Soils can be naturally acid or alkaline, and this can be measured by testing their pH value.

Having the correct pH is important for healthy plant growth. Being aware of the long-term effects of different soil management practices on soil pH is also important. Research has demonstrated that some agricultural practices significantly alter soil pH.

What is soil pH?

Soil pH or soil reaction is an indication of the acidity or alkalinity of soil and is measured in pH units. Soil pH is defined as the negative logarithm of the hydrogen ion concentration. The pH scale goes from 0 to 14 with pH 7 as the neutral point. As the amount of hydrogen ions in the soil increases the soil pH decreases thus becoming more acidic. From pH 7 to 0 the soil is increasingly more acidic and from pH 7 to 14 the soil is increasingly more alkaline or basic.

Descriptive terms commonly associated with certain ranges in soil pH are:

• Extremely acid: < than 4.5; lemon=2.5; vinegar=3.0; stomach acid=2.0; soda=2–4
• Very strongly acid: 4.5–5.0; beer=4.5–5.0; tomatoes=4.5
• Strongly acid: 5.1–5.5; carrots=5.0; asparagus=5.5; boric acid=5.2; cabbage=5.3
• Moderately acid: 5.6–6.0; potatoes=5.6
• Slightly acid: 6.1–6.5; salmon=6.2; cow’s milk=6.5
• Neutral: 6.6–7.3; saliva=6.6–7.3; blood=7.3; shrimp=7.0
• Slightly alkaline: 7.4–7.8; eggs=7.6–7.8
• Moderately alkaline: 7.9–8.4; sea water=8.2; sodium bicarbonate=8.4
• Strongly alkaline: 8.5–9.0; borax=9.0
• Very strongly alkaline: > than 9.1; milk of magnesia=10.5, ammonia=11.1; lime=12

Measuring Soil pH

Soil pH provides various clues about soil properties and is easily determined. The most accurate method of determining soil pH is by a pH meter. A second method which is simple and easy but less accurate then using a pH meter, consists of using certain indicators or dyes.

Many dyes change color with an increase or decrease of pH making it possible to estimate soil pH. In making a pH determination on soil, the sample is saturated with the dye for a few minutes and the color observed. This method is accurate enough for most purposes. Kits (pH) containing the necessary chemicals and color charts are available from garden stores.

There may be considerable variation in the soil pH from one spot in a field or lawn to another. To determine the average soil pH of a field or lawn it is necessary to collect soil from several locations and combine into one sample.

Why soil pH is important?

Soil pH has indirect yet far-reaching effects on plants. Plant nutrients become available or unavailable according to the soil’s pH level . Yellowing between the veins of young leaves indicates an iron deficiency, a condition arising not from a lack of iron in the soil but from insufficient soil acidity to put iron into a form that a plant can absorb. Most plants thrive in slightly acidic soil because that pH affords them good access to all nutrients.

The darker side of soil pH is plant poisoning.  Too low a pH level can render the plant nutrient manganese available at toxic levels; geraniums are particularly sensitive to this, showing their discomfort with yellowed, brown-flecked, or dead leaves. A pH level that is too low also liberates aluminum—not a plant nutrient—in amounts that can stunt root growth and interfere with a plant’s uptake of nutrients. At a high pH level, the plant nutrient molybdenum becomes available in toxic amounts.

There are a few plants that require a soil pH of 4.5 to 5.5. These “acid-loving” plants include azaleas, rhododendrons, and blueberries. The soil pH for these plants can be lowered by incorporating elemental sulfur (S) into the soil. Since the soil acidifying response to elemental sulfur is slow, it should be applied and incorporated a year before planting. Working Canadian sphagnum peat into the soil is another method to lower pH.

For most plants, however, a soil pH below 6.0 is undesirable. Strongly acid soils need to be limed to raise the pH to near neutral levels. Liming materials include ground limestone which is mainly calcium carbonate (CaCO3) and dolomitic limestone which contains CaCO3 and some magnesium carbonate (MgCO3). Since most Iowa soils do not need to be limed, apply liming materials only when recommended by a soil test.

A highly visible example of the role of soil pH in the availability of nutrients can often be seen in pin oaks. Many pin oaks in the Midwest suffer from iron chlorosis which is characterized by yellowish green foliage. The yellowing of the leaves is caused by a deficiency of iron within the plant. Most Iowa soils contain sufficient quantities of iron for good plant growth and leaf coloration. Unfortunately, much of the iron is in an insoluble form and is unavailable to pin oaks when the soil pH is above 7.0. Iron is readily available to pin oaks at a pH range of 5 to 6.5. Since efforts to correct iron chlorosis are extremely difficult, gardeners should avoid planting pin oaks in alkaline soils.