Natural fertilizers, like composted sewage sludge, cow manure or complete fertilizer blends, provide nitrogen and other nutrients slowly. An advantage of these natural "nutrient suppliers" is that they provide minor nutrients - minerals required in small amounts such as iron or zinc - not usually found in synthetic fertilizers. Natural fertilizers also improve the soil structure.
A disadvantage of natural fertilizers is that usually the concentration of nitrogen, phosphorus and potassium are lower. Therefore, a greater amount of a natural fertilizer must be applied to provide the same amount of nutrients that can be obtained with a lesser quantity from a synthetic nutrient source.