Since posting Soil 101, we have had great questions asked and thought this would be a good addition to the topic. Most of this info was gathered from my personal notes and Soil Science textbook used in my horticulture class in college.
When compared to commercial fertilizers, all manure is relatively low in nutrient levels.
It releases nutrients slowly and only when the soil is warm and moist
enough for microorganisms to break down the components. While each batch
of manure will vary in nutrient content, horse and cow manure have more
similarities than differences.
Manure isn't made up of feces and urine alone. By the time
it reaches your garden, manure may include animal feed, bedding, soil,
plant matter and many other materials. The amount of nutrients in the
manure will be directly affected by the quality and composition of the
animal's diet, as well as its size and physical condition. You would
have to analyze a sample to know the exact breakdown of its nutrient
content. For most home gardeners, that level of detail isn't necessary.