Reduction of pH levels tends to be a more expensive and complicated process than increasing them. There are, however a couple of “emergency measures” which are sometimes used to good effect. These are outlined later.
There are no specific chemicals which can safely be applied to the soil to make it more acid, so the acids must come from increased quantities of organic matter as they are decomposed by soil bacteria. The use of acid natured fertilisers will also help.
The speed at which organic matter breaks down is affected by soil type, temperature and bacterial content, so the amounts required can only be approximated.
To reduce the pH by approximately 1.0 unit, use:
Peat - dug into your soil at the rate of approximately 2.5 lbs per square yard
Compost - dug into your soil at the rate of approximately 14 lbs per square yard
Manure - dug into your soil at the rate of approximately 5 lbs per square yard
When you want to increase the nitrogen level in your soil, use a fertiliser that exerts an acidic effect, such as Sulphate of Ammonia. If it is used at around 2 oz per square yard, this should reduce the pH by about 1.0 although the effect is short lived, and you might not want to use so much fertiliser anyway.
Change the soil
One possible alternative if only a section of your garden is required for acid loving plants is to change the soil by building a raised acid border on top of the existing soil. Such borders are generally made of peat block retaining walls and filled with peaty, acid soil brought in from elsewhere. The beds should be at least 12 inches (300 mm) deep above the existing ground level and should not have the imported soil dug or mixed with the existing soil.
Although not always easily available, it is possible to lower soil pH with Flowers of Sulphur or Ground Rock Sulphur. American research led the way in this technology, although some British research has been undertaken. Suggested rates are: To reduce pH by 1.0 units (e.g. to go from 6.5 to 5.5), apply Sulphur at: 1.2 oz per square yard on sandy soils, or 3.6 oz per square yard on all other soils.
Ideally the sulphur should be thoroughly mixed into the soil before planting, but it can be used a s a top dressing hoed or forked into the soil around existing plants, in which case a booster dressing may be needed every few years. The greatest benefit comes when it is thoroughly mixed in amongst the soil particles. Clearly there is a trade off here with the risk of mechanical damage to the roots of existing plants.
It is advisable to check your pH at least once each year to see how you are gong on.
Plants which have been planted in a soil which is too alkaline for them can have their problems eased by an annual application of Sequestrine of Iron (available from Garden Centres and shops), although this is not a permanent solution.