Answer: This is a topic that is sure to get folks excited and the debate lingers on, so I'm not sure I can help you resolve your difference. But here are the researched facts:
Horse manure that has been composted in an area out of the elements is a good fertilizer to use in all areas of your garden. Keep in mind that some key elements, such as nitrogen, potassium and sulphur might be missing since mammals secret those elements through their urine. You might want to mix in some grass clippings or other yard waste to build up these other elements.
There is a debate about whether fresh horse manure is such a wise choice to use in the garden, however. This is probably one of the biggest reason gardeners have turned up their noses at using what is most certainly a readily available and free source of fertilizer. We live in an area that is home to many horses and horse farms. If you don't know someone with a horse, someone you do know does know a person who would give you all the free horse manure you desire simply for the hauling.
Since a horse tends to be an inefficient "composter," weed seeds it eats in hay or grass can pass through the horse in its waste. When you use fresh manure that contains weed seeds, weeds could sprout and grow in your garden. This is easy to remedy, however. Simply compost the fresh manure for a period of time in the same way you would compost other garden waste.
Build yourself a 3 foot long, 3 foot wide and 3 foot high bin to contain the droppings. Allow the manure to reach internal temperatures of around 140 degrees F for three days, turning it as often as necessary to keep the temperature up. This will kill the weed seeds and pathogens. You will need to keep turning the pile so that material on the outside can be moved to the inside of the pile, resulting in everything getting well composted.
If your pile smells foul, it has gone anaerobic and you need to turn it right away. A healthy compost pile should smell -- well ... healthy. You will need to keep the pile moist, like a wet sponge, but not so wet it is dripping. Cover the pile with some old cardboard to keep it from drying out or getting too wet and to keep the heat up in the pile.
After it has composted or aged for 2-3 months it will be free of weed seeds and pathogens and will make a nice mulch for your garden.
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