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
The soil sampling can be done at any time of the year, although autumn and spring are the optimal times, as winter can be too wet and in summer the ground may be too hard.
1) Take three surface soil and three subsoil samples from each site (as described in points 1-5 the Taking soil samples activity guide in this Vitinote series). Make sure surface soil and subsoil are not combined so that they can be analysed separately.
2) Crush large aggregates and remove any gravel so that you have a fine mix to test.
a) Using a colorimetric test kit
• follow manufacturers instructions or if not available the general procedure is as follows.
• Put half a teaspoon of soil on the plate.
• Add enough dye to saturate the sample, mix well
• Sprinkle barium sulphate onto the soil mix and allow the colour to develop.
• Compare the sample colour with the pH colour chart.
• Record results.
b) Using a portable pH meter
• Refer to instructions and calibrate your pH meter prior to each use.
• Unscrew jar lid and fill the lid level with soil. Do not compress the soil. Pour into jar.*
• Add 5 jar lids of distilled water and screw lid on tight. Shake for 5 minutes then allow to settle for
• Rinse the pH meter electrodes in distilled water and dry gently with a tissue.
• Take a reading by immersing the electrode in the water above the settled soil as per manufacturer instructions. Make sure the electrodes are fully covered. Take care to minimise electrode contact with soil at the bottom of the jar.
• Gently stir the solution with the electrode while allowing the reading to stabilise. Try not to unsettle the sediment. Record results as pHw.
• Rinse electrode before next reading.
pH Affects Nutrients, Minerals and Growth
The effect of soil pH is great on the solubility of minerals or nutrients. Fourteen of the seventeen essential plant nutrients are obtained from the soil. Before a nutrient can be used by plants it must be dissolved in the soil solution. Most minerals and nutrients are more soluble or available in acid soils than in neutral or slightly alkaline soils.
Phosphorus is never readily soluble in the soil but is most available in soil with a pH range centered around 6.5. Extremely and strongly acid soils (pH 4.0-5.0) can have high concentrations of soluble aluminum, iron and manganese which may be toxic to the growth of some plants. A pH range of approximately 6 to 7 promotes the most ready availability of plant nutrients.
But some plants, such as azaleas, rhododendrons, blueberries, white potatoes and conifer trees, tolerate strong acid soils and grow well. Also, some plants do well only in slightly acid to moderately alkaline soils. However, a slightly alkaline (pH 7.4-7.8) or higher pH soil can cause a problem with the availability of iron to pin oak and a few other trees in Central New York causing chlorosis of the leaves which will put the tree under stress leading to tree decline and eventual mortality.
The soil pH can also influence plant growth by its effect on activity of beneficial microorganisms Bacteria that decompose soil organic matter are hindered in strong acid soils. This prevents organic matter from breaking down, resulting in an accumulation of organic matter and the tie up of nutrients, particularly nitrogen, that are held in the organic matter.