pH: do you really care?

We can look up lists of plants recommended by local experts but if we want to get the best performance from our plants, and especially from the more sensitive plants, then we probably need to know more about pH. pH in soil depends on parent-rock, climate, topography, biological activity, human interference, and time, and affects soil properties and root function.

Chemically, pH represents the free hydrogen ion (H⁺) concentration in the water within soil. Extremely acid soil has a pH of 1, and the range for most soil solutions is between 2 and 10.5, beyond which protoplasm becomes damaged. Normally humid region soils have a pH between 5.0 and 7.1, arid regions from 6.9 to 9.0. At the acidic extreme, aluminium, manganese and iron can be toxic, and at the alkaline extreme, iron, manganese and phosphate become unavailable. A pH of 7, as in distilled water, indicates a concentration of 10^-7 hydrogen ions in moles per litre (and 10^-7 hydroxyl ions/litre). Maximum pH (minimum pOH) on this logarithmic scale is 14, so, for example, water with a pH of 6 is 10 times as acid as water with a pH of 7, a huge difference  (would that be one spoon of sugar, or ten?).

Experts give pH values of 6.0 (slightly acid) to a high of 7.5 (slightly alkali) for ideal conditions for perennials but this value must depend on the plant group studied: lawn grasses and vegetables require similar chemistry. At this acidity, essential nutrients are adequate for plant growth, micronutrients are sufficient, and fungi and bacteria are also present. The essential elements for plants, including nitrogen, phosphate and potash, each have their own specific pH at which they are most readily available to plants. A good average is 6.2, considerably more acidic than the typical Calgary soil that has a pH of 8.0! Increased rainfall and higher silica sand in the foothills\92 soils causes the soils become more acidic, and indoor plants enjoy a pH of 5.5 to 6.5.

\93Acid lovers\94 include azaleas and rhododendrons (both of which need iron), lady\92s slipper and blueberries: acid soils reduce the development of potato scab. Conifers like acid soils, and the lack of water below each tree tends to increase the acidity within the drip line. The presence of aluminum produces complex chemistry in clay-rich acid soils. Native plants of course are adjusted to our soils. Dianthus and legumes including perennial peas like an alkaline soil, although alkaline soils tend to bind nutrients and prevent their use by plants. High pH &/or bicarbonate concentrations appear to produce iron and manganese chlorosis.

We need to collect more rain-water in our gardens for financial and chemical reasons. Less alkaline soils would give us better performance. Calgary has an average precipitation of 399mm (15.7\94), i.e. we live in an arid climate in which evaporation and evapotranspiration exceed precipitation. The amount of dissolved salts in our soils is important whether those salts are carbonates (e.g. dolomites and limestones) or sulphates, so electrical conductivity is another useful tool in characterizing soils. Rain-water is a dilute solution of carbonic acid, typically with a pH of 5.65: \93acid\94 rain has more sulphur dioxide content. The pH range for the City of Calgary is 7.06 to 8.37, with calcium and magnesium as two common constituents, and calcium carbonate content up to 263 mg/litre. In 2001, tap water from Bearspaw reservoir was slightly more alkaline (pH 7.2 to 8.3) than Glenmore reservoir with a pH of 7.2 to 8.0, although the former was better in that it had fewer salts. Both the Bow and the Elbow rivers become less alkaline during spring run-off.

Our soil can be acidified by adding humus, acidic peat moss, sphagnum moss, used tea leaves or coffee grounds, rotted conifer bark or wood chips, or by adding pure agricultural sulphur or a little aluminium, iron or calcium sulphate. High nitrogen fertilizers reduce alkalinity Some organic compounds associated with peat have the ability to chelate ions, that is, to pry loose the aluminium and iron from clays and other minerals in solution with pH values at which the ions otherwise would be insoluble. Potting mixtures for planters and pots include dolomite, which neutralizes the acidic peat base of the mixture but in our gardens we normally would not need to add any form of lime. One thing to be aware of is that boulders in rock gardens with alkaline soils should ideally consist of quartz sandstone, granite, gneiss, or \93Rundle Rock\94, rather than limestone, dolomite, or calcareous sandstone that could leach calcium carbonate. A final sentence: keep on composting!

May 14, 2002                                         

Revised July 9, 2002.

Glynn Wright                                                                                  

Further reading:

Curtis, Helen, 1983, Biology.

Tan, Kim H. 1982, Principles of soil chemistry.

Wild, Alan, ed., 1988, Russell\92s Soil conditions & plant growth.

Notes from Curtis, 1983.

Concentration of H ⁺ ions, in moles per litre    

Approximate pH values