If you’re starting to take gardening seriously, you really should have an occasional soil test done. It’s important to understand the soil you garden in, especially just when you’re starting a new flower bed or vegetable garden.
The test will tell you, among other things, whether the soil’s pH is too high or too low (that is, whether it is too alkaline or too acid) and if any vital minerals are missing. By correcting your soil’s flaws before you even begin to start planting, you’ll get much better results. And even if you decide to learn to live with the soil conditions Mother Nature gave you (or, more likely, with the poor quality subsoil left you by the contractor who removed your property’s topsoil during construction); at least you’ll know where you stand and you can then choose plants that will do well under those conditions.
What Do Soil Tests Analyze?
In general, a soil test will include the following elements:
1. Soil type/particle size analysis: is it clay, sand or loam? Does it hold water and minerals or allow them to drain away? Is there enough organic matter? (Normally, there should be at least 5%.)
2. Acidity/alkalinity: the test will measure the soil’s pH to determine its acidity or alkalinity. The pH scale for garden soils ranges from about 4 (very acidic) to a bit above 8 (very alkaline), with 7 being neutral. In general, plants require a slightly acidic pH, from 6.0 to 6.9, to do well, but acid-loving plants (rhododendrons, blueberries, heathers, etc.) prefer soil that is distinctly acid (a ph of 4.5 to 6.0). The test also looks at the buffer pH, a factor that will help determine the amount of amendment you’ll need to add to bring the soil the desired pH.
3. Phosphorus (P) Content: it will measure the amount of phosphoric acid present in the soil. Phosphorus is vital to rooting and to overall plant growth.
4. Potassium (K) Content: the test will determine the available potassium in the soil. Potassium is also vital to plant growth.
5. Secondary and Minor Nutrients: these are minerals that are essential to the plant’s health, but needed in very small quantities, such as calcium, magnesium and iron (the ones usually tested for.) Calcium favors the growth of young roots and promotes ripening. Magnesium plays an important role in the green coloration of the foliage, ensures proper ripening of fruit and promotes the plant’s absorption of phosphorus and nitrogen. Iron promotes the development of darker green leaves and is an essential element for plant growth.
Don’t underestimate the importance of minor nutrients, even if the plant only requires trace levels of some of these minerals: their absence will cause a deficiency and lead to poor growth and reduced yields.
Not Included in the Soil Test
It may seem odd that the laboratory soil tests don’t check the soil’s nitrogen (N) level even though it is the most important mineral plants need to grow well. But nitrogen is also the most volatile mineral and its content rises and falls depending on many factors, including temperature, moisture level, and microbial activity. Soil nitrogen test results are therefore only valuable for a short time and even then, only under the conditions under which the test was made. For that reason, the results of any test will always recommend the addition of some quantity of nitrogen, but by using other criteria to estimate the situation in your garden rather than actually testing for nitrogen.
Where to Go for a Soil Test
In the United States, you can probably have your soil tested via your local county extension service. In Canada and most other countries, contact a garden center. Even many hardware stores offer a soil test service. Note, though, that you won’t find a small laboratory in the center of the store in full view of everyone. Instead, you have to ask an employee about the service.
In general, you will be given a bag or container in which to put your sample and a form to be completed, including what you intend to grow in the garden whose soil you want analyzed.
Then simply take the kit home, take soil samples as recommended below, and follow instructions on how to submit them. (Sometimes you bring the samples in, other times you mail them.)
What About Home Soil Test Kits?
You’ll see these in your garden center, in box stores and on-line: little soil test kits looking much like test kits for swimming pools. You simply add a bit of soil and water to a test tube, add the chemical, shake, wait for particles to settle and then compare the color of the resulting solution to a chart of some sort.
However, gardens are much more complex than swimming pool water and, quite frankly, these tests are not going to be of much use.
First, they’re hard to interpret: if your color vision isn’t excellent, you may not be able to tell the difference between light vomit green and medium vomit green.
And what do the results mean? They may tell you that there is a problem, not how to fix it.
Say you learn your soil is too acidic. What do you do about it? After all, different soils react differently to different amendments, so which one should you be using? How much should you apply? And how often? A laboratory soil test will tell you exactly how much of which product to apply and even suggest the frequency of applications (for example, so much lime per year for 2 years or 3 years) so you can change the pH slowly and not harm the soil’s beneficial microbial flora.
Home soil test kits leave you struggling to figure out what to do. Even a soil expert would have a hard time working with them! I suggest avoiding them!
When to Do a Soil Test?
There is no specific season for having soil tested. You can do it at any time of the year … at least, when the ground is not frozen or sopping wet. Most gardeners have theirs done in the spring, just before the growing season starts. Autumn, however, is perhaps the best time: if you need to apply lime, which is a very slow-acting product, a fall application will give it time to work all winter so the soil will be showing results by spring.
If you have recently limed your yard or garden, it would be better to wait 5 or 6 months before doing the test. And wait a few weeks after applying fertilizer or compost as well. By then, the soil will have regained its balance and therefore the test will more accurate results.
Taking a Sample
Each test should be of one garden only, whether a vegetable garden, a lawn, or a flower-bed, as the recommended treatments will vary depending on the plants grown. If you have a special garden with soil needs outside of the normal range, for example a rhododendron garden or a potato bed, a separate test will be necessary. Serious gardeners will often do one test a year, each time in a different garden, starting with the most urgent (a lawn in poor shape, for example) to spread the cost of testing and treatments over time.
Before you begin, thoroughly wash your sampling tool (professionals use a core drill, but a garden trowel is the home gardener’s usual choice), rinsing well to remove all traces of soap. Take the equivalent of a few large tablespoonfuls about 6 to 12 inches (15 to 30 cm) deep, in the root zone. Do not use surface soil: it often contains elements that can skew the results.
Pour each sample into the test bag or container, then take other samples at different locations of the same garden (vegetable garden, lawn, flower bed, etc.) for a total of about six samples (more if the garden is very large): this will ensure a more comprehensive picture of the situation than a single sample.
If necessary, remove any stones, sticks, etc. from the sample, wearing gloves (do not touch the sample with your hands, as they can contaminate the soil and affect the results), then blend well. Each test requires about 1 cup (250 ml) of soil; if you have too much, remove the surplus.
It’s also very important to fill in the form that accompanies the kit. First, to identify yourself so that the results get back to you, but also so the lab knows what you want to do with the garden. That’s because there will be different recommendations for a lawn, a flower-bed, a vegetable garden or even a space that will be dedicated to a specific group of plants.
Now return the sample. You can normally expect to learn the results in about two weeks. The cost? It may be free in the US! The going rate in Canada in 2019 seems to be about $25.
Expect very detailed results with specific recommendations, like “apply product X at such a dose and such a frequency.” In high-rainfall areas, soils tend to be acidic, so there may be lime to apply; in low-rainfall areas, soils tend to be alkaline, so sulfur will likely be recommended. In either case, there may well be fertilizers and organic amendments such as compost to apply. Don’t be surprised to discover that the recommended treatment for your lawn is not the same as for the flower bed or vegetable garden, because each of these environments has different soil needs.
Every garden really should be tested every 4 or 5 years. That way if there are any changes occurring (for example, the soil is starting to return to its original pH), you’ll be aware of them before the plants begin to suffer. You’ll also be forewarned if your soil’s mineral supply is becoming exhausted.
For a vegetable garden, test the soil even more often, every 2 or 3 years, because you won’t want to skimp on the quality of its soil, not if you want vegetables of the highest quality!
The average home gardener can follow the above rule for his lawn (a test every 4 to 5 years), but then, most gardeners are content with a lawn that is green and fairly weed free. If you want a perfect golf green type lawn, though, biannual or annual testing may be required. When you’re a lawn nut, nothing is too good for your personal green space!
Having your soil tested? It’s what you do when you want the very best results in your garden!