Soil pH.

Peter Miles
7 min readAug 21, 2020

A measure of the acidity or alkalinity of soil.

The acidity or alkalinity of a soil is measured on the pH scale from 0 to 14, this can be measured using readily available and inexpensive pH test kits. The pH can be changed by the application of lime to increase the pH (alkaline) or sulphur to decrease the pH (acidic). The pH is important in soil nutrition as it effects the availability to plants of soil nutrients.

Testing a soil sample.

Many years ago, in Mount Barker, South Australia, my neighbour Tim was a keen grower of vegetables in his home garden but slowly over a few years his new vegetable seedlings wouldn’t grow very well and appeared stunted and being unhealthy would get attacked by insects. He told me about this over the fence one day and he said he added cow and chicken manure but it didn’t help. Now the area where we lived had naturally occurring acidic soils, because of the higher rainfall, we lived next to a row of pine trees, the needle shaped foliage of which are slightly acidic and these were falling on his vegetable patch and also the animal manures he added are often slightly acidic. So, these 3 factors, acidic soil, pine needles and animal manures, combined together had made his soil too acidic. After some discussion we decided upon adding agricultural lime to his vegetable patch soil, lime is alkaline and served to balance the acidity and bring his soil closer to neutral which the plants prefer. Tim then had success growing healthy plants and continued to be a keen grower of vegetables.

The short story above is about acidity or alkalinity of soil. This can be expressed on the pH scale with values of 0 to 14, with 0 being very acid, 7 neutral and 14 very alkaline.

Examples of approximate pH or acidity/alkalinity of common substances are:

0 Hydrochloric acid HCl

1 Lead acid car battery

2 Stomach acid

3 Vinegar, lemon juice, soda drinks

4 Tomato juice

5 Black coffee

6 Human saliva

6.6 Cow’s milk

7 Neutral. Pure water

7.4 Blood

7.5 to 8.4 Sea water.

8 Egg white

9 Baking soda

10 Milk of magnesia

11 Soapy water

12 Ammonia — household bleach

13 Drain cleaners

14 Sodium hydroxide NaOH caustic soda.

Plant species will generally grow in the range from 5 to 8 with most preferring 6.5 to 7.5 pH.

The pH, representing the Hydrogen ion concentration, is one of the important factors that affect plant growth. The p stands for potential of Hydrogen and is the common logarithmic (based on 10) counting scale used with H representing the Hydrogen counted, with more H in the acidic range and more OH Hydroxide ions in the alkaline range. A negative logarithm is used which results in a low pH indicating more Hydrogen ions. In the laboratory these numbers are very small, for example a soil may have an H count of 0.0000007mole (mole is a measurement used for numbers of atomic size particles, a mole contains 6.022x1023 molecules and is called Avogadro’s number). This in turn can be written as 10 to the power of -7, but can be tedious so the value is converted to whole numbers, 1 to 14, using a logarithmic scale, p. The pH scale being logarithmic means that a change of pH from 7 to 8 for example, is actually a 10 times value change.

Note that pH is correctly written with a small p and capital H. PH is quite incorrect as it indicates P phosphorus bonding with H hydrogen, which doesn’t happen except in rare cases of bonding with Phosphine.

The pH is important in soil nutrition as it effects the availability to plants of soil nutrients. Plant absorb some nutrients by excreting hydrogen ions from their roots to exchange with the nutrients bonded to soil particles. If the soil is too alkaline it will already have high levels of hydrogen ions and not release nutrients to the roots. If the soil is too acidic nutrients like aluminium and manganese can become too soluble and make too much available and therefore be toxic to plants.

Alkaline substances are also referred to as basic or base, for example basic soil is alkaline soil. An old term for alkaline soil is ‘sweet soil’.

Plants will generally grow in the range from 5 to 8 pH with most preferring 6.5 to 7.5 pH.

Plant crops which will grow in really acid soils such as 5.0 to 5.5 include blue berries and sweet potato, acid 5.5 to 6.5 soil crops include corn (maize) and beans and slightly acid at 6.5 to 7 neutral soil crops include alfalfa (lucerne), asparagus and sugar beets.

Ornamental plants such as Azalea, Rhododendron and Camellia prefer acid soils of pH 5 to 6 while some plants such as Abelia, Canna and Wisteria will tolerate soils with pH up to 8.0.

Effect of soil pH on nutrient availability (Landon, 2014).

Raising the pH; excess acidity in soil can be corrected by the application of agricultural lime which is pulverized or crushed limestone rock, calcium carbonate CaCO3. Pulverizing the rock to fine, small particle sizes creates a large surface area that can come into contact with the soil particles.

There are 3 types of lime, the aforementioned agricultural lime, this can then be heated in a kiln to create calcium oxide CaO called burnt lime or quicklime and then if water is added it becomes calcium hydroxide Ca (OH)2 called slaked lime or hydrated lime. Quicklime and hydrated lime are used in the building industry to make mortars to bond together bricks and stones but should not be used on soils as they are too strong in their effect and quick acting and will have a detrimental effect on soil microorganisms. Agricultural lime will take time to have an effect on the pH of a soil and hence is best applied at the start of the rainy season when rain can help carry it into the soil and react with the soil particles. Apply agricultural lime at about 1 kilogram per square metre but it is best to follow the directions on the container.

Dolomite lime is calcium magnesium carbonate Ca Mg (CO3)2 and does have an effect of increasing pH in an acid soil but is best avoided in most situations as it contains too much magnesium.

Gypsum is calcium sulphate dihydrate CaSO4.2H2O and doesn’t have a direct effect on the soil pH, it is used as a soil treatment to improve infiltration rates and drainage in clay soils. It enables the soil particles to flocculate together to form larger aggregates which therefore have larger pore spaces. This is particularly the case in soils with too much salt, sodic soils, as sodium (salt is sodium chloride NaCl) is limited in its ability to flocculate soils, whereas calcium in the gypsum is more able to flocculate soil particles.

Lowering of the soil pH, making a soil more acidic can occur through the use of chemical fertilizers, the use of manures on soil and just through irrigation. Intentional acidification of the soil can be done with the application agricultural sulphur S.

You can test your own soil pH with generally available and relatively cheap soil pH test kits. These are a chemical kit which include an indicator liquid that is mixed with a small soil sample, to make a solution, then dusted with a white powder which changes colour and that is compared to a colour chart with the different colours indicating the pH. See photograph above.

A pH test kit.

Be careful with the selection of soil samples to be tested. In the past I have tested sandy soil where I was living near the coast, these soils are nearly always alkaline with a pH of around 8 but my test result was 5.5, quite acidic. On closer examination, in my soil sample was a small piece of cow manure which I had top dressed on the soil the year before and this is what was giving the acidic reading, not the soil particles. Another test gave a result of 8.5 which is more what is expected from such a soil.

It is best to do several tests of different soil samples from over your site and also at different depths, at the surface and at 2 or 3 centimeters down. Push to one side any organic matter on the surface before testing, as although this is very good for the health of soil it will give an inaccurate measurement of your soil pH as mentioned above.

In summary, acidity or alkalinity of a soil is measured on the pH scale from 0 to 14 and soil pH is generally between 5 and 8. The pH is important in soil nutrition as it effects the availability to plants of soil nutrients.

References:

Landon, J. R. (2014). Booker tropical soil manual: a handbook for soil survey and agricultural land evaluation in the tropics and subtropics (2nd ed.). New York.: Routledge.

Zumdahl, S.S. and DeCoste, D.J. (2011) Introductory Chemistry: A Foundation (7th Ed.). Brooks/Cole, USA.: Cengage Learning. Book.

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Peter Miles

45 years in Environmental Science, B.Env.Sc. in Wildlife & Conservation Biology. Writes on Animals, Plants, Soil & Climate Change. environmentalsciencepro.com