I CAN shed light on Coun Orrell's and Jeremy Fox's confusion about methane gas.

Methane is a natural gas produced mainly by bacteria which live in oxygen-free and low oxygen environments.

This is why methane bubbles out of waterlogged bogs, seasonally flooded forests, reservoirs and lakes and landfills.

There are also large gas-fields of 'fossil' methane, buried in rocks and now running out as we use the gas as fuel and chemical feedstock for many materials including fertilisers.

When the gas gets into the atmosphere it slowly changes into carbon dioxide and water - but before it does so, it has a powerful 'greenhouse effect', trapping heat 23 times as effectively as carbon dioxide (New Scientist, 8/10/05) and thus warming the planet.

Biodegradable or organic matter initially decays in air to carbon dioxide and water, and only if the locally available oxygen is exhausted do the anaerobic bacteria grow, multiply and give off methane.

This happens in landfill containing biodegradables when it is compacted and methane escapes before it is capped. Turning your compost heap or adding scrunched-up cardboard keeps the pile aerobic, allowing composting to work.

The reason our garden waste is now being collected separately is because too much of it is going in landfills, causing pollution and filling them up unnecessarily.

The garden material will not be 'stockpiled' in a way which lets it get anaerobic. Professional composters mix, shred, turn and water their heaps so they heat up, rot down quickly and remain aerobic and sweet. The compost produced, once tested for contaminants, is good for gardens and allotments.

However, this is an energy-intensive process and it is better if people compost at home or use a community composting site. York Rotters can help you home compost (phone number: 01904-411821).

John Cossham,

Garden House,

Hull Road, York.

Updated: 09:10 Tuesday, October 11, 2005