Using compost before it is ready can damage plants. Undecayed “brown” materials in the soil can temporarily reduce plant available nitrogen. Undecayed “green” materials can harbor pests and diseases. Immature compost can also introduce weed seeds and root-damaging organic acids.
Compost is ready when:
It smells earthy; not sour, putrid or like ammonia.
It no longer heats up after it is turned or wetted.
It looks like dark soil.
It is crumbly, and not identifiable as food items, grass, leaves, etc.
If the compost is warm or hot to the touch it is not ready. To test if the compost is ready do the "smudge" test. Rub a sample of compost between your fingers. The compost should be fragile, flaky and leave a dark smudge on your fingers.
Other Readiness Tips
A properly made heap will reach temperatures up to 160º F in several days. During this time, you will notice settlement in the pile. This is a good sign that your compost pile is working properly.
The composting process may last from a few weeks to many months, depending upon the types of materials you use, water requirements, and how often you turn your pile.
Apply the finished product to your garden or around trees and shrubs by turning the soil and applying 1- to 3-inch layers of compost, working it into the soil well.
Compost can be shoveled out of a pile or bin and used just as is, especially for mulch. Remove undecayed objects by sifting them through a screen. If you are using compost in preparing soil for planting or sodding, sift it through a one-inch mesh screen. Compost used in potting mixes or as top-dressing on lawns is commonly sifted through a 3/8-inch or 1/2-inch mesh screen.
Make a simple screen by mounting hardware cloth or other durable wire mesh in a sturdy wooden frame that will fit neatly onto the wheelbarrow or other container into which you will screen the compost. Spread compost onto the screen in a thin layer and shake it. You can work the finer material through the screen with a paddle if it is clumpy. Add the oversized material that remains on top of the screen to a new pile to help it start composting faster.