The pH scale measures the hydrogen-ion concentration in a given substance from 0 to 14. With a device for measuring this concentration, we can tell how “sour” or “sweet” a given soil may be.
Why is pH important? First, it tells us what chemical nutrients can be available to plant if they are present. A pH test does not tell you, for example, that phosphorus is present, but only if the soil is not too acid or too alkaline to allow phosphate to be released.
The point at which all elements are available to plants that do not require highly acid soil has its center at pH 6.5. On the pH scale, 7.0 is considered neutral. Thus somewhere between 6.0 and 7.0 is perhaps the best point to strive for in altering soil acidity.
Acidity decreases the availability of elements directly and reduces the activity of soil bacteria until at 4.0 such activity might cease entirely. Since organic matter is available to plants only after bacteria have digested it first, the effect on growth is obvious. Another effect of pH is the release of toxic elements such as aluminum, which is released at a low pH (below 5.5) and also at a high pH (8.5). The prevalence of disease is also affected by pH, both by acid and alkaline soils. Scab on potatoes occurs only at a pH above 7.0
In developing your soil into a suitable planting area, it is important to know its pH, which is the indication of the acidity or alkalinity of a given soil. Many plants such as broadleaf evergreens and Azaleas must have low pH soil or soil that is quite acid, while other plants like lilacs, peonies, and many kinds of grass need higher pH soils to grow properly.
Raising the pH with Lime
Lime is an extremely important ingredient in gardening. Low pH soils may need Lime to raise the pH and to supply calcium to some plants. It also contributes to the building of the clay particles, giving better texture to heavy clay areas. Liming the soil is a yearly ritual for many gardeners because this addition helps vegetable gardens and lawns immeasurably. But not all plants or garden areas should be limed. Azaleas, Camellias, Rhododendrons, hollies, laurels, boxwood, centipede grass, and other broadleaf evergreens need acidic soil. Before liming any areas containing these types of plants, do a pH test.
The form of Lime which gardeners use is agricultural limestone, a very finely-ground limestone. Pelletized limestone is another form of Lime. It has the advantage of being faster-acting and easier to apply, especially when using a fertilizer distributor.
Occasionally, hydrated Lime is found in garden shops and nurseries. Much faster-acting than ground limestone, this lime form is often used in preparation for planting tomatoes, which need a quickly-available source of calcium, or plants like clematis and lilacs which need a rapid change in the pH.
Never use builder’s Lime, which may be called burnt or slaked Lime. This form absorbs water rapidly, producing high amounts of heat which will burn the roots of plants.
When applying Lime, remember that agricultural limestone is really finely-ground rock and is slow-acting. Plan ahead when using agricultural limestone, applying it well ahead of the time of planning. Apply hydrated Lime when you need the calcium or a pH change in a hurry.
Calcium may also be added to garden areas by using bone meal which as rapid-acting and also has some nitrate nitrogen.
Correcting High pH
Occasionally soils may be too alkaline to grow many of the acid-loving plants which are such an important of landscape material. Many of these plants grow best when the pH is below 6.5. Higher pH soils may be treated with aluminum sulfate to lower the pH. The use of acid-forming fertilizers will also help to lower the pH and keep it in the right range for these acid-loving plants.
High pH conditions are often discovered when acid-loving plants are chlorotic, a condition indicated by yellowing between the veins of the leaves. Chlorosis is most often due to a lack of iron in the plant. Soil which is too alkaline has the normally available iron bound in compounds which are not usable by the plants. These plants should be fertilized with an acid-forming fertilizer or the soil treated with aluminum sulfate, both of which will reduce the pH and free the iron for use.
You may correct the immediate problem of chlorosis by the use of an iron compound applied as a spray or as a drench around the roots. However, this is a temporary solution, and if the chlorosis returns, undertake the more permanent solution of lowering the soil pH with acid-forming fertilizers or aluminum sulfate.
Of the vast majority of plants grown, comparatively few show made preferences for decidedly acid or alkaline soils. The vast majority exhibit a wide tolerance.
Plants That Grow Well on Acid Soils
Andromeda, Arbutus, Azalea, Beans, Bent Grasses, White Birch, Blackberry, Buckwheat, Camellia, White cedar, Cineraria, Cowpea, corn, Cranberry, cucumber, Cyclamen, Daffodil, dewberry, Endive, Ferns, Fir, Hemlock, Huckleberry, Hydrangea (Blue), Lily, Lupine, Millet, Mountain-laurel, Scrub Oak, Orchard Grass, Orchid, Parsley, Pea, Peanut, Pepper, Pine, Potato, Pumpkin, Radish, Raspberry, red-cedar, Rhododendron, Rye, Skimmia, soybean, Spruce, Strawberry, Tobacco, Turnip, Hairy Vetch, Violet, Watermelon, Wintergreen, Wheat, Zinnia.