It is a rich source of nitrogen, and it supplies some of the essential micronutrients, including iron. Fish emulsion, a well-rounded fertilizer, is a partially decomposed blend of pulverized fish. The odor is intense but it dissipates within a day or two. Manure is a complete fertilizer, but low in the amounts of nutrients it can supply. Manures are best used as soil conditioners instead of nutrient suppliers.
Sewage sludge is a recycled product of municipal sewage treatment plants.
Two forms are commonly available, activated and composted. Activated sludge has higher concentrations of nutrients than composted sludge and is usually sold in a dry, granular form for use as a general purpose, long-lasting fertilizer. There is some question about the long-term effect of using sewage sludge products in the garden, particularly around edible crops. Heavy metals, such as cadmium, are sometimes present in sludge, and may build up in the soil. Sludge use should be guided by results of chemical analyses of the sludge in question.