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Mineral amendments

Applying a mineral amendment
Photo: Jardin botanique de Montréal (Michel Tremblay)


Gypsum is crushed calcium sulphate from mines (in which case it is grey) or industrial by-products (in which case it is white). It contains 22% calcium and 17% sulphur. Adding gypsum will improve the structure of “sodic” clay soils, which are high in sodium. It can also be used to neutralize soils where deicing salts have caused high sodium levels. Gypsum has very little effect on soil pH.

Gypsum should be scratched into the top few centimetres of soil in spring. How much you should apply depends on your soil test results.


"Lime" refers to raw products, such as horticultural or agricultural lime, dolomitic lime or marl, or processed products such as slaked or quick lime, which are obtained by heating limestone or calcium carbonate (CaCO3). Processed lime products are not recommended, because they are caustic.

Depending on how much you apply, lime will lower your soil’s acidity. Because this amendment supplies calcium, it will lighten compacted soil by promoting the formation of aggregates. For magnesium-deficient soil, add dolomitic lime, which contains magnesium carbonate and calcium carbonate in a ratio of 1:4.

Because lime breaks down slowly, its effects can last 3 to 4 years, depending on how much you apply and your soil type. Always have your soil tested before adding this amendment.

You can apply lime at practically any time of year, but August to October is the ideal time. It is best to apply it in spring to sandy soils. Note that you should wait at least one month before fertilizing freshly limed soil.

Lime should be worked into the top 10 to 15 centimetres of soil. If you need to add large amounts of lime, or to adjust the pH value by several units, it is best to do so in stages to avoid overliming problems.

Wood ashes

The best wood ashes for improving soil, i.e. the wood ashes with the highest mineral content, come from hardwood. They supply valuable amounts of calcium, potassium, phosphorous and several micronutrients. Never use ashes from painted or treated wood, coloured paper or plastic, all of which contain toxic substances.

Wood ashes are extremely alkalinizing because of their high calcium content. They are fast-acting but not long-lasting in the soil.

Wood ashes should be worked into the soil or compost in spring. They should not be used to amend soil for growing acid-loving plants or seeds.

Caution! Wood ashes are highly soluble and saline. Excessive amounts of salts in soil interfere with plant growth and will often destroy microorganisms. Repeated applications of large amounts of wood ashes may also cause toxins such as lead and cadmium to accumulate in the soil and interfere with plants’ assimilation of minerals. This amendment should be used in moderation.

Recommended application rates:

To gradually increase pH by 0.3 to 0.4 units (for instance, to go from pH of 6.2 to 6.5):10 kg/100 m2

To maintain pH, once you have reached the desired level: 5kg/100 m2


Sulphur used as an amendment is available in powder (micronized sulphur) or granular form. It is used to acidify soil. Sulphur has a medium- and long-term effect on soil. This means that it should be applied one or two years before planting. For established plants, spread the recommended amount on the soil at the drip line and work it into the top 7 or 8 centimetres.

Sulphur may be applied in spring or fall, depending on your soil test results. If you need to apply large amounts, do so in two stages, once in early spring and then again in fall.

Granular sulphur works more slowly than powdered sulphur, but is longer-lasting. It is also easier to apply, leaches out of the soil more slowly and produces less dust. Inhaling sulphur powder may irritate respiratory passages.

Iron sulphate

Iron sulphate is very useful for lowering soil pH quickly, especially when plants show signs of an iron deficiency. How much you should apply depends on your soil test results. Its effects on soil are not as long-lasting as those of sulphur.

Aluminum sulphate

Aluminum sulphate is another way to lower soil pH. Ideally, this amendment should be used only with blue hydrangeas (Hydrangea macrophylla), because aluminum is toxic to many plants. How much you should apply depends on your soil test results.

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