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Compost

Also known as Garden compost, Green compost, Humus

Growing mediumDepends
Materials breaking down in a compost bin Photo: Ksd5 · CC0

In short

Compost is decomposed organic material such as garden waste and food scraps. It adds nutrients, organic matter and beneficial microbes to potting mixes and garden beds, but on its own it is too dense and variable for most pots. In British English, “compost” also means bagged potting mix.

How it looks

Finished compost is dark brown to almost black, crumbly and resembles dark topsoil, with an earthy, forest-floor smell. It should pass through a 3/8-inch (about 1 cm) screen with no rocks or rubbish. Unfinished compost still shows recognisable scraps and may smell of ammonia or sulphur.

Signs

Immature compost can stunt or burn seedlings because of phytotoxins and nitrogen tie-up. Compost made from manure or mushroom waste can be salty, causing leaf burn.

What favours it

Finished compost typically has pH 6.0–8.0. Nutrients are released slowly by microbial activity, more slowly in cool conditions.

Prevention

  • Use only mature, earthy-smelling compost in pots.
  • Blend compost into potting mix rather than using it alone; SDSU notes gardeners use 10–50%, with about 25% most common.
  • Use manure-based compost sparingly because it can be high in salts and nutrients.
  • Avoid home-made compost for seed sowing because of damping-off and weed risks.

Treatment

Start with the first step and move down only if the problem persists.

  1. If plants show salt burn, flush the pot with plenty of water and repot into a lower-compost mix.

Pros and cons

Pros

  • Supplies nutrients and organic matter.
  • Stimulates soil microbe activity; compost teas and extracts have been found to inhibit some plant pathogens such as Fusarium oxysporum.
  • Recycles local waste and has a small carbon footprint when made at home.

Cons

  • Variable quality, nutrient level and pH, especially home-made or green-waste compost.
  • Too dense on its own; packs down and reduces aeration in pots.
  • Immature or manure-based compost can harm plants through phytotoxins or salts.
  • Can contain weed seeds and pathogens if not well made.

Look-alikes

Worm castings
Castings are finer, more uniform granules made by worms, usually used in smaller amounts.
Garden soil
Garden soil contains mineral sand, silt and clay and is heavier than compost.

Typical uses

Vegetables, herbs, annuals and garden beds; used in moderate amounts in container mixes. Not suited to carnivorous plants, which need nutrient-poor media.

Sources

  1. Compost — Wikipedia
  2. How to use compost in gardens and landscapes (EM 9308) — Oregon State University Extension
  3. Using Compost in Raised Beds and Containers — South Dakota State University Extension
  4. Growing Media (Potting Soil) for Containers — University of Maryland Extension
  5. What are the alternatives to using peat-based compost? — RHS

Written in our own words from these sources. Check a product label before using any treatment.