Soil pH is an indication of the level of acidity or alkalinity present in soil measured in pH units. Technically defined soil pH is the negative logarithm of the hydrogen ion concentration. The pH scale goes from 0 to 14 where 7 is considered neutral pH, below 7 (7-0) the soil is becoming more acidic and above 7 (7-14) will be more alkaline or basic. An increase in hydrogen ions makes the soil pH more acidic, which hopefully helps to clarify the dense definition. For reference, below is a list of some commonly used descriptors for pH levels and examples of items which exist naturally at those levels.
- 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
Soil pH greatly affects plant growth by changing or limiting the availability of nutrients and minerals and uptake. Nutrients must be in solution to be available for uptake by plants, and minerals and nutrients are generally more soluble in slightly acidic soil. For example, phosphorus, which is not often readily available in soil, is soluble at a pH level of 6.5, just barely acidic. While metals, aluminum, iron and manganese, can be present in high concentrations in strongly acid soils, pH of 4-5, and this can detrimental to plant health. A range of pH levels between 6 and 7 is ideal for solubility of necessary nutrients to plant health and vigor.
Microorganisms in the soil, such as beneficial bacteria, are affected by pH levels as well. Strongly acidic soils can impede the processes performed by decomposing bacteria, this prevents organic matter from being broken down. If organic matter can’t decompose it will build up in the soil, binding up nutrients, especially nitrogen, making them inaccessible for plant uptake.
Soils tend to become acidic for several reasons; rainwater can leach ions (calcium, magnesium, potassium and sodium) from soil, carbon dioxide resulting from decomposition and byproducts of root respiration can bond to form a weak acid and/or nitric or sulfuric acid can be form d from oxidation of some components in fertilizer and decaying organic matter. A simple soil test can be performed to gauge the pH level of your landscape soils. If necessary, amendments are available which can adjust pH towards a level more suitable for healthy plant growth. Lime treatments are an efficient and effective way to increase soil pH, it also provides essential nutrients calcium and magnesium and helps make phosphorus more readily available for plant uptake by accelerating decomposition. Sulfur is a common addition to soils which need pH lowered, as sulfur oxidizes hydrogen ions are released, however oxidation is the product of microbial activity and so is slow acting. Lime treatments may be most effective if applied in fall, while sulfur applications may be held off until spring when microbes become active again. Consult with your arborist regarding soil testing and pH adjusting applications, they will be able to develop a plan for your soil’s specific needs.