How Do I Know When My Compost Is Ready to Use?

Using compost before it is properly finished can actually harm plants rather than help them — immature compost continues to decompose in the soil, consuming oxygen and nitrogen in the process, and may contain compounds that inhibit seed germination and root growth. Finished compost, on the other hand, is one of the most beneficial things you can add to a garden. Knowing the difference between the two is straightforward once you know what to look for.

What finished compost looks like

Finished, ready-to-use compost is dark brown to black, crumbly, and moist but not wet. It has a uniform texture — the original materials (food scraps, grass, leaves) should no longer be identifiable as individual pieces. There may be some woody or fibrous pieces that have not fully broken down — these can be sieved out and returned to the heap for further composting. The volume of the finished compost will be dramatically reduced from the original input — a full bin typically produces a fraction of its original volume in finished compost.

The cress test

A simple test for compost maturity: fill a small pot with the compost and sow a few cress seeds on the surface. Water and place in a warm spot. If the seeds germinate and grow normally within a week, the compost is safe to use. If germination is very slow, patchy, or seedlings yellow and collapse shortly after germination, the compost is still immature and needs more time. This test costs almost nothing and takes only a few days.

What to do with nearly-finished compost

If the compost is mostly finished but still contains some recognisable pieces, sieve it — use the fine material immediately and return the coarser pieces to the heap. Alternatively, use nearly-finished compost as a mulch on top of the soil rather than digging it in — the remaining decomposition happens on the surface without affecting soil nitrogen or root systems. As a surface mulch, compost does not need to be perfectly finished to be useful.