Gardening
Soil pH

The term pH refers to the activity of hydrogen ions. It is a very important factor in many chemical reactions involved in the soil, inside organisms (including us), and in industrial processes. In these activities, pH has a regulating role almost as important as temperature.

Most plants grow best in a ‘neutral’ pH range. If your soil is outside of this range, you can amend the pH of soil, or you can specifically seek out plants that suit your local conditions. Either way you should have a basic understanding of what pH is, how it affects your plants and what you can do to manage it.

The pH scale is a measurement of acidity or alkalinity. Acids have the property of producing hydrogen ions. The concentration of ions they produce varies depending on wether they are strongly acid (high concentrations of hydrogen ions) or weakly acid (low concentrations of hydrogen ions, high hydroxyl ion concentration). For practical purposes, the scale starts at 0 and goes to 14. Pure clean water is neutral, at pH 7. It is a logarithmic scale, meaning that the difference between one number (eg 9) and the next (10) is a ten times increase in ion concentration – pH 10 is ten times as alkaline as pH 9. pH 10 is one hundred times more alkaline than pH 8.

A very acidic substance like lemon juice is around pH 2.6. Orange juice is around pH 4.

A very acid soil is around pH 4. Only specialised bog plants can grow in a severe pH of this value. A more normal acid soil may go as low as pH 5. Slightly acid soils (around pH 6) are more normal in the garden situation, and some vegetables such as potato and spinach are well suited to this condition.

Neutral soils are around pH 7. Many books will refer to pH 6.5 to 7.5 as ‘normal’, because most plants in common cultivation prefer this range.

Alkaline soils are common in limestone areas. A pH around 8 is reasonably common and easily managed. Over pH 8 constitutes a very alkaline soil, such as may be found on the Eyre Peninsula (SA).

Baking soda is a common alkaline substance. It has a pH of 8.5. Ammonia is strongly alkaline.

Alkaline soils are sometimes called ‘sweet’. Acid soils are called ‘sour’.

The pH of soil can be measured with pH test kits, portable pH meters and litmus paper.

Limus paper contains a pH sensitive dye, which changes colour according to the pH of the material tested. Mix soil with water to form a thick, muddy slurry. Let it stand for about one hour and dip the paper into the slurry, leave it for one minute and remove. Rinse it with distilled water and observe the colour change.

Soil test kits are generally more accurate. Portable pH meters require careful use and must be kept clean and dry and they don’t produce a better result that the test kit (unless you are prepared to invest a lot of money).

The pH of soil has a major affect on nutrient availability, because it affects which nutrients are able to dissolve in the soil solution. For instance manganese is unavailable at a soil pH below 5.5. Unless plants are specifically adapted to very low pH conditions, chlorophyll production will cease in these conditions.

Gardeners who have a soil in the mildly acid to mildly alkaline range generally don’t have to worry about pH affects on nutrients, especially if they use regular additions of compost. If you have a high acid/alkaline soil, a good Albrecht-based soil test should produce a suitable recommendation for nutrient additions.

Adjusting soil pH

The pH of acid soils is easily ameliorated with additions of finely ground limestone (calcium carbonate) or dolomite (magnesium carbonate and calcium carbonate). Clay soils require much more lime to change pH than sandy soils,

It is harder to modify the pH of alkaline soils, but it can be done with additions of sulphur, or sulphur containing fertilisers. Elemental sulphur (flowers of sulphur) can be used for this purpose. The treatment may need to be repeated periodically. It is also useful to add organic matter such as well-rotted chicken manure. Western Red Cedar chips used as mulch, or composted with the chicken manure and elemental sulphur also assist with reducing pH.

Additions of organic matter generally influence all soils towards neutral, so compost, green manures and crop rotation can also influence pH. Plants will often grow better outside their preferred pH range if the soil is well supplied with humus.

Plants which prefer acid soils

Azalea

Blackberry

Blueberry

Carrots

Corn

Eggplant

Flax

Parsley

Parsnips

Peanut

Potato*

Pumpkins

Rhododendron

Rhubarb*

Shallot*

Sorrel

Strawberries

Sweet potato

Tomatoes

Watermelon*

* very tolerant

Plants which prefer alkaline soils

Alyssum

Asparagus

Bean

Beetroot

Broccoli

Cabbage & Cauliflower

Carnation

Cucumber

Iris

Leeks

Lettuce

Nasturtium

Onion

Phlox

Spinach

Squash

Sweet pea

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