Changing pH

Acid soil can be made more alkaline (or brought closer to neutral) by adding lime. The effect of lime additions can be quite slow, and it is dangerous to add large quantities, because roots can be severely scorched by lime. It is generally better to add small quantities more often, than to attempt to make large changes in pH ‘all at once’.

Ideally lime should be applied two or three weeks before planting. It may be lightly tilled into the soil or just watered in. If lime is to be added to the subsoil, it can be flushed down rip lines.

Do not apply manures and lime at the same time, as they will form ammonia, a gaseous form of nitrogen, and the nutrient will be lost to the atmosphere.

The amount of lime needed depends upon the starting pH of the soil and the clay content. Heavy clay soils will need more lime to make the same change in pH than a sandy soil will (ie the amount required to change pH down one point will be greater for clay soil).

For example, to reduce the pH of a sandy soil by one point will take approximately 2 kilograms of lime per 100 square metres. A loam may take 3 kg and a heavy clay around 4 kg.

Most plants have some tolerance for growing in soil where the pH is not ideal, so being exactly accurate is not necessary, but growing plants well outside their pH tolerance will reduce growth and productivity.

The simplest way to measure pH is with Litmus paper, available from the local pharmacy. A simple pH testing kit is a much more reliable method. They sell for between $25 and $45 and are available from garden centres and fertiliser suppliers. A pH probe is also useful, but they do need careful use and storage to ensure that they remain accurate.

Soils can be made more acidic by adding large quantities of compost, aged manures or other forms of organic matter. If soils need to be made very much more alkaline, additions of elemental sulphur will help. A mixture of chicken manure, western red cedar chips and elemental sulphur is recommended by soil adviser Peter Bennett as the best and quickest way to reduce pH.

Types of lime

Slaked lime:

Slaked lime or calcium oxide is also called “garden lime”. It is a longer lasting form of lime treatment.

Hydrated lime:

Also known as ‘builder’s lime” because it is used in cement. It is OK to use in gardens, but may need to be reapplied annually.

Ground limestone:

Undoubtedly the best form of lime for use in the garden. It is more expensive than the slaked or hydrated lime, but the effects will last for several years.

Ground dolomite:

Similar to limestone, except that dolomite also contains magnesium.

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