A quick review of any table of nutrients will show that there are significant differences in the nutrient levels (and balance) of fresh animal manures. One good source is the website at Cornell University. Rotted manure, which has been allowed to sit, out of the rain yet kept moist and sufficiently packed down to exclude air, is usually richer by weight (than fresh manure it loses much of its weight during the rotting process), and is also more stable in terms of its nutrients, since microorganisms have already had a chance to do some of their work. While the balance of nutrients in manure is relatively good, improper use or storage can cause a significant loss of nitrogen.
However, it isn’t necessary to store manure or wait for it to rot. Fresh manure can be spread directly on a new garden; if turned under immediately, there will be only minor losses of nitrogen and other nutrients, though planting should be delayed three weeks to a month to allow it to break down and stabilize. But once the garden has been established, direct application may only be feasible in the early spring and late fall, as fresh manure is too strong for most plants.
Fresh cow or horse manure (without bedding) applied in the fall, at the rate of two to three bushels (about 100 to150 pounds) per hundred square feet, will supply enough nutrients for general vegetable cropping. If stronger manures are used, decrease this volume a bit; if there is a lot of bedding mixed in, increase it. One important point: fresh manure should be turned under immediately, or significant amounts of nitrogen will be lost to the atmosphere or to runoff.
Country gardeners may want to keep a few chickens or rabbits, or even a horse; from then on, manure will be available whenever they like. Others, without the room to keep animals, can often locate a livestock or poultry keeper with manure to spare. In fact, they may be glad you asked. For most gardeners, though, the occasional use of purchased organic fertilizers—either dry bagged manure or in granular form—plus composting, may be the most reasonable plan of action. Elsewhere we discuss purchased organic fertilizers in a bit more detail.