it even better!
Finally, get your soil tested by a local lab. Over-the-counter kits are not accurate enough to be useful. Inexpensive lab-based soil tests tell you which nutrients are needed, which are present in excess, and if you have lead-contaminated soil.
Even if you have lived in your home for decades, the effects of construction soil may still be present. Creating healthy soil means that your plants will be better able to defend themselves against pests and disease, along with frost and drought damage. In other words, healthy soil gives you more time to relax!
The government might know more about your soil than you do. Did you know you can access the USDA’s soil map of your property? You can and I am going to show you how.
What are soil maps?
Soil maps, also known as soil surveys, are used by architects and engineers to determine a soil’s ability to support roads or structures. Farmers use soil surveys to help them decide the best use for their land and you can, too. Your soil map can help you with plant selection, irrigation, and other gardening decisions.
Soil maps are the combined information collected by various government agencies on different types of soil. Soil surveys used to be printed in book form by every county. We don’t do that anymore. [Thank goodness!] Now, all the information is found online.
How to access a soil survey
All of the information the U.S. government has about your soil is available at the USDA’s Web Soil Survey page. Because this page isn’t exactly intuitive to use, we will work through it together. Once you open the webpage, click on the green Start WSS button to begin.
Once you are in, you will see five tabs. Those tabs are:
You will automatically be on the Area of Interest page. This is where we will begin.
Area of Interest
Before you can access any useful information, you have to set an area of interest (AOI). To do that, follow these steps:
*If your AOI is too small, you will get a warning. If this happens, make sure you are on the AOI tab, under Area of Interest and AOI Properties, and click the Clear AOI button and start again at step #7, using a larger area.
Your map will have orange lines and reddish-orange numbers and letters marking various soil series, which will be listed on the left. You can click on the soil type links for a surprising amount of information, including:
If you need help, as I did, with some of the terminology, try the USDA Soil Glossary. Now we get into the nitty gritty information. Click on the Soil Data Explorer tab.
Soil Date Explorer
This tab has sub-tabs you can investigate. Under Intro to Soils, you can get the equivalent of a college education on soil, free for the reading. The next sub-tab, Suitabilities and Limitations for Use, returns you to your map with a ton of informational categories on the left. While you probably don’t care about Building Site Development, you still might find it interesting reading. If you are short on time, go straight to the Land Management heading and click on the double arrows to expand that category. [Be sure to check out some of the other headings, as well.]
A list of several sub-categories will open up and you can expand any of them. For each of these sub-categories, you can click on View Description or View Rating. In many cases, you may see “Not rated”. I have to assume that means it is either not relevant, or that it has not been considered worth the investment.
Speaking of investments, were you surprised to learn that our tax dollars are spent on this sort of information collection?
Download Soils Data
The next tab is labelled Download Soils Data. While you can certainly try using it, I had no luck. Apparently, I do not have the proper software to open the downloadable files.
Shopping Cart (Free)
This tab allows you to download a 30-page or so document with all the general information about your soil, if you want it. Personally, I find just playing around on the website gives me more of the information I can use in my garden than the report. If you want your report, click on the Check Out button and then decide if you want it now or later, and click OK.
Since most of this information is collected for farmers, builders, emergency response, and military use, it can be far more than you need in the home garden. But it sure makes for some interesting reading!
Silt refers to minerals larger than clay and smaller than sand. Silt is commonly moved by water and deposited as sediment. Silt is what makes the alluvial soil surrounding rivers so fertile. Silt is also fine enough to be carried surprisingly long distances on the wind as dust.
How silt is formed
As rocks and regolith are eroded by weather, frost, and other processes, larger particles are ground down into smaller, rounded bits. Those smaller pieces become silt. Silt typically measures 0.05-0.002 mm and is usually composed of quartz and feldspar. Because silt moves so easily in water, construction and clear-cutting often result in silt levels that pollute waterways. This type of pollution is called siltation. In home gardens, over-watering can cause similar leaching problems and urban-drool. But silt is good for your plants.
Silt in garden soil
Sandy garden soil loses water and nutrients too quickly, while clay soil holds on too tightly. Loamy soil, in the middle, is ideal for garden plants. Loam consists of 40% sand, 20% clay, and 40% silt.
Silt particles tend to be round, so they can retain a lot of water. This high water holding capacity is made even better because silt particles cannot hold on to the water very tightly. The same is true for mineral nutrients. Roots and microorganisms have an easy time pulling water and food from silty soil. Silt can be beige to black, depending on how much organic material it contains.
Silt is prone to compaction, but not nearly as much as clay. If your soil feels slippery when it is wet, it contains a lot of silt.
Your soil is filled with positively and negatively charged bits of plant food. The percentage of that food being held by soil particles is called its base saturation.
Of course, it’s not that simple. The chemical reactions going on in soil are enough to make a chemist’s head spin. But we are here to simplify and understand, so let’s get started!
Electrified plant food
Plants use electrically charged mineral bits, called ions, as food. The negatively charged bits (anions) are usually found floating around in water. The positively charged bits (cations) attach themselves to soil particles, which are negative charged. Those soil particles have a certain number of electrical charges that can attract minerals. That number is referred to as its cation exchange capacity. The number of those attachments being used is its base saturation.
Playing the percentages
There is some crazy math and lab work involved with calculating base saturation, but we can leave that to the experts. Most soil test results will list separate base saturation percentages for calcium, magnesium, and potassium. Don’t be confused by the fact that these numbers do not add up to 100%. Hydrogen and sodium have been omitted. But what do these percentages tell you?
When the charges of soil nutrients are out of balance, plants cannot absorb what they need to thrive. It doesn’t matter if a nutrient is present if the net electrical charges are wrong. If most of the nutrients in your soil are negatively charged, all of the positively charged bits will be able to connect, leaving many negative bits hanging in isolation. Those leftover minerals impact soil pH.
Base saturation and soil pH
Base saturation measures the number of non-acidic, positively charged bits in a soil sample. That’s why it is called “base” saturation. There are also acidic positively charged bits. Soils with a high base saturation have lots of those acidic, positively charged bits lying around unattached. The more loose acidic bits laying around in the soil, the lower the soil pH.
Using base saturation numbers
Soil test results will tell you how much of each plant nutrient is present and base saturation percentages. One thing you might see is an excessive amount of a nutrient but a normal base saturation percentage. How is this possible? Again, it goes back to electrical charges. Say you have a ton of calcium, a positively charged mineral, but the calcium base saturation is normal. This happens because other charged particles are also present. They can block the excess bits from connecting with anything. Or, there may not be enough negatively charged soil particles available. You need to use both the actual mineral levels and the base saturation percentages when deciding on whether or not to add fertilizer.
This post is an oversimplification of an extremely complex topic, but it is accurate enough to help you get the most out of your soil test results. Soil tests cost around $25 and are worth every penny.
Your soil has a characteristic known as bulk density.
Put simply, if you take a scoop of soil, it will weigh something. If you take a scoop of different soil, it will have a different weight. Those weights are a measure of the material held in that space. No surprise, right?
Also known as scoop density, this measurement tells you how tightly your soil is crammed into a space. It also tells you a lot about your soil’s permeability (ability to drain), infiltration (rate of drainage), porosity (the number of macropores and micropores), soil texture (sand, silt, and clay), and soil structure. This is important information for plant roots.
Another non-surprise: soil is heavy. The weight of the top soil pushes down on the soil below it. That layer pushes down on the layer below that, and so on. This means that soil becomes more and more dense, the deeper you go. This is one reason why so many plants keep their roots near the soil surface.
Bulk density is measured in grams per cubic centimeter (g/cc). Bulk density generally ranges from 1.0 to 1.25 g/cc. Sandy soil tends to have high bulk densities (1.3-1.7 g/cc), while clays and silts normally have moderate densities (1.1-1.6 g/cc). Soils that contain more organic matter tend to have lower bulk density values. Lower bulk densities allow for proper drainage, reducing the chance of fungal disease and helping plants overcome the negative effects of mud and drought.
Too much stuff
If a soil’s bulk density is higher than 1.6 g/cc, your plants are going to have a hard time. Compacted soil restricts the free movement of roots, air, and water. High bulk densities can also prevent germinating seeds from making it to the surface with enough energy to thrive.
What is your soil’s bulk density?
The USDA provides instructions for a DIY bulk density test, but I have to warn you, your kitchen will stink after you bake or microwave a soil sample. A far easier and more pleasant method is to send a sample to a lab. For the price of a bag of fertilizer, your can learn a lot of good stuff about your soil. Soil tests tell you about nutrient levels, the cation exchange capacity, pH, and base saturation numbers, along with bulk density.
Case in point
In 2015, my soil’s bulk density was 1.18 g/cc. By 2019, it had changed to 0.95 g/cc. What happened?
In 2015, my soil test indicated an extreme overabundance of every nutrient, except iron, and compacted clay. [The overfertilizing was done by the previous owner.] To counteract the compaction and the lack of iron (a nutrient needed by plants to help them consume other nutrients), I applied foliar sprays of chelated iron and mulched the heck out of every soil surface with aged compost and chicken bedding.
The iron sprays allowed my plants to make use of and extract the abundant nutrients, bringing them closer to normal, balanced levels. The composted manure and other organic materials created more spaces between soil particles, making it easier for roots, gases, and water to move around. Four years later, all of my plants are growing better and my soil organic matter (SOM) levels went from 3.5% to 7.6%.
If your soil is too dense, your plants can’t thrive. If you know your soil’s bulk density, you can take action to improve it.
Have you ever noticed how the larger bits come to the surface when you shake a container of soil?
This is called the Brazil nut effect. I have no idea why.
Crusting is a type of soil compaction.
When we say soil is compacted, we are referring to all of it. When compaction occurs below the soil surface, it is called hardpan. When the problem is at the surface, we call it crusting.
Healthy soil is lumpy. These lumps are called soil aggregates. Soil aggregates are made up of different sized minerals, bits of organic matter, and spaces, called macropores and micropores. Those spaces are critical to soil and plant health, as they provide pathways for air, water, and roots.
When surface aggregates are broken into smaller and smaller bits, the soil particles shift around, dry out, and realign into a smooth, plate-like structure, called a crust. As that crust dries out even further, cracks commonly appear. These cracks are nearly always at 120° or 90° angles.
Types of crusting
Soil crusting can be classified as chemical, biological, or physical. Chemical crusts are the result of salt or other mineral deposits on the surface that commonly occur in arid regions. Biological crusts are generally caused by algal deposits left behind from slow-draining ponds and they tend to be lumpier than other soil crusts.
Physical crusts may be structural or depositional. Depositional crusts are the result of fine soil particles carried in runoff being deposited over an area. Structural physical crusts are more likely to occur in the home garden. Crusting is particularly common in clay soils because the particles are already so tiny. Flat clay particles average less than 2 μm and are attracted to one another by electrostatic forces. Silt is boxier and 2 to 50 μm, while sand particles are larger than 50 μm. Neither silt or sand particles are attracted to one another electrically. If your clay soil contains high levels of magnesium and/or sodium, the odds of soil crusting are even higher. [What does your soil test say?]
What causes structural crusting?
Rototilling and rain are the two most common causes of crusting. Frequent digging or rototilling disrupts microorganism populations and breaks up soil aggregates. Those aggregates are needed to allow air and water to move through the soil. Soil microorganisms are partly responsible for maintaining those soil aggregates and for feeding many of your plants.
As heavy rain (or sprinkler water) falls, each drop hits the topsoil and breaks up soil aggregates into smaller and smaller particles. These smaller particles are more prone to compaction and surface crusting.
Problems with crusting
Compacted soil makes it difficult for water, air, and roots to move through. It also slows soil gas exchanges and drainage. Crusty soil slows water infiltration and makes life very difficult for germinating seeds and young seedlings. In fact, crusting can stop germinating seeds from getting to sunlight altogether. Crusting also increases the chances of runoff and urban drool. If the soil below has reached its watering holding capacity, crusting can prevent evaporation, causing roots, worms, insects, and microbes to drown.
Soil crusts are rather fragile. As they are damaged, they tend to break apart, allowing the soil to erode very quickly. [My Burner readers know what I mean. Pre-event, the Black Rock Desert crust is firm and dust levels are relatively low. As traffic picks up, the surface crust is damaged and dust storms can become rather impressive. For you non-Burners, just think of the Dust Bowl of the 1930s.]
Correcting crusty soil
Patches of crusting can be corrected by lightly breaking up the soil surface and planting cover crops, green manure crops, or cereal grains. You can also top dress the area with aged compost or manure, or reduce damage by mulching.
How to prevent crusting
Rather than rototilling or digging, use mulch to encourage worms and soil microorganisms to do the work for you. Also, after harvesting an area, cover it with straw, mulch, or a fast-growing cover crop to absorb rain droplets and prevent erosion and compaction.
Soil organic matter (SOM) is a category found in soil test results and it is critical for good soil health.
Soil organic matter levels can range from practically nothing to as much as 90%. Deserts are at the low end of the scale, while low lying, wet areas (think peat bogs) are at the high end. Most topsoils range from 1% to 6% soil organic matter. Soils containing 12% to 18% organic matter are called histosols. Histosols tend to be acidic, low in nutrients, and have poor drainage.
Components of soil organic matter
Soil is made up of minerals (45-49%), water (25%), air (25%), and things that were or are alive. These lifeforms can be insects, plants or animals, in various stages of living or decomposing, microbes, and any substances created by those living things. These lifeforms, both alive and dead, and their secretions and exudates, are what make up soil organic matter.
Soil organic matter is approximately 5% living things, 10% fresh residue, 33-55% stabilized organic matter, and 33-50% decomposing organic material.
Organic matter and soil health
Maintaining healthy soil is a big part of the Integrated Pest Management (IPM) practices that allow us to grow plants with a minimum of chemical interventions. Healthy levels of soil organic matter provide biological, physical, and chemical benefits to your soil. Sufficient soil organic matter improves soil structure and water retention and infiltration. It also increases soil aggregation, or clumping, which increases the number of macropores and micropores through which water, air and roots can move. Organic matter improves soil biodiversity, and the absorption and retention of pollutants, while reducing soil compaction, crusting, and urban drool. Organic matter also creates a buffer against changes in soil pH.
Organic matter and plant health
As plants, animals, and insects decompose, a variety of compounds become available to plants, increasing soil fertility and nutrient cycling and storage. These compounds include carbohydrates (sugars and starches), fats, lignin, proteins, and charcoal. As these compounds are broken down further, or mineralized, they increase your soil’s cation exchange capacity. This means plants are better able to absorb atoms and molecules of plant food through root hairs. Insufficient soil organic matter can cause mottling and other signs of nutrient deficiency.
Soil organic matter also acts as a carbon sink, reducing the amount of carbon in our atmosphere. As a major player in the carbon cycle, soil organic matter is believed to hold 58% of the Earth’s carbon. We can help keep it there (and out of our air) with no-dig gardening and cover crops.
How to increase soil organic matter
Before increasing anything in your soil, send a sample to a lab for testing. There is no other way of knowing what, exactly, is present without a soil test. It would be rare for most soils to have a problem with increasing organic matter levels, but it’s better to be safe than sorry. Plus, then you’ll have all that other great information!
You can increase organic matter levels in your soil with these tips:
Remember, soil organics matter!
Eating lead-based paint is a bad idea. You don’t want it in your garden soil, either. But how do you know if it is there and what can you do about it if it is?
Lead is a soft, heavy metal that has been used to make paint, pipes, bullets, batteries, pewter, leaded glass, and in gasoline. Lead is still used to make high voltage power cables, lead-acid batteries, solder, and wicks for cheap tea lights.
Damage caused by lead
Lead is a neurotoxin that accumulates in bones and soft tissues, causing brain, kidney, liver, reproductive system, digestive system, and nervous system damage. It also reduces intellect and is believed by some to be associated with increased rates of crime and violence. Many historians attribute the Fall of Rome to the fact that their pipes many of their food containers were lined with lead.
Most countries have banned the use of lead in products that might cause exposure, but not all. Countries such as China, India, and Indonesia still use lead in many products which is why it is important to verify that planting containers, coffee cups, and other food-related items are safe to use. Red and yellow ceramics are the most likely to contain lead.
Where does lead come from?
Lead was added to gasoline as an anti-knock agent in 1921. By the 1970’s, over 75% of the U.S. population had elevated lead levels in their blood. That number dropped to just over 2% twenty years later, after lead was removed from gasoline. All those fumes, spewing forth for over 50 years, contained lead. That lead settled on roads, yards, gardens, and fields. Rain and irrigation water leached some of that lead into rivers, lakes, and oceans.
Lead can also find its way into your garden soil by sanding, chipping, or sandblasting lead-based paint from older buildings, or when old lead pipes, roof flashing, or lead-batteries are allowed to sit on the ground and break down.
How much lead is in your soil?
Lead occurs naturally in the soil. While there is no safe level of exposure, natural concentrations range from 10 to 30 parts per million (ppm). Areas where leaded gasoline was still in use in 2014 were found to have lead levels of 100 to 1,000ppm. Homes painted with lead-based paint that were located near high traffic roads could have had lead levels as high as 3,000ppm.
Until you get your soil tested, there is no way of knowing how much lead is there. I use the UMass Extension Soil Testing Lab. My soil test lists anything below 22ppm as acceptable. My results were 2.1ppm in 2015 and 2.0 in 2019. Lab-based soil tests are inexpensive and they provide valuable information both for your plants’ health and your family’s health.
How to manage lead contaminated soil
If your soil is contaminated, your biggest health risk is breathing in dust that contains lead. One of the easiest ways to reduce the risk of inhaling lead dust is to grow cover crops or mulch over the area. You can also cover the contaminated area with 4” to 6” of clean soil, to reduce the risk of dust.
You can also use certain amendments that bind to the lead, making it less likely to be absorbed by plants or released into the air via dust. Lead will bind to organic matter, such as aged compost, but this treatment needs to be repeated as the compost breaks down. Depending on your soil’s phosphorus levels, the addition of more phosphorus may improve the binding action. Too much phosphorus is bad for plants, so check your soil test results before using this method.
As soil pH increases, becoming more alkaline, plants absorb more lead. Maintaining a soil pH of 6.5 to 7.5 is ideal, both for plant health and to reduce lead absorption.
Can I grow edible plants in lead contaminated soil?
Plants can grow in soil with lead levels as high as 500ppm. Lead moves very slowly through plants, staying mostly in the roots. According to the University of California Department of Agriculture and Natural Resources, “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 [when] grown in lead-contaminated soils.”
Crops that should never be grown in lead contaminated soils include leafy greens, such as chard and collards, and root vegetables, such as beets, carrots, potatoes, and turnips. These crops are better grown in raised beds with clean potting soil.
Also, if you know your soil contains high levels of lead, be sure to wash all produce thoroughly to remove any lead dust that may be present.
Finally, pencil leads have never been made from lead. They are made with graphite.
Now you know.
Acidification is a process that lowers soil pH.
Soil can be alkaline, acidic, or neutral. The pH scale ranges from 0 (acidic) to 14 (alkaline), with (neutral) 7 in the middle. Soil pH dictates the availability of many nutrients to your plants’ roots.
Your soil can be packed full of important minerals, but the wrong pH can make it impossible for plants to reach that bounty. According to my 2015 soil test, my soil had a pH of 7.7 and very little iron. Plants need iron to absorb many other essential nutrients. By lowering the pH, or acidifying, my soil, I can make the iron more readily available. By 2019, the soil pH had a pH of 6.2, which makes many more nutrients available. If you live in an area with alkaline soil and want to grow acid-loving plants, you will need to acidify your soil.
Which edible plants prefer acidic soil?
If all of your plants prefer your soil’s current pH, you are in luck. It’s really the easiest way to go. Most garden and landscape plants prefer a pH range of 6.2 to 7.3. Acid-loving plants include:
Moderately acid-loving plants that prefer a pH of 5.5 to 6.5 include apples, basil, carrots, cauliflower, corn, cucumber, dill, eggplant, garlic, melon, peppers, pumpkin, rhubarb, winter squash, tomato, and turnips.
Factors of acidification
There are three factors that determine the amount of acid needed to lower soil pH. Some of this stuff gets deep in the world of chemistry, but I think I have sorted it out well enough. [If you understand these things better than I have explained, please educate us all in the Comments section!]
How to acidify soil
While using the above information will give you more accurate data, you can gently acidify your soil by applying elemental sulfur (S) in stages. As the sulfur oxidizes, it turns into sulfuric acid, acidifying the soil. Changing soil pH takes several months to accomplish and it tends to require regular monitoring and adjustments. Since soil pH is a function of geology and climate, it will be an ongoing process. Just be sure to read and follow the package directions.
Fertilizers and acidification
Nitrogen has a powerful impact on soil pH. The form of nitrogen you use makes a difference. To lower the pH of your soil, use ammonium-based fertilizers, rather than nitrate-based fertilizers. Your blueberry plants will thank you.
What's the pH of your tap water?
Clay soil is prone to compaction.
Healthy soil is loose enough to allow roots and earthworms to move around freely, while still providing support and structure. Unlike sand (which has its own problems), compacted soil has too few macropores and micropores (larger and smaller spaces) between soil particles. These spaces are needed to hold air, water, and nutrients for plant roots.
Compacted soil can prevent water from moving into the soil (infiltration), through the soil (permeability), and out of the soil (drainage). Standing water can drown plants and create mosquito habitat. It can also make life difficult for tender new seedlings trying to get a healthy start by reducing nutrient uptake and poorly anchoring plants to the ground. Soil compaction hurts mature plants, as well, by reducing nitrogen levels in the soil, as well as other nutrients.
What causes soil compaction?
Every step you take presses down on the soil beneath your foot. Healthy soil can spring back. Soil that is walked on too frequently loses that ability and it becomes compacted. Other common causes of soil compaction include:
Plants that counteract compaction
Deep taproots can help break up compacted soil. Put these plants to work for you, rather than compounding the problem with further digging. Adding these plants to your landscape can help reduce compaction and improve soil structure:
Other tips to reduce soil compaction
The very best thing you can do for compacted soil is cover it with a thick layer of aged mulch or some wood chips and leave it alone for a while.
Humus is the magical dark stuff of soil that helps plants grow. Or is it?
Microorganisms in the soil facilitate the decomposition of plants, bug bodies, and other living things in a process called humification. Humification occurs in the soil and in compost piles. Along with microorganisms, worms, nematodes, and other tiny critters help this transformation along. The black, rich, earthy smell from premium soil is the humus, or organic matter. (The black color is from carbon.)
When you add organic matter to the garden or compost pile, you are feeding the organisms that make nutrients available to your plants.
Infiltration rate is a measurement of how quickly water can enter soil.
Infiltration rates are reported as the depth water (in millimeters) can reach within one hour. For example, an infiltration rate of 10 mm per hour means that a 10 mm layer of water on top of the soil will take one hour to soak in. Understanding the infiltration rate of your garden or landscape can mean the difference between irrigation and flooding.
Soil types & infiltration
When soil is extremely dry, it won’t absorb water at all because it becomes hydrophobic. Before that point is reached, water is absorbed quickly in what is called the initial infiltration rate. It happens quickly because the macropores in the soil contain only air, giving the water plenty of places to go. As the pores begin to fill, absorption slows down to a steady rate called the basic infiltration rate. According to the Food and Agriculture Organization of the United Nations (I never knew there was such a group until today!), different types of soil have different basic infiltration rates:
Clearly, taking an infiltration test can help you have a better understanding of what type of soil you have. It also helps you to select plants that are best suited to your soil.
Benefits of better infiltration
Understanding and amending the infiltration rate of your soil can provide many benefits:
How to conduct an infiltration field test
Generally speaking, most of these tests are done with specialized equipment. You can, however, follow these steps to perform a modified version that will give you useful information:
Another testing method you can try only requires a shovel and a watch:
If your soil is like mine, compacted heavy clay when we moved in, adding organic material is the best way to improve the infiltration rate. If you plan on installing a rain garden, checking the infiltration rate is critical. Standing water can drown even the healthiest plant, given enough time.
Help your garden and your landscape with improved permeability and infiltration rates. Your plants will be healthier, more beautiful, and more productive.
Every drop of rain that falls on your landscape ends up somewhere. Where that water falls and where it ends up is called a watershed.
Rainwater may be absorbed by plants, sipped by local wildlife, or it may evaporate back into the atmosphere. Rainwater can also wash away valuable topsoil, carrying fertilizers and pollutants into our groundwater supplies, rivers, lakes, and oceans. In fact, the California Native Plant Society tells us that urban drool is the #1 source of ocean pollution. To prevent water waste, pollution, and runoff, a new approach to landscape design was created to protect our precious watersheds.
Watershed approach to landscape design
The watershed approach to landscaping uses garden design, installation, and maintenance methods that take advantage of natural processes to create spaces that are water efficient, while providing abundant plant growth, good habitat, and an enjoyable place to be. The watershed approach captures, cleans, and collects rainwater to slow, spread, and redirect its flow in ways that reduce the need for other irrigation. The benefits of using a watershed approach include:
How much rainfall do you get?
Every yard is different, but your average 2700 sq. ft. roof in the Bay Area can collect more than 25,000 gallons of water each year! You can use the USGS rainfall calculator to determine how much rain water falls on your roof in any given storm, simply by entering your home’s footprint dimensions (length x width) and the amount of rainfall measured by you* or reported by your local news station. During a storm that drops one inch of rain onto a half-acre lot turns out to be nearly 14,000 gallons of water! Rain barrels, ponds, swales, and filtration tanks are all different ways you can collect rainwater.
Where does that water go?
All too often, rain water falls on buildings, roads, and concrete, where it collects pollutants and debris, and carries them to our groundwater. Our garden plants never have the opportunity to soak it up. On the other end of the spectrum, rain water either floods an area, carrying away valuable topsoil, or it gets stuck in one place, where soil, plants, and organisms begin to rot. Using the watershed approach removes those problems by studying where water comes from, where it goes, and taking actions that redirect water flow to be more efficient and environmentally sound.
First flush and absorption areas
First flush refers to the first 3/4 to 1 inch of rain that falls after a dry period. This rain water contains higher levels of pollutants and debris than the rain that follows. Can you filter those pollutants out or redirect this water to less vulnerable areas? After that water is dealt with, how much permeable soil is needed to absorb your expected rainfall? First, you will need to know how deeply your soil absorbs water. You can determine this by going outside after a few days of rain and digging in with your shovel. How far down did the rain actually go? This number can help you determine how big of an absorption area you will need for the expected rain.
Example: You live in a 1,000 square foot house in San Jose, CA, where you receive an average of 15 inches of rain each year. Using the USGS rainfall calculator, you would discover that your house can collect 9,351 gallons of water in a year. To absorb all that water, you would need to divide the volume of water by 7.48 for a per foot absorption area. (There are 7.48 gallons of water in a one cubic foot of space.) This gives you 1250 square feet needed to absorb all that water, assuming that your soil absorbed water down to a depth of one foot. If it only went down 6 inches, the 1250 sq. ft. figure would have to be doubled. If you don’t have that much space, how can you prevent runoff? What if that’s not enough water?
What are other sources of irrigation water?
Be sure to check with your local municipality for laws regarding water collection (there have been countless wars started over water rights). You can collect water from your bath or shower, as it heats up, in a bucket. You may be able to redirect the outflow from your washing machine to irrigate ornamentals. Even the water left over from cooking pasta and vegetables makes useful water for the garden.
How much water do you really need?
There is no excuse for wastefulness when it comes to water. You might be surprised at how little water you and your garden actually need. Our household has reduced water consumption to only one-fourth of what it was three years ago and we get more production from the garden! This is possible by:
Other factors to consider when using the watershed approach:
Start using the watershed approach in your yard by asking yourself these questions:
What’s really nice about the watershed approach is that it takes advantage of natural processes that have evolved over thousands of years to work without any help on our part. Native plants and those suited to your microclimate require less care, which translates into less work, less expense, and a healthier environment.
Activity: Inventory your landscape’s water needs
Every drop of water that you are able to use more efficiently protects the environment and your bank account.
Perlite has a distinctive feel, lightweight and crispy, but what is it and how do we use it in the garden?
(This one's for you, Jim!)
What is perlite?
Perlite is actually a form of obsidian. Obsidian is a dark volcanic glass that forms when lava cools quickly. Obsidian is very brittle and extremely sharp. It was commonly used to make cutting and hunting tools by primitive peoples. [Back in the early 80’s, I had an archeology professor at Seattle Central Community College. He loved to tell us how, when he needed chest surgery, he found a surgeon who was willing to use obsidian tools. Being sharper than surgical scalpels, the obsidian left a scar that was practically invisible!]
How perlite is made
Coming out of a volcano, obsidian is less than 1% water. As it comes into contact with rain and groundwater, it starts to absorb moisture. This hydrated obsidian is mined and then baked in a 3,000°F oven where it pops like kernels of popcorn, growing to twenty times its original size! These glassy kernels look more like a froth of bubbles, but the outer bubbles are broken, leaving jagged edges.
How perlite is used
Perlite has many industrial uses, including insulation, mortar, plaster, and ceiling tiles. The broken glass bubbles of horticultural perlite are used to aerate soil, increasing porosity. Perlite does this by increasing the number of macropores and micropores that carry and hold air and water for plant roots. Perlite can hold 3 to 4 times its weight in water. Perlite is also used in hydroponic garden systems as a filtration medium. (Did you know that perlite is used to filter beer?) Perlite is also found in many planting mixes. You will see them as small white chunks. Sometimes these white particles are pumice, like the volcanic foot scrubbing stone you buy at the drugstore. Sometimes those white bits are styrofoam. Both perlite and pumice improve soil structure, but styrofoam does not. If you are going to use perlite to improve heavy clay soil, don’t just pour it on top. You will need to dig it in a bit, otherwise you will simply end up with a layer of perlite on top of your clay.
Perlite is an excellent soil additive for roof gardens, balcony plants, and extra large planting containers, because it is so lightweight. Perlite is sterile, inert, and incombustible. Mold and mildew won’t grow on it and pests won’t eat it. Adding perlite to your soil can lighten heavy clay and it can help sandy soils retain more water and nutrients.
We may not need Noah just yet, but many areas are prone to winter and spring flooding.
Years with El Nino events can bring severe rain and flash floods, wreaking havoc with homes, drainage, and the garden. After making sure that your family and home are safe, it is important to protect your landscape and garden from the negative effects of flooding.
Rain and soil
Too much rain at one time can cause mountain creeks and streams to overrun their boundaries, carrying debris, mud, and even more water crashing down into already soggy bottom lands. Soil is an amazing structure, but the bedrock that holds it in place also creates a water barrier that can lead to pooling, flooding and more mud than your landscape can handle.
As we have discussed earlier, permeability refers to the ability of water to drain. Our heavy clay soil does not drain well, which makes it great at holding on to water during the dry months, but creates significant problems when rainfall rates overrun carrying capacity.
Flooding and standing water can drown your plants. Roots need air space to breath and to conduct photosynthesis. Standing water and poor drainage also encourages the development of fungal infestations, mosquito breeding grounds, and disease-carrying pests such as fungus gnats.
Just as over-watering causes leaching of nutrients, salts, and chemicals, flooding can wash away valuable topsoil and pollute local groundwater. When you notice standing water in your garden, it is time to take action.
Floods can be devastating, but you can reduce the negative impact with these simple steps. Keep yourself and your garden healthy and safe!
No, I’m not talking about Medieval medical practices!
Clay soil holds far more water than sandy soils, but every soil has a holding capacity. Once that limit is reached, gravity will pull the water downward into underground waterways where it will ultimately flow to lakes and oceans. As it flows away, the water carries nitrogen, salts, fertilizer, pesticides, fungicides and whatever else was in your soil with it - leading to a potentially dangerous chemical soup that can wreak environmental havoc. Leaching also moves valuable nutrients out of reach from your plants roots.
To avoid leaching, it is always a good idea to water only as much as is needed. You can see for yourself where your irrigation water is going simply by inserting a moisture meter next to the plants you intended to water (but not too close). Many people are surprised to discover that the water intended for their tomato plants actually went in another direction due to hardpan, sandy pockets, or poor soil structure. Improving soil structure with compost can improve drainage and help prevent leaching.
You can grow a surprising amount of food in your own yard. Ask me how!
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