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.