Compost: what, why and which one?

Compost: what, why and which one?

We buy metric tons of compost every growing season, but what is it actually made of and why do we need it?

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What is Compost?

Compost is the name given to decomposed organic matter which, through a process of natural re-cycling, results in a commodity that is rich in the fundamental nutrients for maintaining the essential microbial flora and fauna within soil that ultimately promotes and enables healthy plant growth.

Compost added to soil, ensures that an otherwise inert substrate can sustain the vital food webs that together, maintain and condition the subterranean environment for plant roots to function effectively. Roots that feed the surface plant life that all terrestrial life depends on.


freshly picked carrots

How does Compost work?

Besides providing a vital supply of the micro and macro nutrients that plants require for essential growth, compost creates the perfect environment for vast numbers of soil organisms to thrive within, including the important microbes (bacteria and fungi), some of which form symbiotic relationships with plant roots. The Mycorrhizal fungi for example, improve the effectiveness of root hairs in acquiring water and soluble nutrients, ensuring that the plants are healthy and more likely to overcome problems associated with pests and diseases. Macro organisms, such as the earthworms that feed within the compost, improve the structure of soil by aerating it and improving drainage and root penetration.


worms in healthy compost

Why should compost be used where plants are cultivated?

In uncultivated environments such as woods and grasslands, the process of composting goes on as a natural process, where fallen leaves and other dead organic matter is naturally broken down by the soil organisms and re-cycled back to the plants and the trees. However, in cultivated areas, which includes the majority of home gardens, a cycle of successive new plantings usually occurs, and the old, dead plant material is not usually left to decompose naturally. The soil is therefore constantly being depleted of organic matter and associated nutrients and if healthy plant growth is to be sustained, the organic matter and nutrients need to be replenished, either by adding compost into the soil for new plantings or by mulching the established flowerbeds and borders. For this purpose, many different composts are now commercially available, but these vary considerably in their composition, their performance and their impact on the environment.

What are the different types of compost?

When plants are grown and maintained in pots and containers, a compost is often used on its own, rather than as an additive to soil, and for this horticultural use, a range of different composts exist. Some have been formulated for particular stages of a plant’s growth, such as the fine structured composts that are low in nutrients and are used for germinating seeds. Others have a more fibrous structure and are used for planting-on purposes, whilst others cater for plants with different pH requirements, as well as those that are seasonally maintained in pots and containers requiring specific and regular feeding regimes.

But for the general use of composts, either as growing mediums or for soil enhancement, there are basically five different types:

  1. The Peat-Based Composts – Despite the primary component of these composts being peat, many different formulations and blends are available. These have been designed for different uses and can contain additives such as slow-release fertilisers, wetting agents, pH stabilisers, many of which might have been synthetically produced and therefore not suitable for organic growing. Peat is also obtained by removing it from natural peatlands which are valuable havens for wildlife and therefore its removal has an environmental impact.

  2. The Peat-Free Composts – These composts use natural alternatives to peat as their base component and aim to provide a similar growing medium to the peat-based composts. The alternatives can be coir, wood fibre, or re-cycled composted green waste. Additives are also used in these composts, which could include synthetic fertilisers. Maintaining consistency between batches is also difficult with re-cycled green waste.

  3. Loam-Based Composts - These composts contain soil as their base component. Additives such as peat, sand and grit can then be used to provide a blend which may be enhanced with fertilisers, wetting agents and other types of synthetic compounds. Different blends are available for different growing purposes.

  4. Organic Composts - These composts are solely derived from natural sources. Some of these utilise the depleting natural peat reserves, whilst others use composted bark and wood fibres as their base component. Natural additives from either animal or plant waste provide the nutrients for plants.

    However, a new era of organic composts, derived totally from plants and providing all the vital natural ingredients that are essential for healthy plant growth, has recently been pioneered and perfected by PlantGrow. A Norfolk based company, PlantGrow has developed and utilises an anaerobic digestion process that releases the nutrients that are held within harvested plants and generates high quality natural compost for incorporating in to the soil and feeding plants.
  1. Multi-Purpose Composts – Are blends of other composts and encompass all the different aspects that define the other compost types.


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