The pHacts on pH

The pHacts on pH

By Jim Hole

Whenever a discussion about the effects of soil pH on plants comes up, I'm willing to bet that many gardeners are a little mystified by the issue. Almost everyone knows that pH has some kind of impact on plants, but by and large it seems that people are still unclear on the specifics.

Soil pH really isn't that complicated. It's simply a number that represents the degree of acidity or alkalinity (also called basicity) of the soil. pH scores run from 0 (very acidic) to 14 (very alkaline or basic), with 7 being neutral. (Distilled water has a pH of 7). In the real world, soil pH values are never as low as 0 or as high as 14.

My Cup of Tea

To visualize how pH works, I like to use the tea analogy. Have you ever noticed how quickly and easily sugar dissolves in hot tea versus how slowly it dissolves in ice tea? Soil with a high pH is like cold tea; soil with low pH is like hot tea.

Most, but not all, nutrients are affected by pH as follows: at a very high pH, insufficient nutrients are dissolved for the plant roots to absorb, and the plants suffer. In other words, the tea isn't sweet enough. At a very low pH, many nutrients and some toxic metals are too soluble in the soil—so soluble, in fact, that they may be a little bit too available to the plants, resulting in injury. In this case, the tea is too sweet.

Ideally, your cup of tea should be neither too sweet nor too bland—you want a pH that's "just right." For most garden plants, a soil pH between 6.2-6.8 is ideal. (Some plants, of course, do like the extremes: acidic for azaleas, rhododendrons, and blueberries versus basic for gypsophila, Russian olive, etc.).

When soil pH falls a little outside the desired range for your plants, it can be adjusted. Sulphur, sphagnum peat, and aluminum sulphate lower soil pH, whereas limestone will raise it. Often, garden soils are in the correct pH range, but the only way to be certain is to have them tested. Soil pH test kits can be found at most garden centres, though they vary in quality. I've found the cheap $10 pH meters to be essentially useless. Reasonably good-quality pH meters start at around $50. A mid-range alternative is one of the kits that involve pouring powders into test tubes and using colour as an indicator of pH value. These are, generally speaking, quite good. The kits aren't as precise as a quality meter, but they give a reasonably accurate reading. Soil pH isn't really that complex when you take a closer look. A little basic knowledge will help
you pass the acid test.


pH is an abbreviation derived from the French phrase "pouvoir hydrogène", or the power of hydrogen. It refers to the concentration of hydrogen ions in a solution. pH values are on a negative logarithmic scale. What that means is that the difference between pH 5 and pH 6 is larger than it first appears; 5 is 100 times more acidic than 6; and, going further, 4, is 1000 times more acidic than 6. So if your plants happen to require a soil pH of 6, 5 definitely isn't close enough.

Soil pH has a tremendous effect on soil microorganisms. Some soil microorganisms work hard to convert compost , manure, and other organic matter into usable nutrients for your plants. Such organisms can't survive in extremely acidic soil, leaving your plants with few nutrients to absorb.

Taking a Proper Soil Sample

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.