Doesn't horse manure have to be fairly composted in order to add it to the garden? I've used horse manure frequently in my garden but it's always been added in the fall season before so that it has time to compost over the winter. Nevertheless, I still found some rather "solid" pieces of the manure in the spring.

After a couple of decades of clearing out the horse manure from our neighbor's barn to use on the garden, we had a disaster. About 3 days after we transplanted healthy tomato and pepper plants, the leaves began to curl and shrivel. After some serious research, we discovered a herbicide called Grazon had been applied to the neighbor's hay fields the previous year. Grazon does not break down in the horse's digestive system nor when the manure is composted. The affects can harm your garden (particularly for nightshade family plants) for several years. We dug all the dirt out of our raised beds down to the clay and had to start completely over building our soil. Be aware that this risk exists, even in heavily composted manure. Although good horse manure is our favorite garden enhancement, we can no longer risk using it because of the prevalence of Grazon in our area.