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. Maturity is not the same as quality.
Maturity means the energy and nutrient containing materials have merged into a stable organic mass.
Quality is the chemical composition of that mass. For example, a compost could be mature, but of poor quality, if nutrients had leached away or it contained contaminants. Source materials affect quality. Soluble salts, nutrients and contaminants vary, depending on what the source material of the compost is. While not a human health concern, concentrated soluble salts can cause problems in plant growth.