Organic matter: what is it?
Organic matter is a much used term that can refer to several things, and is easily confused with similarly named activities such as organic gardening. It is no surprise that gardeners are often confused by this very useful piece of horticultural shorthand.
Timing: All year
What is organic matter?
'Organic' means derived from living matter, but in chemistry means carbon-containing compounds that are usually but not always derived from living things. Organic can also refer to unity of things.
'Organic' in horticulture often refers to the practice of organic gardening which relies on materials derived from living things; materials that do not derive from living sources are not allowed - chemical fertilisers, for example. Although they have special importance in organic gardening, the use of organic matter and its importance applies to all gardens and soils.
The organic matter content of most soils is fairly meagre and benefits from being enriched by the gardener. To achieve this manures, composts (garden and green waste) and other materials are added to the soil. These materials are sometimes, and helpfully, called bulky organic matter. They may also be referred to as 'soil improver' or 'soil conditioner'.
The term 'organic matter' is used for both organic matter in the soil (better called soil organic matter), and the many manures, composts (garden and green waste) and other organic materials added to the soil to increase the organic matter content.
All organic matter derives from living things, mainly plants but also animal origin. Some materials are best rotted or composted before use in order to have a favourable carbon:nitrogen ratio.
Fertilisers are materials that contain more concentrated nutrients than composts or manures and may be chemical or organic. Organic fertilisers are derived from living things such as abattoir wastes, agricultural wastes, fish processing wastes, seaweed amongst others. Their nutrient content is usually not quite as high as chemical fertilisers and being concentrated not enough is added to significantly affect the soil organic matter status. They usually release their nutrient content more slowly than chemical fertilisers as they have to rot before the nutrients become available to plants. Slow release of nutrients is also a characteristic of manures, composts and other bulky organic matter and is a desirable horticultural feature that leads to healthier and higher quality plant growth.
For notes on using organic matter in the garden, see our advice page.
Examples of organic matter
Manures and composts widely used by gardeners include:
- Animal manures: faeces, urine and bedding from farms and stables, that may be rotted or unrotted
- Bark chips: chipped bark and other uncomposted woody waste can deplete soil nitrogen so are best used as a mulch on the soil surface
- Cardboard: best composted before adding to the garden
- Coffee grounds: provides low levels of nutrients and can either be used in small quantities around plants or composted before adding to the garden. It's effectiveness as a slug deterrent is unclear
- Fallen leaves: can deplete soil nitrogen if used without composting, either on their owns as leafmould or in with garden compost
- Garden compost: composted garden and household (kitchen) waste
- Green manures: not a good source of organic matter being largely water
- Kitchen waste: widely available but best composted before use
- Lawn mowings: widely available but best composted with a carbon rich material such as straw before use
- Leafmould: fallen leaves rotted on their own with no other additions
- Poultry or chicken manure: only limited quantities can be applied due its richness in nutrients. Most valuable as a compost activator or replacement for fertilisers
- Municipal compost: composted garden, household and trade wastes, usually at higher temperatures than home composting so less troubled by weeds, weed seeds and disease
- Mushroom compost: locally abundant and inexpensive, can be alkaline so best suited for use on acid soils and for vegetable crops
- Peat: gardeners are urged not to use peat to improve soils as better, less environmentally damaging alternatives available
- Soil improver/soil conditioner: alternative names for well rotted organic matter
- Straw: cereal straw depletes soil nitrogen as it rots and is best composted with nitrogen rich materials such as grass clippings before use
- Woodchips: chipped timber and woody garden waste (e.g. prunings and hedge trimmings) deplete soil nitrogen as it rots so best to compost first or use as a mulch on the surface
Examples of less common bulky organic matter include:
- Animal slurries: liquid farm manures usually too objectionable to be used in gardens but widely used by farmers
- Biodigester wastes: liquid wastes from recyclable power schemes, too objectionable to be used in gardens but increasingly used by farmers
- Bracken: locally available
- Guano: bird or bat droppings accumulated over long periods on dry islands or in caves, occasionally available
- Paper wastes: waste product of paper industry usually only available to farmers
- Sawdust: waste material that depletes soil nitrogen as it rots
- Sewage solids: solid materials form sewage treatment works. Usually only available to farmers as potentially hazardous in garden situations
- Spent hops: waste product of brewing industry sometimes offered to urban gardeners
- Wool waste: traditional waste product from woollen industry, occasionally available
Composts, manures and potting media
Manures are strictly speaking derived from animal faeces, urine and bedding, typically straw but sometimes wood chips or hemp fibre. Manure can be 'fresh' straight from the farm or stable, or it can be well-rotted. The latter is much more hygienic and easy to use, but the former can be richer in nutrients. Any manure with recognisable straw or wood chips is best stacked and allowed to rot for a season, ideally under cover or at least covered with a plastic sheet to exclude rain.
'Composted manure' is often offered, usually baled or bagged, and this manure has not just been stacked but it has been turned or mixed, wetted in dry weather, resulting in a very uniform relatively hygienic product that is usually more expensive that manure from the farm.
Compost (not to be confused with 'potting compost') or green waste is organic waste material from gardens or waste collection services that as been stacked, in a garden situation ideally in a suitable compost bin, turned as required and allowed to rot. Garden compost is often very variable in texture, but it is still a good soil improver even when lumpier than expected.
Unfortunately the term 'compost' is commonly used to mean the material used to fill pots, seed trays and containers. Both compost and manure are too rich and too poorly drained to make good potting or growing media. Potting 'composts' also are an expensive and not especially effective way of improving soil.
Unfortunately some weedkillers used in manure and compost production can contaminate the end product causing severe damage to garden plants.
Organic matter can be very polluting. It is good garden practice to cover organic matter to exclude rain until it is used. Organic matter can seep nutrient rich liquid. This liquid 'run-off' and it should not be allowed to enter drains, ditches, streams or other surface water.
Seek advice before using novel materials such as dredgings from lakes or composted industrials wastes as these can potentially contaminate the soil with unwanted chemicals.
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