Compost is the result of organic matter – like food scraps, yard waste, and other biodegradable materials – breaking down into a dark, crumbly, soil-like substance through natural decomposition. This process is largely driven by microorganisms (bacteria, fungi, action, protozoa, nematodes) and larger organisms (worms, insects, arthropods) that transform the material into a stable, nutrient-rich product.
Physical Properties
Texture
- A high-quality finished compost is crumbly and loose, containing a structure that is similar to rich soil. This allows for good drainage and aeration, and supports healthy root growth.
- Moist but not wet: finished compost should feel like a wrung out sponge and have a moisture content of about 35-50%.
- When a finished compost is squeezed, the mixture should hold its shape until broken apart. No water should drip out of the material.
- Good compost is highly absorbent and will easily soak up and hold water.
- I think the best compost looks a bit like a nice chocolate cake: spongy, moist, sweet smelling… mmmmmm.
Particle Size
- In its original state, good compost is fine, yet varied: some of the particles are small enough to blend well with soil but with some larger particles to improve aeration and drainage.
- If the finished compost contains large chunks of wood (bigger than your thumb) or any other materials that are not yet decomposed, it is an indication that the composting process is incomplete.
- Compost is usually screened before package, so it can be harder to determine the state of particles.
Colour
- The nicest compost is usually a rich, dark chocolate-brown colour, indicating high levels of organic matter and humic content.
Odour
- High quality compost will have an earthy, fresh smell – kind of like the forest floor. There should be NO foul smells present, like ammonia, garbage, or rotting, which would indicate the presence of undesirable organisms.
Temperature
- Because finished compost is stabilized carbon and nutrients, it should always be ambient temperature. If you purchase compost that is still warm or steaming, it’s not finished and contains volatile chemicals that can harm your plants and soil biology. Ideally, hot compost is aged/seasoned for several months following the thermophilic phase.
- Just because your compost is ambient temperature, it doesn’t mean that it is good or done. Use all points in this list to evaluate the product.
Chemical Properties
Balanced Nutrients
- A good compost typically contains macronutrients (nitrogen, phosphorus, potassium) and micronutrients (calcium, magnesium, iron, zinc) in bioavailable forms.
- The best composts will contain adequate amounts of all minerals required for life!
Stable Organic Matter
- Although Carbon is not technically a nutrient, it is vitally important to biological life. Compost contains a significant amount of stable carbon and humified material, which resists further decomposition and provides long-term benefits to soil.
- Carbon and Nitrogen will be in a balance of 10-1 – 20:1, allowing for sustained microbial activity and nutrient release.
pH
- Neutral to slightly acidic (pH 6.0–7.5), creating a hospitable environment for plants and soil microbes.
Humic and Fulvic Acids
- Rich in these organic acids, which improve nutrient exchange, root absorption, and soil buffering capacity.
Low Salinity
- Free from excess salts that could inhibit plant growth, ensuring suitability for sensitive plants.
- Some commercial composts that are made with animal manure can contain excessive levels of salt. One way to test, is to grow a tray of seedlings in 100% compost and look for inhibited plant-growth. This is also a good way of checking for persistent herbicides, which can last for several years in the finished compost, surviving both the animal digestive tract and the composting process.