Organic matter

Organic matter is made up of decomposed/decomposing plant or animal material, which is used in gardening in a number of ways:

Below is a table of some of the common organic matter alternatives for gardeners, along with their possible uses and other information. For more detail on their uses, please see the relevant ‘how to…’ section.

Can be used in…
Soil improver Mulch Potting compost Positives Negatives
Bark – composted Yes Yes Yes Good moisture retention.
Excellent for soil structure improvement, particularly on heavy soils.
Can contain toxins, diseases and pests if not from a reliable source.
Bark/wood chippings No Yes No May be acidic if from conifers – good for ericaceous plants.
Can be an attractive and child friendly mulch.
Can draw nitrogen out of the soil as it decomposes.
If from conifers, should not be used around acid hating plants.
Can contain toxins, diseases and pests if not from a reliable source.
Bracken – composted Yes Yes Yes A good way of using this plant which is often considered a weed and cleared for conservation reasons.
Can also be packed around tender plants to insulate them over winter.
Bracken contains carcinogens and therefore should not be gathered in late summer when the spores are released and could be inhaled.
Coir/cocoa fibre Yes No Yes Good moisture retention.
Unlike peat, can be re-wetted easily.
Can lock up nitrogen as it decomposes.
Farmyard (horse) manure Yes Yes No Good moisture retention.
Generally has a good level of nutrients.
Excellent for soil structure improvement.
Can contain weed seeds.
Garden compost Yes Yes No Cheap – in fact it’s free!
Recycling garden and household waste.
Is only as good as what you put in it.
Grass clippings No Yes No High nitrogen content.
Cheap – if you have a lawn.
Useful additive for garden compost to balance out carbon rich materials.
May contain harmful herbicide residues.
Leaf mould Yes Yes Yes Cheap – can be made at home.
Good moisture retention.
Looks attractive as a mulch.
Unless they are from trees growing in acidic conditions (or conifers) they may be slightly alkaline (due to a high calcium content) so good for around acid-hating plants.
Pine needle leaf mould is valued in the propagation and growing of ericaecous plants.
Can take some time to compost – shred thick leaves first and put evergreen/conifer leaves on your compost heap instead.
Peat (sphagnum moss peats or sedge peats) Yes Yes Yes Considered the best option by many gardeners, despite environmental issues.
Excellent for soil structure improvement.
Good moisture retention (more so sphagnum moss peats).
Can be nutritious (sedge peats).
May be acidic – good for ericaceous plants.
Enviromental concerns about the impact of peat farming.
If acidic, should not be used around acid hating plants.
Once dry it is difficult to re-wet.
Seaweed Yes Yes No Can be nutrient rich (particularly nitrogen, potassium, phosphorous, magnesium and micronutrients).
Can be dug into soil/laid as a mulch fresh, there is no need to compost it.
You may wish to wash it in fresh water first to remove excess salt (although the salt levels in seaweed usually aren’t too high).
Spent hops and grains Yes Yes No Cheap.
Recycling an industrial by-product.
Hops are poisonous to dogs.
Spent mushroom compost Yes Yes No Cheap.
Recycling an industrial by-product.
Can be alkaline (if it contains remnants of chalk packaging), good to reduce soil acidity or for lime loving plants.
Good moisture retention.
Should not be used around ericaceous plants or around fruit trees.
Not for use in potting compost as it may contain high levels of salts.
Straw Yes Yes No Use to open up compacted soil.
Can also be packed around tender plants to insulate them over winter.
Can draw nitrogen out of the soil as it decomposes.
May contain harmful herbicide residues.
Little nutritious value.