Soil pH is a measure of the current concentration of hydrogen ions in soil. Hydrogen is universally used to denote an element or particle with a net positive charge. Soil pH is measured on a scale from 1 to 14 where each whole number on the scale is 10 units larger than its preceding number. For example, while 7 is considered neutral, it is 10 times more alkaline than 6 which is considered slightly acidic but ten times more alkaline than 5. Therefore, increasing acidity is indicated as the numbers move below a pH of 7.
This pH measurement is important in determining what kind of plants the soil will support. Apart from other environmental concerns, plants that prefer an acidic solution around their roots generally perform poorly in West Texas. Our soils are simply too alkaline to keep azaleas, magnolias, and rhododendrons happy for very long.
MYTH BUSTING
A common misconception is that acidic mulches can increase soil acidity? This common belief has virtually no research support, yet the myth persists. Try a google search on the topic and you’ll get hundreds of pages of advice on how to increase soil acidity with pine needles and oak leaves.
If soil pH could be reduced by using pine and oak leaf mulches, it would have wonderful implications for the kinds of plants we might be able to grow in our West Texas landscapes. However, researchers have not been able to significantly acidify soil with mulches of pine needles or oak leaves. Even after years of applying these mulches, the soils underneath maintain a pH common to their region. Soils of the Llano Estacado are slightly alkaline, hovering around a pH of about 7.8.
So where does this myth come from? It is true that pine needles have a very acidic pH of about 3.5 when they fall to the ground and oak leaves have a pH of about 5. However, the process of decomposition by weathering and soil microbes generates organic material that is neutral to slightly alkaline.
Slight changes in pH may be temporarily observed in the thin layer of soil immediately beneath decomposing oak leaves and pine needles but the effect would not be significant enough to start an azalea farm in West Texas. Even incorporating acidic forms of organic matter into the soil does little to change pH in the long run. The volume of soil beneath a mulch is simply too massive to be easily changed by even the most acidic sources of organic matter. Even if the byproducts of decomposition were not near neutral, they would quickly be neutralized by soil components.
Soil has a remarkable capacity to resist change. This buffering capacity, as it is known, is one of soil’s most important fundamental chemical properties. Soils are composed of cations (positively charged elements) and anions (negatively charged elements). When a positive ion such as hydrogen is introduced into the soil, it quickly reacts with calcium, magnesium, potassium or another anion and is effectively shielded by oppositely charged components in the soil. A similar reaction occurs when negatively charged compounds are introduced into the soil.
The amount of time and volume of material required to change soil pH is absurdly large in West Texas simply because of the huge reservoir of bases in our soils. In the final analysis we are compelled, by forces mostly beyond our control, to put plants in our landscapes that enjoy our climate and appreciate the pH of our soils. In fact, you may be familiar with the saying, “the right plant in the right place.” Soil pH is one of the factors that limit the selection of plants we can enjoy in West Texas.
