Taking a Proper Soil Sample: Advice and Techniques
by Jim Hole
Good soil is the foundation of a good garden. Without the right balance of nutrients and the right composition for root penetration and water and air movement, plants will never reach their full potential. So when plants experience problems, it makes sense to take a closer look at the soil. A soil test can be a useful tool to determine whether or not your soil has what it takes to support lush, healthy plants. Accurate results, however, demand an accurate soil sample.
To Test or Not to Test
How do you determine whether or not your soil should be tested? Well, if your plants look healthy, a soil test is probably unwarranted. But if you’ve had problems growing plants and you’ve exhausted all possible causes – disease, insect attacks, substandard plant varieties and poor maintenance, a soil test should be your next step.
Getting a Good Sample
A soil test is only as good as the soil sample tested. At least, that’s the principle you should adhere to when you test the soil in your yard.
First, divide your garden into zones. For example, if you have a rock garden with sandy soil, a woodland area in one corner of the yard, and a loamy vegetable garden along the back fence, you should take a soil sample from each of these distinct zones.
Even within zones there can be irregularities, such as a child’s sandbox or a bog that’s restricted to a very small portion of the yard. Avoid taking samples from these spots; they don’t really represent the soil your plants will be using for sustenance, so any results collected from these spots will be extraneous and misleading.
Once you’ve picked out the sampling zones and made note of any irregularities, the next step is to collect enough soil from different parts of the zone to get a truly representative sample. If you have, say, a 6 x 6 metre vegetable patch, I would take at least a half-dozen samples from throughout the patch. Taking only one sample from a zone increases the risk that the sample you acquired was an anomaly, a localized patch of extremely good or bad soil rather than truly representative of the zone. I use a garden spade to collect the soil. Digging up a 20-cm deep profile of soil will do the trick in most yards.
I like to obtain about a half a coffee can’s worth of soil from each zone. Even though this is more than you really need for a soil test, it’s easier to blend together and it’s no harder to obtain than, say a half a cup’s worth. However, any clean container, such as an unused sandwich bag, will do.
When to Sample
Soil samples should be taken when the soil is relatively pristine – that is, not immediately after being heavily composted or fertilized. Soil amendments will give a distorted and unrealistic picture of what caused any plant problems in the first place. Sampling just prior to planting, or as soon as a problem is noticed (as long as you haven’t just amended the soil), will provide the most accurate results.
Garbage In, Garbage Out
Like any scientific test, accurate results absolutely depend upon good data from the field. When you bring samples of your soil to the lab, do your best to ensure the samples are truly representative of your yard and untainted by outside factors such as recently added soil amendments and contaminants from unclean sample containers. In other words, when testing the soil, make sure that the soil that gets tested is as representative of your trouble spots as possible.
What’s Being Tested?
After collecting soil samples, most gardeners will need to take them to a lab or a well-equipped greenhouse for the actual test. Typically, once the samples are in the hands of technician, he or she will run a series of tests. From a gardener’s perspective, the most useful tests measure the salt content and acidity/alkalinity (pH) of the soil. Salts are typically nutrients and other chemicals present in all soils. Too low a value often indicates a low-nutrient soil, while a high value indicates a salty soil that inevitably leads to burned roots.
The pH values that aren’t in the correct range – generally between 6.0-6.5 – affect the uptake of nutrients by plant roots. Depending on whether the soil is too acidic or too basic (high alkalinity), the plants will, respectively, absorb too much or too little of the nutrients in the soil, leading to nutrient overdose or nutrient starvation (there are exceptions to this rule, but it’s a good general guideline).
Knowing the acidity and salt content of your soil ensures that you’ll make the right decisions when it comes to solving any problems. If your salt content is too high, you must avoid adding any additional fertilizer or rich manure. Irrigating heavily will leach the excess salts deep into the soil and away from the salt- sensitive roots of your plants. Furthermore, various soil amendments are available at garden centres to bring the pH back to the proper range.