A Beginner's Guide to Soil PH

PH is the measure of a soil's acidity or alkalinity, in chemical terms, the presence and type of certain hydrogen bonds. PH directly affects how plants use important nutrients, and whether they will grow at all. Some extreme conditions of alkalinity, like the Borax filled deserts of America's Death Valley or extremes of acidity such as lakes filled with acid rain make growing nearly anything a challenge.

Fortunately, your garden is neither a desert nor a bog, so minor adjustments will probably be all that is necessary. Most plants grow best at a neutral PH, that is to say a PH of 7 on the scale of 0 to 14, the PH of pure distilled water. Most rainwater will be somewhat acidic since as it passes through the atmosphere it picks up carbon dioxide to form a mildly acidic substance called carbonic acid. In extreme cases, large amounts of carbon in the air such as those created by burning fossil fuels leads to so-called, acid rain, the condition that has been killing fish in many formerly pristine mountain lakes and eating ancient marble statuary.

Gardeners may have their soil professionally tested for PH and mineral content or use one of many home testing kits. However, the educated eye can make a reasonable guess about the probable soil PH of a given area.

For example: How wet or dry is your area? If you are planning to xeroscape your land and plant native cacti, you probably are not surprised to learn that your dry land is somewhat alkali. That is to be expected in desert areas where rainfall is scarce. No water means little carbonic acid from rainfall. Additionally, the presence of certain underlying minerals such as limestone or borax, both of which are highly alkali and common in desert areas will add to the alkalinity. This soil will have a PH of greater than 7.

On the other hand, maybe your land is waterlogged. In that case, expect a lower PH of under 7. This low PH combined with water; means you may want to look at plants that like boggy ground, either that, or improve drainage before planting. Soil amendments that improve texture and aeration will be necessary. PH alone is not your sole concern. Lack of oxygen and the presence of plant pathogens that just love the moisture will be the primary concern here. Naturally-occurring peat in a true bog will significantly lower PH as will decaying leaves of many kinds.

How about an acid condition that is not really so extreme? Many plants, such as blueberries, azaleas and rhododendrons love mildly acid conditions native to their homes. For example, in Long Island, the native rhododendrons share space with oaks and scrub pines whose leaves and needles further enhance the acidity that the plants love. When these plants are cultivated in gardens with neutral soils, additional acid is required so that they can use the iron that they need to grow. A very famous example of a commercial feed for such plants is Miracid. Gardeners have been known to add iron to such soils by burying nails and the like, but Miracid is cheaper and less painful than a tetanus shot.

What about neutral soils? In moderately watered areas where organic amendments like decaying leaves or needles are counteracted by alkali minerals like limestone, the soil PH will be very close to 7 or that of distilled water. This is the ideal soil for most cultivated plants. One hint that you have neutral soil is the color of your hydrangea bushes. In a neutral PH they will look lavender. More acid soil gives them a blue color while alkali soil tints them pink.

Try to work with nature in your plantings. Remember, PH is just one of a complex of things that affect your plant's growth. Yes, you can change the PH, but can you also change the climate, natural rainfall, chemical composition of the soil, sunlight and other factors? The closer you work with the natural conditions of your area, the greater the success and lower the expense.

Before purchasing expensive foundation plantings, drop by your nearest botanical garden. Many of them have demonstration plots where you can see potential additions and judge their hardiness, appearance and mature size before committing. Don't be afraid to speak to experienced nurserymen in your area to determine what has worked well for neighbors.

If, after assessing your conditions and choosing your plants, you decide that a minor tweak of PH is in order, here is a list of some common soil amendments to change your PH:

To make soil more acid: oak leaves, pine needles, coffee grounds, peat moss, Miracid fertilizer.

To increase alkalinity: egg shells, lime, wood ash, ammonia

Don't go overboard, and remember to use organic materials as well as chemical fertilizers when adjusting PH since the overuse of chemicals combined with lack of composed material will eventually deplete the soil of nutrients, reduce aeration and drainage and kill good microorganisms.

Since many of the substances listed above are also valuable sources of the major plant nutrients such as Nitrogen, Potassium and Potash you are probably already using them. Just be aware that you might want to keep the alkali-rich wood ash away from your acid-loving blueberries and rhododendrons. Don't overuse leaves, coffee grounds or peat moss for acidity unless you are planting a bog.

Sources:

http://www.nps.gov/history/history/online_books/deva/section3e1.htm

http://www.elmhurst.edu/~chm/vchembook/184ph.html

http://www.hort.purdue.edu/ext/woodash.html