When it comes to soil tests, there is a frightening amount of inaccurate and outdated information online. So be very careful before trusting a youtube video or blog post about this subject.
But soil tests are essential if you want to understand the health of your garden. The problems that we see with the naked eye tell us that something isn’t right below the ground. Soil tests will get us to the root of our problems. This means we can treat causes rather than symptoms of stressed plants.
I’m currently training to be a soil technician, so I spend most of my time reading the latest peer-reviewed articles on soil health or studying microorganisms beneath my microscope.
I’m going to share what I’ve learned about the different types of soil tests so far, so you know which ones are actually worth your time, money, and energy.
The Importance Of Soil Tests
You can’t regenerate your soil without soil tests. It’s like if you went to the doctor with a terrible rash all over your body. He or she might prescribe you all kinds of different medications, but your rash hasn’t gone away.
The only way for that doctor to find out what’s wrong with you is to run more tests. If they don’t, they will keep guessing at cures, but your problems are going to worsen until someone gets to the bottom of your problems.
What Does a Soil Test Show?
Different soil tests can show different things. Here are some examples of what soil tests can be checking for:
- Biology (bacteria, fungi, protozoa, nematodes etc).
- Nutrients (e.g., phosphorus, nitrogen, potash)
- Soil type (clay, sand, silt, or loam)
- Contaminants (you might live near a nuclear power station and want to check for radiation).
- pH (how acidic or alkaline is your soil)
None of the soil tests are fundamentally better or worse than the other. It depends on what you need to find out. But there are some misconceptions about soil testing, particularly when it comes to testing for nutrients.
Benefits of a Soil Test
Talking of those misconceptions, let me explain the benefits and limitations of biological and nutrient testing. Hopefully, this will clear up any confusion.
By testing for biology, you are looking at the root of your problems rather than the symptoms. This is because a healthy community of microorganisms is the key to plant health.
Bacteria and fungi form a symbiotic relationship with plants. They cycle nutrients and form a shield to stop pests and pathogens from getting to your plant.
Fungi can be sent off to collect water for the plant that it is living with. Good bacteria can also shift the pH in your soil and eat away at compaction.
By testing for biology instead of pH or nutrient levels, you would be zooming out on the problems your garden is facing.
There is no way to know what biology is living in your soil without getting a shadowing microscopy test. These cost around $100 to $125, at least in my neck of the woods.
Nutrient testing does have a place in the world.
The Melich 3 is one of the most comprehensive nutrient tests because it will tell you about different micro and macronutrients and the cation exchange capacity of the soil (The cation exchange refers to how readily your soil can hold on to nutrients).
Then you’ve got more specific tests, such as the Bray P1 or Olsen Sodium Bicarbonate test. Both of these test for phosphorus, but for different soil pH.
The benefit of these tests is they can tell you what nutrients you’re lacking. So say you’re halfway through the growing season and your plants are all dying, you could find out what nutrients are missing and try and do some fire fighting with fertilizers.
This means you can save the crop you’ve worked hard for, but it’s not a long-term solution. (More on that soon!)
Soil Test Limitations
When testing for microorganisms with a shadowing microscope, the test will only tell you about the functional group of organisms present.
For example, it can tell you that you have root-feeding nematodes, but it won’t tell you which species and genus of root feeder you’re dealing with.
In practical terms, this doesn’t make much difference to you. Because by restoring a healthy community of microorganisms, your beneficial nematodes will outcompete the root feeder either way.
Another limitation is that the biology can change in the soil sample on the way to the lab. Healthy soil needs plenty of oxygen. If your soil sample takes a few days to get to the lab, it can run out of oxygen on the way, and the beneficial organisms can die.
So looking at the microscope, your technician will think your soil is much less healthy than it really is.
You need to make sure you leave plenty of air space in your shipping container to account for this.
People love to test for nitrogen and phosphorus, then tell you that your soil is depleted and you have to buy nutrients. But this is quite outdated advice.
The truth is, plants need 16 different types of nutrients. So if you dump a load of nitrogen on your garden, is that going to help? What about zinc? The boron? The magnesium? The calcium?
It’s not feasible to keep spreading all 16 essential nutrients on your land. Besides, applying nitrogen fertilizers selects for weeds.
This is because weeds love nitrates (the type you get in fertilizers) whereas vegetables and fruits do much better with nitrogen in the form of ammonium (the type that microorganisms fix in the soil for you).
Types of Soil Tests
I’ve already given you a bit of background on soil tests, but I’ve put together a table to help you understand a little more about the process.
Options for the Home Gardener
If you want to test soil for yourself, you’ve got a few options.
You can get home testing kits to check for nutrients or pH. These are not expensive, and it can be interesting to know what kind of nutrients you’ve got in the soil.
Just remember that nutrient deficiencies and pH changes are a symptom of imbalanced biology. So you can treat the symptom by adding fertilizers, but it won’t address the cause.
You can test for compaction using a penetrometer. They aren’t crazily expensive (here’, but you might want to go in with some friends because it’s a bit excessive to have one for every single yard.
You just push them against the ground, and the pressure gauge will tell you what the compaction level is.
How Much Is A Soil Test Kit?
The price of a soil test kit varies. You can expect to buy a home pH testing kit from around $10 and a home nutrient kit from $20 up. Kits that test for a broader range of nutrients are going to be more expensive.
How To Use a Soil Test Kit
Each soil test kit will come with specific instructions, but here’s a general idea of how to use a soil test kit.
With a pH test, you mix a small amount of soil with the soil testing solution. It might come in liquid form, or you might get a powder that you have to mix with water first.
After you’ve mixed your soil and solution, it will turn a particular color. You compare the color to the swatch to find out the pH of that soil. The process is similar for most home nutrient tests.
Remember that you need to scrape the first couple of inches from the top of the ground when you take your soil sample. This will remove the leaf litter or other organic matter that is not an accurate representation of your soil.
Are Home Soil Test Kits Accurate?
The accuracy of a soil test kit depends on the brand. A study by the American Society for Horticultural Science showed that home soil kits varied from 33% to 94% accurate.
The best performing brands were La Motte and Rapitest. Nitty-Gritty and Soil Kit performed poorly.
Best Home Soil Test Kits
If you’re going to get a home soil test kit, this is the one that I recommend.
Rapitest soil kits performed well in the study done by the American Society for Horticultural Science (the tests were up to 92% accurate!).
This kit will let you test for pH and three of the essential nutrients for plants: nitrogen, potash, and potassium. It is helpful to know what’s going on with these nutrients, but don’t forget they are only 3 of 16 nutrients that plants need.
Working out the nutrient levels is a good and affordable clue about your overall soil health. If everything is looking good without the use of fertilizers, then you don’t need to worry.
But if your soil can’t cycle nutrients by itself, you might want to test the biology.
How Often Soil Should Be Tested
I would test my soil at least once a year.
For example, I would use a nutrient test a couple of weeks before I started planting my first seedlings. This will be a good clue as to whether I need to fork out for a soil biology test.
If you can afford it, I would test for biology once a year. The tests are more expensive than soil home tests, though.
You could save biology tests for when something seems wrong, but you will need to do a follow-up after you’ve taken steps to restore biology to make sure your plan really worked.
When Is the Best Time to Test Your Soil?
For biological testing, you don’t want to be testing in the cold months. Many microorganisms go dormant or migrate deep into the earth in the winter, so you wouldn’t be getting a realistic picture of what’s going on.
You might like to test for nutrients before you start spring planting, so your readings aren’t getting influenced by any amendments.
Understanding Soil Test Results
Beware of oversimplification of soil test results. Getting an acidic pH reading doesn’t necessarily mean you need to dump a load of lime on your garden.
A low nitrogen reading doesn’t mean you need to get inorganic fertilizer down immediately. Compaction doesn’t mean you should grab the rotavator (actually, that will make it worse!).
So if you are having persistent problems, I recommend you speak with a soil consultant or permaculture expert. If possible, try and find an impartial consultant who isn’t doubling as a salesperson for any amendment companies.
If that’s too hard to find, at least make sure you read up on soil health, so you’re not going into the process vulnerable (reading list at the end of the article).
How To Test Your Soil To Determine Soil Type
You can test your soil type by yourself, and you don’t need fancy equipment.
- Take a glass jar or plastic bottle and fill it one-third with soil.
- Top it up with water, then shake hard until the earth and the water are well mixed.
- If you place the jar on a surface and let it settle, you’ll see three distinct layers.
The bottom layer will be sand, the next up will be silt, and the top layer will be clay.
You can now measure them all to work out the percentage you’ve got of each.
Armed with your ratios, you can work out your soil texture by referring to the soil texture pyramid.
How To Tell If Your Soil Is Acidic or Alkaline
The best way to tell if your soil is acidic or alkaline is to use a soil pH test.
Make sure you take samples from a few different places in your yard because you’ll find different pockets of soil pH depending on the biology or amendments in each tiny soil aggregate.
Best Soil pH Test
I recommend this pH test because it’s accurate, affordable, and easy to use. Most pH test kits are pretty much the same, though. So don’t worry if you bought a different one already, just go with what you have!
I wish I could say that soil testing was as simple as “pop your soil in a solution, and all your problems are solved”. The truth is, soil testing is a science that is constantly evolving, and we can’t fully understand such a complex, living medium with a $10 kit.
But the fact that you read this article means you already have a better understanding of soil health than 99% of the population.
So don’t feel disheartened by what you don’t know, but feel inspired that you’re leading the charge on soil health and testing.
No matter what soil testing route you go down, I hope you have a lot of fun in the process!
- Soil Biology Primer, Dr. Elaine Ingham
- Teaming With Fungi, Jeff Lowenfels
- Teaming With Microbes, Jeff Lowenfels
- Mycorrhizal Planet: How Symbiotic Fungi Work with Roots to Support Plant Health and Build Soil Fertility, Micheal Phillips
- Growing a Revolution: Bringing Our Soil Back to Life, David Montgomery
- No Dig Gardening, Charles Dowding
- Gaia’s Garden: A Guide to Home-Scale Permaculture, Toby Hemenway
- Soil Science for Gardeners, Robert Pavlis