What is Compost? - Different types of compost explained!

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Compost and composting is an extremely wide subject and can prove a little bit confusing at times. Below you will find all the information you need to make the right decision when it comes to selecting the right compost for your garden, whether it’s buying ready-made bags, selecting an additive or making it yourself.

 

Compost is loosely described as ‘decaying plant material that is added to soil to improve its quality’.  When good, its appearance should be that of a brown crumbly mass in which no original waste plant or animal material is distinguishable. It generally has an earthy smell, is dark brown/almost black, is damp, sticky or pliable and comprises mostly of fine particles. But here’s the issue - compost varies -massively!  Think of it as scale – at one end, immature compost impedes (hinders!) plant growth, in the middle it is OK (helps) and at it’s very best it is “black gold” and the gardeners best friend with a positive effect on plant growth.

 

PAS 100 compost

This is the most common type of compost, often found in garden centres.  The goal of the PAS100 standard is to remove ‘poor’ immature compost, which causes gardeners problems, and to ensure that our green waste is recycled and can be used in gardens with benefit.  This variety is for general use and is usually fairly low cost.

 

Potting “composts, seedling “composts” and John Innes 1,2,3

The supplier is “designing” a growing media that functions as well or better than soil for seed germination, potting up or container growth. Ingredients for these composts vary greatly and can include all or some of the following ingredients bark, loam, coir, grit, sand, peat, wood waste, food waste, manure. These varieties are great for filling pots and using in seed trays but can prove to be a very expensive soil improver! (technical note – none of these will be labelled PAS100)

 

Multi-purpose composts (sometimes sold as “all purpose” compost)

Traditionally the main ingredient of all-purpose compost was peat – but peat is being phased out and coir (coconut husk fibre) or wood pulp fibre is replacing it. 

Here the supplier is trying to make one compost fit all needs – potting, seedlings, soil improver. A lot of these are based on UK sourced raw materials (i.e. organic matter, green waste compost) mixed with other ingredients like coir, bark, wetting agents etc to enhance performance, which can be a very challenging balancing act. 

 

Boosting nutrients in the soil

Our garden soil loses nutrients every time a crop is harvested, but there are lots of things you can do to give it a boost. Organic matter is generally added directly to garden soil to enrich and improve its structure; it helps the soil to retain moisture and provides beneficial nutrients. If you have a compost bin or composter at home then you will already be producing organic matter by adding grass cuttings, fruit and vegetable peelings, garden prunings, fallen leaves, tea bags, egg boxes etc., once well-rotted it can be dug into the soil and act as a conditioner, helping to improve its overall health which will have a beneficial impact on the plants or vegetables you grow.

 

Other Additives - Humus & biochar

One of the most beneficial by-products of homemade compost is small amounts of colloidal humus. This brown/black substance provides a multitude of benefits (read more about humus here) and helps improve soil to the best it can be, improving plant fertility and resilience. Adding humus directly to your composter or the soil is a great way to boost your organic matter and overall soil health, another way is to add biochar, which is a form of charcoal used to improve compost and topsoil. This fine black powder is also massively beneficial providing excellent water retention properties, it improves the nutrient supply to the roots, it also supports the formation of colloidal humus and has some extremely good environmental credentials; capturing carbon to offset your carbon footprint (to read more click here)

 

Manures

Well-rotted horse, cow and chicken manure are also excellent additions to the soil and will add nutrients and improve the texture and structure of the soil, however, if you are an organic gardener be aware that additives from animal feeds. Antibiotics, and/or supplements (e.g. vitamins and minerals) that might not meet the organic origin definition may be present.

 

Fertilisers

You might be wondering if you need to use a fertiliser if you are adding organic matter (compost) and other soil improvers that provide so many benefits. If you take nutrients away from the soil (i.e. in your crops and cut flowers) then you need to add them back for the following year. A good organic system will replace the nutrients via compost (organic matter). However, when heavily cropping you may need a little help. You can top up the macro-nutrients (NPK) using either a solid (powder) fertiliser or a liquid fertiliser. There is growing evidence that adding nutrients (as chemicals) decrease the microbial activity in the soil. There is growing awareness that most nutrients get to plant roots via microbial and mycorrhizal fungi interactions. SO you need a healthy microbial soil, not just a soil loaded with nutrients. The good news is that certain types of biochar appear to help this soil microbial system prosper. SoilFixer is not alone in believing this ‘biological’ impact will be the unique contribution biochar makes to plant growth and soil improvement!

 

Further reading

What is super compost and how to make it - https://www.soilfixer.co.uk/SF60-super-compost-Detailed-FAQ

How to make the best use of your compost - https://www.soilfixer.co.uk/Making-best-use-of-your-compost

Reusing potting compost - https://www.soilfixer.co.uk/Reusing-Potting-Compost