Research

This work evaluated seven garden mulches for a range of characteristics. They were:

• cocoa shells,
• coarse chipped conifer bark,
• fine chipped conifer bark,
• wood chips,
• garden compost,
• horse manure,
• black polythene

Over a period of 12 months the following criteria were investigated:

• mulch appearance,
• rate of decomposition,
• weed suppression,
• soil moisture retention,
• surface soil temperature mediation,
• surface soil pH,
• nutrient content,
• organic matter content.

Results revealed that the differences in chemical composition of the mulch materials significantly affected their longevity as well as nutrient content and pH of the underlying soil.

Moisture retention and soil temperature

Data relating to moisture retention and soil temperature were inconclusive. The main differences observed were between the bare earth control, black polythene and the organic mulches as a group.

Weed suppression

Similarly, weed suppression data highlighted the value of mulching as a practice compared to bare earth, but no significant difference was observed between the types of mulch.

Acidity/alkalinity

Horse manure was the most alkaline mulch, garden compost was neutral in pH, and the wood and bark mulches were acidic. Mulching raised the soil pH, i.e. it became more alkaline, even if the pH of the mulch was initially acidic. This has implications for the use of mulches to create acid beds. Clay soils and those rich in organic matter are more greatly buffered against pH change than sandy soils.

Nutrients

The initial nutrient concentration of garden compost was in the mid-range of the mulches, except that its carbon content was lower than the rest. The cocoa shell mulch was particularly high in nitrogen, potassium and magnesium concentration. The horse manure had the highest phosphorus concentration, whereas bark and wood mulches were very low.

The initial nutrient content of the mulch affected the final soil nutrient content.

There was no change in soil organic matter.

There were no signs of immobilisation of nitrogen, which is a concern particularly with woody mulches of a high C:N ratio. However this may have been masked by the extended period of the experiment, and the limited mixing of the mulch and soil.

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