Examples of organic mulch materials include: bark shredded or cut into chunks of various sizes; wood chips or shavings; sawdust; compost; grass clippings; leaves; peat from stem material of various waterside plants; pine needles; and straw (Anon. 2006).
Since organic mulches are derived from plants, and are commonly used as garden composts, their decomposition over time may enrich or otherwise alter the properties of soil. For instance, adding organic mulch may improve soil structure. This is achieved by the decomposition of the organic mulch where by-products of microbial organisms such as Actinomycetes sp. and Azotobacter sp. promote the stability of granules in soils (McCalla 1950). Mulches also help preserve soil structure by acting as a cushion against compaction (Trowsdale & Simcock 2011). The use of compost as mulch if properly done, has the additional benefit of killing weed seeds which may be present in the mix (Duryea et al. 1999a).
Soil pH can also be modified by organic mulches. Wood-based mulches may reduce soil pH (Billeaud & Zajicek 1989) but not in all conditions (Cregg & Schutzki 2009); using sphagnum moss could exacerbate this effect due to the acidophilic nature of associated microbial activity (Pankratov et al. 2008). However, if compost mixes are used as mulch, a slight increase in alkalinity may be observed as the decomposed mixture ‘ripens’ (Lalande et al. 1998).
Organic mulch supports soil microbial populations as it acts as a nutrient source, conserves moisture and maintains a relatively constant temperature (Almeida et al. 2011). Whilst the increase in soil biodiversity may be encouraged for promoting nutrient cycling, using poor quality mulch may risk the introduction of pathogens and pests (Miyasaka et al. 2001; Jordan & Jones 2002).
Urban soils may be more prone to toxic contaminants as by-products from industrial activities. The application of organic mulches such as from leaf (Salim & Abu El-Halawa 2002) and
wood (Kiikkilä et al. 2002) materials may serve to decontaminate soils by forming stable complexes with heavy metal ions. This efficacy of mulch in soil decontamination has been comparable to the use of activated carbon. Such decontamination effects not only benefit plant life directly, but also represent another way that mulch bolsters soil ecology.
Application of mulch is usually done in late spring before the soil gets too dry. However, adding mulch during the rainy spring season may cause soil to be too wet and not provide sufficient root aeration (MSU Extension 1998). A second application in autumn is often necessary before cold winters to reduce heaving of tree saplings during the freezing and thawing cycles which damages plant roots (Goulet 1995). The recommended mulching depth using organic mulches should be maintained at 5.0-7.5 cm for effective weed control (Greenly & Rakow 1995; Jordan & Jones 2002; Penny & Neal 2003).
Commercially sold organic mulches are often formulated from materials unavailable to the public in large quantities. These are often by-products from cropping processes and hence may be limited in availability and also localised to the immediate community after prior arrangements were made.
Cocoa bean shells are an example which have been popular where they are available, as they have an attractive appearance, and many people enjoy the aroma of cocoa as it decomposes and adds nutrients to the soil. Unfortunately, there is concern that pet dogs may develop a taste for this mulch and eat it in quantities that lead to gastro-intestinal upsets or even fatal toxicity (Hansen et al. 2003).
Waste wood from sawmills, fallen or pruned trees, unwanted crates and pallets may be traditionally bound for landfills but a modern focus of recycling gives these sources of wood a new lease of use in the form of mulch. These may range from sawdust, wood chips, shavings, and chunks of wood of varying sizes. Bark mulch is sold separately from wood mulch, and may be more popular due to perception its appearance is more natural.
There has been some controversy that the use of wood-based mulches will lead to induced nitrogen deficiency (Bollen & Lu 1957) as a result of the proliferation of soil microorganisms metabolising cellulose. Recent work by TerArvest et al. (2011) showed that soils under wood chip mulch had a high cumulative carbon mineralization and a high C:N ratio, which resulted in some nitrogen immobilisation due to microorganisms using up soil nitrogen for their growth. However, the same paper also found that apple trees in the two wood chip treatments ranked in the top four of the 13 treatments in both fruit yield and tree growth. This favourable outcome of wood chip
mulching in apple orchards was repeated by Choi and Rom (2011) who reported higher leaf nitrogen content and greater trunk diameters of apple trees after 2 years under wood mulch. Atucha et al.
(2011b) also reported a positive nitrogen balance in fruit, leaves, and wood prunings from apple trees mulched with wood chip, despite not being supplemented with nitrogen fertiliser for two years, compared with trees receiving herbicide treatments in bare soil. The literature to date appears to indicate nitrogen immobilisation within five months of using wood mulch, but after two years or more, the nitrogen is made available for plant assimilation.
Another concern raised against the use of wood mulch is the possibility that it may lead to a termite infestation (Schroth et al. 1992); increased population of invertebrates (Jordan & Jones 2007); or even promote fungal growth by harbouring spores and providing a suitable substrate (Brantley et al. 2001), some may find these fungal growths interesting while others find them unsightly. A survey of 2,500 wood chip samples to be processed into mulch led to 114 species of fungi being found (Hoover-Litty & Hanlin 1985).
The claim of termite infestation is disputed by some researchers, who argue that termites are not attracted to wood due to high carbon and lower nutrient content (Long et al. 2001). This backs up findings which confirmed that termites prefer a diet with higher nitrogen and phosphorus content, and will consume wood mulch which satisfy this criterion (Duryea et al. 1999b). To minimise termite infestation in areas of known risk, high carbon wood mulch such as bark material can be used if desired.
Bark mulch may also serve a protectant function against herbicides when applied around the base of trees with shallow roots. This is due to adsorption of herbicide to mulch which reduces the availability, persistence and phytotoxicity of some herbicides (Grover 1971; Smith & Skroch 1995). However, it is hoped that the application of mulch will minimise the need to apply herbicides at all, not only protecting the environment but also reducing management costs as well (Gardiner & Yeiser 1998).
Paper is a highly processed material made from wood, and can be used as mulch when collected from recycled material. It can be used simply by laying down old newspapers on the ground, or purchased as a dedicated landscape product (Crown Zellerbach Corp 1954). Plain newspapers have proven effective in weed suppression (Shogren & David 2006), for more than 3 months and up to two seasons per application (or 16 weeks as determined by Runham et al. (2000)), provided they are wetted after laying, and pressed down with a lawn roller to prevent being wind- blown and better resist weathering (Pellet & Heleba 1995). The drawback is of course in aesthetics,
which necessitates the covering of newspaper mulch with other materials to improve its appearance and to keep it in place. The natural degradation (Brault et al. 2002) of paper mulch may or may not be an advantage, depending on management viewpoint. Oiled paper is available as an alternative which retards the natural degradation of paper (Andersen et al. 1996; Shogren 1999).
To improve aesthetics, paper mulch can also be used in combination with other mulches. Schonbeck (1999) found that two layers of newsprint under straw mulch significantly enhanced weed suppression.
In New Zealand, a commercially available product known as EcoCover is available. This has been tested to give good weed suppression ability for up to 6 months. The degradable nature of this produce is deemed to be advantageous when laid around trees, as compared to black weed mat (Harrington & Bedford 2004).
Being wood-based, there is concern for the potential of induced nitrogen deficiency from the use and subsequent breakdown of paper mulch, as was discussed above for wood mulch. Some research found no conclusive evidence for this (Pellet & Heleba 1995; Jenni et al. 2004); another found that any adverse effect to plant growth was only observed when paper pellets were applied as a layer 50 mm thick but not when only 25 mm thick (Smith et al. 1997). Follow-up research also reiterated that effective weed control was achieved when paper pellets were added at 25mm depth to containerised plants (Smith et al. 1998). Prevention of weed germination by paper pellets was achieved by the absorbance of water which caused the pellets to swell and form an interlocking mat. This effect is contingent on the pellet being of an optimal size and the absorbance property of the paper material used. Separate research in container plants found that paper mulch “retained” nitrogen if fertiliser was top-dressed over paper mulch; but this effect was eliminated if the fertiliser was incorporated into the potting mix instead (Glenn et al. 2000). This again suggests that wood- based mulches have no harmful effect when applied only to the soil surface.