'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.