Alkaline Soils

As I attempt to dig planting holes on my property at this time of year I am reminded once again of just how poor my soil is - at least "poor'' in comparison with those beautiful rich, fluffy loams I see in gardening magazines and on television. This prompted me to see what I could dig up about soils on the Internet. My search was rewarded with an excellent Web site maintained by the Cooperative Extension of Colorado State University (http: //www. coIostate.edu/Depts/CoopExt/PUBS/pubsmenu.html.)

Water, as most of us know, is made up of hydrogen and oxygen in a ratio of two parts hydrogen to one part oxygen (hence the famous formula H2O). At any particular time in a container of pure water, most of the water molecules remain bonded together while a very tiny number break apart into free hydrogen ions (H+) and free hydroxyl ions (OH-). Normally there are equal concentrations (about one part in ten million) of each of the hydrogen and hydroxyl ions. A solution with equal numbers of both ions is said to be neutral, but if something upsets the balance and selectively removes either hydroxyl or hydrogen ions, the solution is said to become either acidic or basic (alkaline). Acid solutions have a larger concentration of free hydrogen ions, and alkaline solutions have a larger concentration of free hydroxyl ions.

The relative acidity or alkalinity of a solution is expressed as pH, which is a measure of the relative number of free hydrogen ions. The pH scale is a little tricky because it's inversely proportional to the concentration of hydrogen ions, that is, a higher pH number indicates fewer rather than more hydrogen ions. (For the technically inclined, pH is the negative of the base 10 logarithm of the reciprocal of the concentration of free hydrogen ions). A solution with a pH of 7 is said to be neutral. Solutions with pH values higher than 7 are alkaline, and those with pH values less than 7 are acidic. Another twist to the pH scale is that each unit value on the scale indicates a tenfold increase or decrease in the number of free hydrogen ions. For example a solution with a pH of 6 is ten times more acidic than a solution with a pH of 7, and one with a pH of 5 is ten times more acidic than a solution with a pH of 6 and 100 times more acidic than a solution with a pH of 7.

The details of the chemical reactions that determine the pH of soil are complicated, but generally where there is more than about 20 inches of rainfall in a year, the soils tend to be acidic. Where there is less than 20 inches of rainfall in a year, such as here in the High Desert, the soils tend to be alkaline. This is important to the gardener because certain plants - especially those that are most familiar to people who grew up in the Eastern United States where the soils tend to be acidic - are adapted to grow in acidic soils and will not do well in alkaline soils.

Gardeners trying to grow plants in the alkaline soils of Cochise County can pursue two possible strategies. The first is to attempt to alter the pH of their soils by the use of amendments such as sulfur so that they can grow the acid-loving plants they remember from another place. The second is to go native and grow only those plants that are adapted to our alkaline soils. Although attempts to lower the pH of soils can be successful in the short term, they require the repeated addition of huge quantities of amendments to maintain even a small change in pH. Ultimately the gardener will lose to Nature. The better strategy is to leave the soil pH alone and plant only those plants that are adapted to the soils of our area.