Soil reaction or pH is a measure of the `active' hydrogen ions in the soil water (solution). Water is often thought of being composed of only water molecules (H₂O), but water also consists of hydrogen ions (H⁺) and hydroxyl ions (OH^-).

In this aqueous solution the product of the concentration of hydrogen (H⁺) and hydroxyl (OH^-) ions is about 10 ^-14 micrograms per liter of solution (ug/l). When the concentrations of the two ions are equal as with distilled water (10^-7, ug/l) the solution is neutral and it's pH is 7.

The concept of 10^-6 being larger than 10 ^-8 may be difficult to understand until you write out the corresponding numbers. This is easily done by moving the decimal point the corresponding spaces to the left.

Since pH is based on log 10, each unit change in pH (i.e. pH 7 to pH 6) is a 10 fold change in hydrogen ion concentration. Accordingly any changes made in the soil to affect pH will need to take this into account. If the initial soil reaction is 7.5, as is common with many of the soils in the valley's of Western Colorado, and the desire is to drop the pH to 6.5, a 10 fold change in the pH of that soil must be made. If the desire is to change the soil to a pH of 4.5 to allow the proper growth of blueberry and heather, the soil reaction must be changed by a factor of one thousand (1,000).

The ability of a soil to resist a change in pH is termed its buffer capacity. Usually the greater the clay and/or organic content of a soil the greater will be it's buffer capacity. In the valley areas of Western Colorado, the high clay content and the natural calcium carbonates in the soil greatly influence this buffering effect. Consequently changing the pH of these soils is extremely difficult if not impossible.

The soil's reaction can influence the availability of nutrients. Where an adequate supply of essential nutrients is maintained as is common with greenhouse and pot plants, plant growth is typically not related to pH except at both extremes of the scale. In natural soils, however, pH can have a great affect on plant growth due to nutrient availability.

A soil's pH is not a fixed quality but changes as the cation exchange capacity (CEC), moisture content, salts and carbon dioxide concentration changes. When it is considered necessary to change a soil's pH, it may be possible to add lime to raise the pH or sulfur to reduce the pH. Please keep in mind, however, that the buffering effect of some soils may severely restrict your ability to change the pH. Check with your local Extension agent if you have questions on this.