Research

For most garden plants, optimum growth is achieved when garden soils are maintained at a pH between 6.0 and 6.5. However, lowering soil pH is desirable where acidloving plants are to be grown. To achieve this gardeners are usually advised to apply an acidifying chemical such as elemental sulphur, and/or incorporate into their soil organic amendments which have a similar acidifying effect. Many organic materials are recommended for this purpose including manure, mature garden compost, pine needles, composted or weathered bark and sawdust, and oak and beech leaf mould.

In the United Kingdom, by far the most popular material for this purpose has been peat, due to its wide availability and low cost relative to other soil amendments. However, RHS policy supported by many RHS members is that peat should not be used as a soil amendment due to the effects its extraction has on wildlife habitats.

Enquiries to the Advisory Service at RHS Garden Wisley regularly ask about increasing the acidity of soil without resorting to peat. Increasing amounts of time, effort and money are being spent on developing peat-free alternatives.

Most work to date has focussed on container growing media while quantitative information on the performance of suitable alternatives to peat for creating 'acid-beds' is more scarce.

Soil amendments

Four different amendments were chosen;

1 Sphagnum moss peat

2 Finely chipped conifer bark

3 Oak sawdust composted with ammonium sulphate

4 Powdered elemental sulphur

The initial soil pH was ~6.5 and each organic acidifying treatment was mixed with the topsoil to create amendment/soil mixes in a 1:1 ratio (v/v). Each mix was used to fill five, plastic, 20 litre containers giving five replicates of each treatment. For the sulphur treatment, powdered sulphur was applied at 15g pot -1 and thoroughly incorporated. This rate was equivalent to an application of 0.75kg S m -3 , slightly higher than the recommended rate for application to loam-based compost for ericaceous plants. Five control pots of unamended soil were also included.

Acidity of the soil

Once incorporated the bark, peat and sawdust amendments immediately caused significant pH reductions relative to the control, which were maintained throughout the course of the experiment. The sulphur treatment took six weeks to bring about a significant reduction in pH relative to the control. This acidifying action was then maintained throughout the remainder of the experiment.

The delay in the acidifying effect of sulphur powder is due to its requirement for conditions that encourage sulphur-oxidising bacteria to flourish i.e. a warm, moist, aerated environment.

Benefits of alternative materials

Whilst peat is an effective acidifying material, the composted sawdust, conifer bark, and sulphur treatments are all promising alternatives and may be considered less damaging to the environment. The conifer bark used in this study acidified the soil, is environmentally-friendly and is widely available to
gardeners. Further benefits of conifer bark are the nutrients it adds to the soil as well as improvements in soil structure and water retention etc.

The long-term effects of acidifying materials are important. The timescale of this experiment was short (approximately 4.5 months). This study was the first of several investigations with different amendments and different timescales being examined.

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