Composts vary greatly, depending upon what goes into them and how they are processed. Quality also varies depending on maturity, pH, presence of weed seeds, concentration of toxic substances, and the population of soil-dwelling organisms, such as earthworms, insects and microorganisms. Higher quality compost is not too wet, is mature and has good water-holding capacity and nutrient availability. Maturity makes a difference. Use of immature composts can cause problems.
Maturity means that the compost has decomposed extensively and has become fairly stable.
Immature compost may still contain some plant inhibitors and excessive soluble salts. When immature compost is added to the garden, its bacteria compete with plants for nitrogen in the soil. The result is unhealthy plants with symptoms such as yellow leaves or stunted growth. If compost is still hot, smells like ammonia, or you can still recognize the original form of organic matter, then it is not ready to use. When in doubt, let compost mature longer.