The term pH refers to the acidity or alkalinity of a substance. In chemical terms, it measures the hydrogen ion concentration. The pH is rated on a 14 point scale with the midpoint i.e. pH 7.0, being neutral. Values lower than 7.0 are considered acid and those with a number higher than 7.0 are alkaline. (BTW - That term came from the name of the baseball Hall of Fame right fielder for the Detroit Tigers - Al Kaline...Just kidding.)

Anyway, the scale is what is called logarithmic. That means that each unit is not just a one point jump but is rather a 10 times increase or decrease. For example, a pH of 6.0 is ten times more acid that one of 7.0. A pH of 5.0 is 100 times more acid than 7.0 and 4.0 is 1,000 times as acid as 7.0. Wow!

As you can see by the scale, although the differences between numbers seems great, they are actually not too dramatic. Orange juice that we have with our breakfast has a pH around 4.0 which, as we mentioned, is 1,000 times more acid than a neutral substance with a pH of 7.0. That is the difference between drinking pure, distilled water and some good O.J.

So, what does all of this have to do with our plants? Well, each plant has an ideal soil pH range in which it will be able to thrive and do its best. It all has to do with chemistry and the way nutrients are absorbed into the plant roots. For this process to take place, the soil solution (water mixed with the nutrients) has to be at a certain pH level. If it is too high or too low, the transfer from the soil into the plant roots will not take place.

So, if the soil pH is wrong for a specific plant, it will show nutrient deficiency symptoms even though there may be adequate levels of that nutrient in the soil around the roots. The plant will just not be able to absorb it until the pH is changed.

The vast majority of plants that we will be growing in our beds and borders need to have a soil pH in the slightly acid range of between 6.0 to 7.0. Under these conditions, the plant will most efficiently absorb the nutrients it needs for growth and health. The further the pH varies either higher or lower from the ideal range, the more difficult this process becomes for the roots. As you can see on the chart on this page, the nutrients required for plant growth are all highly available in the ideal range for most plants. Many plants can tolerate and survive in a pH outside of their ideal range but they will not thrive and grow to their best ability.

Acid Loving Plants - A few plants that we might like to grow are called "acid loving" plants. Many of these belong to the plant family, Ericaceae and are sometimes called Ericaceous plants. Included in this group are Rhododendrons, azaleas, boxwood, heaths and heathers plus a few others. These, and plants such as pin oaks (Quercus palustris) and blueberries (Vaccinium), require a much lower pH (more acid) soil to thrive. They need a pH in the range of 4.5 to 5.5 to do their best.

A classic example of a pH problem occurs when you try to grow acid loving plants in an alkaline type of soil. The leaves of rhododendrons and pin oaks show dramatic nutrient deficiency symptoms under these circumstances. In summer, the foliage will turn a yellowish green color but the veins of the leaf will be a bright, dark green. This is a symptom of iron deficiency but usually, a soil test will show that there is plenty of iron in the root zone. The problem is that at a pH outside their desired range, these plants simply cannot absorb the iron.

Problems? How do you know if you have a pH problem? Well, you can wait until you see the nutrient deficiency signs but a better approach is to get your soil tested. This will not only tell you the level of your soil acidity but, what to do if you need to change it. We will discuss soil testing in the Preparing the Soil Section later.

As we discussed previously, one of the key differences between plants and animals is that plants make their own food through a process called photosynthesis. In order to complete this process, the plant must have chlorophyll, water, heat, light and nutrients in the proper amounts. If even one of these 5 factors is below a minimum level for that plant species, no growth will take place even if the other 4 factors are at perfect levels.

Related Articles: Soil pH - Acid Soils - Alkaline Soils - Lime