Making the most of green waste

Trials have shown that, processed properly, green compost can make a viable alternative to peat.

Green waste is often composted at waste treatment plants. However, some growing media manufacturers are composting at the site of manufacture. This undoubtedly gives them more control as to the feed-stock that is used and how it is composted.

Turned or mixed windrows are used — the "open turned windrow" system is the most common means of composting green waste in the UK, because it is a versatile system, allowing feed-stock to be added to windrows as it comes on to the composting site.

The open windrow system is also cost effective, requiring only concrete pads and machinery. Green waste is initially shredded as it is put into the windrows for composting, then screened down to approximately 25mm. It is then sold as mulch or recomposted and typically screened to 10-12mm. Most is sold at 0-10/12mm. Screening and therefore maturation time is often determined by the market demand.

Green compost that is to be used in growing media must be mature. Immature compost can lock up nutrients, particularly nitrogen, as the result of rapid microbial activity. Depending on the effectiveness of the primary composting process, two to nine months of curing are usually required to produce compost that is stable, cool, odour-free and dry at 20-30 per cent moisture.

Commercial green-waste compost only bears a superficial resemblance to the organic composts growers have historically used. Many commercial compost plant managers view their operations as disposal facilities rather than production facilities. If green compost is to be used in growing media there should be a more significant focus on quality.

High-quality green compost should comply with the Waste & Resources Action Programme’s (WRAP) Guidelines for the Specification of Composted Green Materials used as a Growing Medium Component, which builds on the BSI PAS 100 standard, setting limits for all the requirements of growers as end users.

There is scope to use a percentage of the higher-quality green waste in growing media, providing a consistent supply can be obtained, particularly for more vigorous species. The most common problems with green compost are high pH, high nutrient levels, high bulk-density and sometimes a high initial salt content (if the compost is not completely cured) resulting in high conductivity. For these reasons, using green compost in media for ericaceous or other salt-sensitive plants is not recommended.

The high bulk-density of green compost can pose problems in growing media. The fresh density of mixes used for container substrates range from 200-400g/litre. The densities of green composts are much higher due to the amount of soil around the roots of bedding and other plants that green compost is made from. Organic matter -content is often less than 30 per cent. There is evidence that particles of less than 1mm in diameter within the compost fill the air spaces within the mix, thereby reducing the air-filled porosity. As a result, limits may need to be set to reduce the proportion of fine particles, or alternatively these could be screened out. This could help reduce the amount of mineral matter, potentially reducing bulk density and improving -drainage and air capacity.

To lower alkalinity, most green composts can be supplemented with sulphur or acidic potting amendments. This will lower the pH to between 5.5-7, depending on the crop. Due to the other characteristics of green compost, such as low air-filled porosity, it is more practical to dilute green composts with lower nutrient, lower bulk-density material such as wood fibre, bark or coir to produce a suitable growing medium.

Industry sources suggest it is likely that only larger processors operating sites composting at least 25,000 tonnes a year of green waste will be able to invest in the necessary machinery, monitoring and testing required to ensure the production of consistently higher-quality materials. Composted material has to be mixed with other ingredients to make growing media with suitable physical and chemical characteristics.

Trials carried out by WRAP have shown that media containing at least 25 per cent composted materials are producing plants that are marketable to the same grades as the peat-based media. Compost mixes containing green compost also tend to produce more compact plants than 100 per cent peat mixes. At this rate of inclusion, bulk density should not be too much of an issue when it comes to transporting finished plants, as it is often crop bulk rather than weight that affects lorry fill. Growers producing larger specimen stock may need to think about extra weight in relation to handling stock.

There is evidence that the incorporation of green compost into growing media can suppress liverworts. In trials, no liverwort was found on plants grown in green compost or mixes containing it; however, liverwort was found on plants grown in peat-based growing media. This will be of interest to many nursery stock producers, for whom liverwort is a major problem. Liverwort suppression by green composts may be due to the high level of biological activity in the mix, the drier surface of the pot or high levels of salts.

Green compost at 100 per cent did not slump in the pots, although in some, it did shrink away from the edge of the pot, making watering difficult. However, this is unlikely to be such an issue when it is mixed with other materials.
The benefits of liverwort suppression are huge when you consider the number of man-hours required for its removal.

When green compost is used as pot mulch at potting, it sets with a relatively firm crust. It does not, therefore, fall away from the compost when mature plants blow over. Growers also have the benefit of disease suppression at the compost surface, with water splashing up into the plant canopy each time the crop is irrigated. As with any product, there are drawbacks: its high pH renders it unsuitable for use on ericaceous crops; it contains small bits of plastic plant labels and fragments of glass — the latter has the appearance of coarse sea salt with no sharp edges, which could be considered unsightly, although both contaminants are so small they are relatively inconspicuous.

A growing-media mix containing green compost will have a have a higher population of microbes than a peat mix, which may be advantageous for suppression of root pathogens in the same way that media containing bark suppresses root pathogens.

The heating process of composting, where temperatures of up to 65°C are reached, results in pasteurisation that kills pathogenic micro-organisms and most weed seeds. This allows beneficial -organisms, such as certain bacillus bacteria, to accumulate. These bacteria are thought to be at least partially responsible for the disease supressiveness of some composts. It is thought that beneficial micro-organisms out-compete those that cause disease.

The use of green compost may therefore have the potential to reduce pesticide -requirements. Green composts have been proven to suppress Phytophthora -nicotianae and Rhizoctania solani by Warwick HRI. This ties in with many leading growers’ -experiences with compost tea -(derived from green/humic composts) where Botrytis and Rhizoctonia are suppressed, allowing significant reductions in pesticide inputs.

Researchers believe there are three mechanisms behind the natural fungicidal property of compost: the action of a specific microbe (specific suppression), competition for nutrients (general suppression) and, to a lesser degree, production of fungus-inhibiting by-products (chemical suppression).

Eradicating pathogens and nematodes

The eradication of pathogens from organic waste during composting is due to:

•    heat generated during the thermophilic phase of the composting process;
•    the production of toxic compound such as organic acids and ammonia;
•    lytic activity of enzymes produced in the compost;
•    microbial antagonism, including the production of antibiotics and parasitism;
•    competition for nutrients;
•    natural loss of viability of the pathogen with time;
•    compounds that stimulate the resting stages of pathogens into premature -germination.

Heat generated during the thermophilic, high-temperature phase of aerobic composting appears to be most important for the elimination of plant pathogens.

The moisture content of organic waste can influence the temperature tolerance of micro-organisms. The occurrence of dry pockets within composting material is probably the main cause of pathogen survival where eradication is expected.

Probability studies have shown that the risk of pathogen survival in windrow systems is small, provided the windrows achieve the stipulated average temperature of 64-70°C and duration of 21 days, and are turned at least the specified number of times.

For most pathogenic fungi, oomycetes, bacterial pathogens, nematodes and some plant viruses, this peak temperature for a duration of 21 days was sufficient to reduce numbers to below, or very close to the detection limits of the tests used.

Some viruses, such as Tobacco Mosaic Virus (TMV), require a higher compost temperature for eradication but can be -eradicated after 26-weeks composting at 31°C. One site has claimed that temperatures of around 75°C have been reached in windrows and that composting takes at least two months, during which time windrows are turned at least three times. This should provide green compost with a small risk of pathogen survival, suitable as a -growing-media constituent with many valuable qualities.

Green compost brings many benefits when used as a growing-media constituent and mulch; it is sustainable, usually locally produced and is competitively priced.

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