Now that you know that pH is important and can be measured the next questions is how to alter it to suit the crops we want to grow if it isn’t right. Fortunately it is possible to alter soil pH by adding various minerals. As previously mentioned, the soils in Australia tend towards the acid side of the scale. They may be that way naturally, be acidified by the use of chemical fertilisers or they may become that way over time due to the build up of humic acids from use of organic fertiliser and mulch. For this reason we will concentrate on neutralising acid soils rather than alkaline ones as this is the situation you are most likely to come across.
The magic ingredient for “sweetening” sour or acid soil is lime. Unfortunately the word “lime” can indicate a number of different chemical compounds –
Quicklime
Chemically this is calcium oxide which is VERY alkaline and can burn your hands. It will reduce soil acidity very rapidly but is rough on the soil organisms, the plants and you!
Slaked Lime
Also called slacked or builder’s lime, it is quicklime that has been “slaked” by adding water to it to form calcium hydroxide. It is not as rugged as quicklime but it is still very alkaline. It won’t burn your hands but it will burn the plants and give the soil organisms a hard time. Slaked lime will reduce soil acidity quickly.
Agricultural Lime
You guessed it! This is the one for you. It is ground up limestone and chemically speaking is calcium carbonate. It’s effect on pH is slow but it is safer all round for use than quick or slaked lime. It won’t burn your hands or rub out your soil organisms either.
Dolomite
This can also be referred to as lime although it is actually a mix of calcium and magnesium carbonates. It acts in much the same way as agricultural lime but it can be particularly valuable in soils with a magnesium deficiency.
When lime is mentioned from now on you know I mean agricultural lime.
Applying Lime
Lime is best added by broadcasting by hand over small areas or by a spreader over larger areas, then digging it as deeply as required for it to be distributed throughout the plants’ root zone. It is best to apply it in autumn or early spring and left for as long as possible before planting to allow enough time for it to react and raise the pH. If you use a green manure crop as part of your rotation the lime should be applied when the green manure is turned under. It is also possible to top-dress with lime at other times of the year without damaging any growing crops, but it will take much longer for the lime to raise the pH.
How much lime you need to add to your soil to achieve the desired pH depends on a number of factors, the starting pH being an obvious one. A less obvious one is the “buffering capacity” of the soil, which is the ability of a soil to resist a change in its pH due to the nature of the soil. In practice, what this means is that a soil high in organic matter and/or clay content will require more lime to achieve a desired pH than will a sandy soil under the same conditions. As a guide a soil which is rich in clay may require 300 to 450 grams of lime per square metre to raise it’s pH from 5.5 to 6.5, while a sandy soil would require 150 to 200 grams of lime per square metre for the same pH change. Rainfall must also be considered as the lime will be washed through the soil profile more quickly in a high rainfall area.
Lowering pH
It may be that due to a previous land owners overuse of lime that your soil has an alkaline (high) pH and you wish to lower it. In this case the easiest method to acidify your soil is to apply and dig in elemental sulphur which is available from hardware and garden supply shops. The same rules apply here as for lime, a clay soil requiring more than a sandy soil. As a guide, to reduce soil pH from 7.5 to 6.5 a clay soil needs 90 to 100 grams of sulphur per square metre whereas a sandy soil needs only 30 to 60 grams of sulphur per square metre.
