Compost maturity and stability are not the same

A mature compost is usually stable, but a stable compost may not be mature. Yet, both products have their uses. Though the terms “maturity” and “stability” are often used interchangeably to describe compost, they should not be.


Look at a red and green tomato. Both are stable and edible. But the green tomato won’t be mature until it turns red. This work-in-progress tomato is a bit on the tart side with firmer flesh that holds up when fried. The mature red one is sweeter, softer, and makes a great sauce.

As distinct products, mature and immature composts have their specific characteristics and uses, too. But like red and green tomatoes, they’re definitely not the same.

Compost maturity and stability

MATURITY – All organics will eventually decay until nothing remains but atoms. The trick is to reach a degradation phase where the easy stuff is gone, leaving only dark, slow-to-degrade, earthy-smelling material behind. That’s a mature compost.

Between the raw waste and finished compost, however, are a series of degradation steps that aren’t that beneficial to plants. In an immature state, compost can release compounds harmful to plants, fight with plants for oxygen, and pull nitrogen out of the soil.

Compost maturity is best determined by testing, which is a good reason to insist on seeing a recent lab report for the compost under consideration. Maturity indicators on lab reports include:

  • C:N ratios
  • Germination rates
  • Oxygen uptake

Maturity assumptions based on curing time are also recognized within the industry, but may not be as reliable as testing.

STABILITY – If a compost passes the maturity test, it is a stable, market-ready product. In a mature compost, microbiological activity slows because all the “easy” food has been consumed.

But there are conditions within the composting mass that can cause product to enter a stable state without reaching maturity.

Compost that has been dried to remove moisture, for example, makes it lighter for shipping, but can exhibit reduced biological activity, as well. The same thing happens if the pile is deprived of oxygen.

Unfortunately, once moisture or air has been reintroduced, microbial colonies can reestablish and return to active feeding. Pathogens can rebloom and odors resurface as the composting process resumes.

Germination tests remain one of the best indicators of mature stability. If the compost exhibits no indications of phytotoxicity in conjunction with good pH ranges and slowed microbial activity, then the product has probably passed into the mature range.

If trying to evaluate stability while standing next to a pile in a landscape supply yard, look for:

  • A light, porous, evenly-textured product that encourages good air flow
  • A compost with sufficient moisture to stick together when squeezed in the palm of the hand without crumbling or dripping water
  • A pleasant, earthy scent

Selecting the right product

In the absence of testing information, the easiest way to gauge a product’s maturity is to smell it. Compost that smells like soil has likely reached a stable, mature state and is ready for use anywhere and by anyone.

Product that still retains some pungency isn’t stable or mature. It’s not quite ready for unrestricted use. But, provided it has met minimum quality standards for pathogen and vector reductions (as specified by regulations), the compost can be applied in rural areas away from sensitive noses where its higher NPK value is much appreciated by farmers.

Time and nature will finish the job of product maturation and stabilization.