Ashes should not be added to compost systems because they fill in air spaces and can suffocate your compost. More importantly, they are alkaline in nature and would upset the near neutral pH balance of the compost. However, they do contain potassium, a valuable plant nutrient, and can be directly added to acidic soils at planting time to help increase fertility. Wood and peat ash are safest. Coal ashes may contain heavy metals and other toxins so they should not be used for gardening.