Coconut coir (‘coconut fibre’ or simply ‘coir’) popularity has grown exponentially in recent years and is the most used alternative substrate both on its own in hydroponic production or in growing media mix. This is due to its good air and water holding characteristics, its ease to re-wet without losing its structure and light weight.

Coir is the fibrous pith in the coconut husk, a co-product of the coconut production industry. Once extracted from the husk, it is processed into different sizes from larger chunks or chips, to finer ground material, and frequently compressed before transportation.

These processes rely on high water and energy usage, and leaching, to get rid of unwanted sodium and nitrogen in the substrate is a cause of pollution. Coir production has the highest impact on ‘ecosystem quality’ and could also be improved in term of social impacts. It is imported and therefore has increased costs and carbon footprints, and can face challenges around availability. The potential for increasing the overall supply of coir is limited and largely determined by the overall demand for coconuts.

Green compost is used in soil improvers and growing media, primarily for the hobby market.

In the UK, ‘green compost’ is produced from separately collected plant wastes (e.g. parks and garden plant waste), in accordance with the British Standards Institution (BSI) PAS 100, which should limit problems linked with plant pathogens, human pathogens, and weed contamination.

While there is availability of this material, its suitability for growing media can be affected by the need for consistent quality and availability at appropriate times of the year. Green compost’s high electrical conductivity (EC) and potassium (K) concentration, as well as a low (5–10%) carbon efficiency and a relatively high pH, are limiting factors. Therefore green compost can be used in growing media mixes (up to 50%), but not as stand-alone substrates, which is more common in commercial soilless productions. The distance between sources of this high-bulk density material and manufacturing facilities needs to make economic sense. More importantly, contamination – both physical (e.g. unwanted plastic shredded into the mix) and from herbicides residues – is a significant challenge with green compost, that needs to be considered.

Horticultural rockwool (or stonewool) use as a substrate for the propagation and production of crops has expanded since the 1970s, due to its water-absorbent capacity. Rockwool growing plugs or slabs are very popular in hydroponic setups, as they offer both an excellent air-water ratio and a natural sterility, making them less susceptible to disease.

Rockwool is a fibrous “wool” or flock material derived from a mixture of volcanic rock, limestone, and coke (baked coal). It is heated to between 1,500 and 2,000 degrees Celsius, creating tiny fine fibres that are spun into what we know as rockwool. Under these extreme conditions, near irreversible chemical bonds form.

These irreversible bonds means that rockwool does not breakdown over time. On the plus side this enables plugs/slabs to be reused (after sterilisation treatment such as steaming). However, that stability of the rockwool material, also means that it isn’t biodegradable, making disposal in landfills an issue, and currently there are only few (and expensive) recycling programs available in the industry (e.g. to make bricks or insulation for the construction industry).