Thanks for writing. Horse manure, when it's fully rotted (after about a year) is a really wonderful soil amendment. If you have a place you can make a pile of horse manure and let it rot, you can definitely enrich your soil with it. You just can't use it fresh --- it has too much salt in it, usually, and may harm your plants. That's why it needs to be aged.
---Justin, Senior Garden Editor, BHG.com"