Cow manure, depending on bedding amounts, weighs in at a dismal 0.5% nitrogen, 0.5% phosphorus, and 0.5% potassium, low in all three elements. Be sure to cure cow manure by giving it plenty of time in your compost pile. Horse manure usually scores slightly better in all categories with a 1.5–1.0–1.5 N-P-K rating and a shorter composting time.

In general, fresh manure from cows and horses contains the same amount of nitrogen. But because horse waste has significantly less water content than cow manure, when it dries it contains almost twice as much nitrogen as cow manure.