Changing from Peat based to green waste compostst...?

T

The closing of one era to the acceptance of a new age will always meet with resistance and a well rounded argument against accepting new regimes; successes built on past activities will be hard to dislodge, the arguments for protecting the environment, healthier living, better crop management and protection will meet with a number of well trodden verbal retorts.       
Composts are very different from peat, and need to be handled differently from peat based growing media. Composts have naturally high nutrient levels and relatively high bulk density, and for this reason, rates of compost inclusion in a growing medium are limited, usually to a maximum of 40-50% by volume, depending on the type of growing medium to be made. The compost must therefore be blended with an appropriate low nutrient/low bulk density material such as peat, bark, coir fibre or wood fibre in order to reduce the bulk density of the mix and, if necessary, reduce the nutrient content. High nutrient levels are associated with a high total salt content (measured as Electrical Conductivity (EC) ) and such levels can damage plant roots, germinating seedlings and young plants. Growers must recognise that growing media based partly on composts differ from those based on peat, and they are likely to need to modify nutritional regimes to reflect the differences in compost-based growing media. High bulk density tends to make the mix less free-draining, it also causes problems with handling of plants in pots and higher transport costs. Growing media based partly on compost is likely to behave differently from peat-based composts when watered. Growers may have to adapt their irrigation regimes in order to ensure that crops receive the correct amount of water.
Some compost can increase problems with insect pests such as sciarid and shore flies when used in certain situations (usually production in glasshouses or polythene tunnels, where warm temperatures and moist conditions can be ideal for the insects to breed).
Growing media are handled by both the gardening public and by workers on nurseries, therefore any ingredients used in them must be safe from a human health point of view and free from human pathogens and any glass or sharp material. Although the standards for BSI PAS 100 quality composts stipulate absolute minimum levels of glass, compost producers wishing to supply into the growing media sector must consistently produce products which contain no sharp materials of any sort. Contaminants such as plastic, whilst not dangerous, are also undesirable because the visual appearance of the end product is important.
The specific end use of the growing medium must be taken into account when using compost
as an ingredient. Crops which are sensitive to high pH and high EC (for example Azalea and Rhododendron) are less suited to being grown in media containing composts than other species. Crops grown to schedules and stringent supermarket specifications are also in a higher risk category (for example indoor pot plants). This is mainly because there is often little flexibility in terms of time or crop performance if the crop grows more slowly than the rate which is expected in a crop grown in more familiar peat-based compost. If a crop does not meet the required standard by the appropriate week number during the year, there are few alternative markets for it and the crop may have to be disposed of, with obvious financial implications. There are however, good commercial examples where more sensitive plant species/types are being grown successfully in growing media based partly on composts.