Plants require nitrogen to grow, but typical sources of nitrogen—chemical fertilizers—have been demonized for the harm they cause the environment. Manure is a natural source of nitrogen that, used correctly, is safe and causes less environmental damage than chemical fertilizers.
Purpose of Nitrogen
The N in the classic fertilizer formula NPK represents nitrogen, the mineral plants require most from the soil. Nitrogen is an essential component of protein and DNA, and it assists plants in using energy effectively. Although nitrogen is the primary component of the Earth's atmosphere, atmospheric nitrogen requires "fixing" before it can be used by plants. Many gardens, therefore, need nitrogen added to the soil through fertilizer or compost. Manure is a natural source of nitrogen, although nitrogen content varies depending on the species.
Rabbit and chicken manures have extremely high nitrogen contents. Cow, duck, horse, pig and sheep manures have roughly half the nitrogen content of chicken manure. Manures vary as well in their contents of other nutrients, so testing your soil before you begin a fertilization program will help you to make the most effective choice of which manure to use.
You should not apply raw manure to a garden bed. Manure should always be composted before use. Composting reduces ammonia and soluble nitrogen levels that can burn plant roots and hamper seed germination, kills any weed seeds found in the manure, and destroys pathogenic bacteria.
Compost manure using hot composting methods by increasing the amount of "green" nitrogen-rich ingredients in order to boost bacterial reproduction. The temperature of a hot compost pile will kill any pathogens. If you don't have the time to devote to hot composting, composting expert Barbara Pleasant recommends incorporating manure into a normal compost heap, but allowing plenty of time for the compost to mature. Helpful soil bacteria will slowly out-compete the pathogens. You can also purchase mature, ready-to-use manure from a gardening supplier.
In natural ecosystems, as animals forage for grass and food, their droppings restore nitrogen and other nutrients to the soil. Using natural fertilization methods, such as manure, eliminates the need to rely on chemical fertilizers, which can have devastating effects on natural ecosystems. Nitrogen runoff from chemical fertilizers, in particular, acts as a pollutant, causing algae blooms that result in aquatic dead zones. At the mouth of the Mississippi River, a dead zone roughly the size of New Jersey can be largely attributed to nitrogen fertilizer runoff. Applying manure in compost boosts plant nutrition while reducing the risk of damaging nitrogen runoff.
As noted above, applying raw manure to your garden can introduce too much soluble nitrogen and burn plants, transmit weeds, and introduce potentially harmful pathogenic bacteria, so manure should always be used with care. Always wash any produce picked from gardens where even mature composted manure has been applied. Additionally, if you are acquiring raw manure from a farm, ask about the use of herbicides on the animals' feed. Some herbicides endure in the manure for years and will halt growth in your garden if applied as part of compost.