Manures vary in nutrient content according to the animal source and what the animal has been eating. Commonly available manures include horse, cow, chicken, and sometimes sheep. As it is aged, leached, or composted, nutrient content is reduced. However, the subsequent reduction in salts will reduce the chances of burning plants. Fresh manure should not be used where it will contact tender plant roots.
Manures are good soil conditioners.
Manures should be incorporated into the soil to conserve nitrogen. If left on the soil surface, up to 25 percent of the ammonia nitrogen can be lost within 2 days and 75 percent or more can be lost within 1 month after application. Sewage sludge is a recycled product of municipal sewage treatment plants. Composted sludge is usually sold in a dry, granular form for use as a general purpose, long lasting, nonburning fertilizer. There is some question about the long term effects of using sewage sludge products in the garden, particularly around edible crops.