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Lesson 9 – Part A: SOIL pH
Lesson 9 – Part B: TESTING SOIL pH
Lesson 9 – Part C: The pH SCALE (FOR SENIORS)

 
PREPARATION FOR LESSON 9:

 
Please read through Lesson 9 text for students.

For this lesson, you will need: 

• A kit to test the pH of soil in a garden bed

• A plastic bucket

• A hand trowel

• A small quantity of water

A Manutec soil pH test kit was used for this lesson because it is simple to use, and is widely available through major hardware stores and nurseries. It was developed by the CSIRO almost fifty years ago, and has proven to be reliable. 'Spike' pH testers require mathematics to work out the average pH of soil in a bed after taking six or more samples.

In Part B of the lesson you can test some soil in school garden beds or, if you don't have beds that need testing, you could ask students to bring in a small soil sample from home, and test a few samples of soil that show differences in colour. If you decide on testing various samples, the lesson can be conducted in the classroom.

Part A – SOIL pH

Soil pH is the measure of the acidity or alkalinity of soil from an extremely acid pH of 0 to an extremely alkaline pH of 14. A soil pH of 7 is neutral, neither acid nor alkaline.

The pH scale is shown as negative logarithms so that the more hydrogen ions in topsoil, the lower the pH number. Because soil pH is expressed as a logarithm, a pH of 6.0 is ten times more acidic than a pH of 7.0, and a pH of 5.0 is a hundred times more acidic than 7.0. Adjusting soil pH without the buffering effect of decomposed organic matter is difficult.

It is not important for young students to know how the pH scale is calculated. It is more important that they understand the strong influence that soil pH has on pest and disease resistance in plants. 

Soil pH controls the availability of nutrient minerals in soil. Depending on the level of acidity or alkalinity of soil, varying amounts of different nutrients can be taken up by plant roots. At some pH levels, nutrients can become bound to other elements, or to soil, and become "locked out" and unavailable to plants.

When soil has an unsuitable pH for a plant species, adding extra fertiliser to counteract deficiency symptoms will be ineffective because the plant will be unable to absorb it from the soil.

All the major nutrients are only freely available to plants within a narrow soil pH range of 6.5 to 7.5, where essential trace elements are also available. Well-made compost has a pH of around 6.5, making it invaluable in assisting with the availability of major plant nutrients, and regulating the absorption of trace elements. Plants without access to a well balanced diet have weak immune systems and are unable to produce the compounds that deter pests. Soil pH also controls the number of microorganisms that improve soil structure.

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Part B – TESTING SOIL pH

The process of testing soil pH is covered in the lesson.

Students should only use the test kit under strict supervision. The purple liquid will stain hands and clothes. The powder in the puffer bottle is barium sulphate – a radio-opaque powder commonly used for gastro-intestinal scans. It can irritate eyes and should not be inhaled. 

ADJUSTING SOIL pH

If the pH of garden beds needs adjusting, organic gardeners have a distinct advantage over conventional gardeners. Adding mature compost to topsoil when preparing beds will help to lower the pH of alkaline soils, and raise the pH of acidic soils, as well as buffering plant roots from an unsuitable pH in surrounding soil. As a consequence, fewer amendments will have to be used to further adjust soil pH.

Where the amount of mature compost is limited, green manures will break down to add nutrients, microorganisms and humus to topsoil. Worm castings are colloidal (gluey) humus. They, and other solid organic-allowed fertilisers, provide nutrients in an easily absorbed form. Ideally, garden beds should be prepared a month before planting to allow soil chemistry to achieve a balance.

Do not attempt to make extreme changes to soil pH at one time. Test soil one month after application of any amendment to see if further applications are necessary.

TO RAISE SOIL pH

In all acidic soils, pH can be raised by the combined use of organic matter and the addition of calcium ions in the form of dolomite or lime.

Agricultural Lime – (Calcium carbonate) is finely ground limestone. Mined limestone, i.e. not chemically treated, is a safe choice to raise pH in garden beds. It takes several weeks to have an effect, but it is longer acting than other sources of lime, and can be thoroughly mixed into a full watering can and watered in around plants. This is a safe method for students to use.

It takes less lime to raise the pH of sandy soils than it does to change clay soils. Apply as recommended in the test kit for sandy, loamy and clay soils. The heading 'organic soils' in the Manutec leaflet refers only to very acidic soils. To avoid over-liming soils, apply at the rate for 'Loams', which are loose soils of clay and sand, especially those containing organic matter and very fertile. Test topsoil pH again one month after application and add a further application of lime or dolomite, if necessary.

[Note: I have not found the application rate recommended by Manutec for 'organic soils' to be accurate if topsoil contains a moderate amount of well-made aerobic compost. It may have been calculated for soils where only uncomposted manures have been added.]

Dolomite – (Calcium magnesium carbonate) is limestone with magnesium, and is applied in the same way. It is a good way to raise soil pH on sandy soils with fairly low organic matter content because both calcium and magnesium leach easily from these soils. In soils with high magnesium content, such as in parts of South East Queensland, agricultural lime is the preferred way to raise soil pH.

Quick Lime – (Calcium oxide) is made by heating limestone in a furnace to remove carbon dioxide. It is very caustic and unsuitable for garden use and not allowed in organic farming.

Hydrated or Slaked Lime – (Calcium hydroxide) is also known as builders' lime. It is produced by slaking quick lime in water. This lime can burn plant roots and should not be used on beds that contain plants or organic matter. Gloves and a mask should be worn when applying hydrated lime as it dries the skin and throat, and it is not allowed in organic farming, so we do not recommend this lime for school gardens. 

TO LOWER SOIL pH

Adding organic matter as compost and green manures without including lime or dolomite can be enough to adjust the pH of slightly alkaline soils because organic matter produces hydrogen ions as it decomposes.

Elemental sulphur (flowers of sulphur) will assist organic matter in reducing soil pH in more alkaline soils. Elemental sulphur is available from produce stores, and some nurseries.

Aged, crumbly manure from cows, horses and sheep that is free of pesticides and herbicides can be used more liberally on alkaline soils before growing a green manure grain. It has been calculated that 2–3 kilos of manure per square metre of bed area will reduce soil pH from 8.0 to 7.0 as the manure breaks down.

Please note – Lime sulphur is a fungicide, not a soil amendment.

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Part C – THE pH SCALE

This section is optional and is included for older students, who are able to understand more about soil pH.