Nutrient values of manures vary greatly, depending on the diet and age of the animals, and the nature and quantity of bedding in the mix. There’s also freshness to consider: the older the manure, the fewer nutrients it will contain. On the other hand, the older it is, the sooner it will be safe to use and the less fragrant it will be.
These variables make precise nutrient listings impossible, but there are significant differences between manure types that are useful to know.
Poultry: Hen dressing, as it’s known in the country, is higher in nitrogen than other common manures. It also contains a significant amount of phosphorus, and some potash. Chicken manure from a farm where birds run around in straw will be considerably less potent (and probably less full of antibiotics) than manure from an egg factory where birds live crowded together in wire cages. Chicken manure is famous for burning plants if it is used when too fresh.
Sheep: Comparatively high in nitrogen, an excellent source of potash, with moderate phosphorus. Manure from sheep fed hay and grain will be more potent — and more likely to be available — than manure from animals that live on pasture.
Horse: About half as rich in nitrogen as chicken, with a good amount of potash but only a modest dose of phosphorus. Amounts of bedding vary greatly, which means potency does, too. Horse manure can be a powerful carrier of weed seeds.
Cow: Cow manure has the lowest nutrient numbers, in part because there is so much bedding mixed with it. But that low nutrient concentration makes it safe to use in unlimited quantities. Try to find manure that’s mixed with straw or shredded newspaper, rather than the more common sawdust. If you get the sawdust kind, expect it to take a year before it starts to deliver results.
Specialty: Rabbit manure is very high in nutrients and less likely to cause nitrogen burn than chicken manure. Most rabbit owners know this and do not give it away. Bat guano is like supercharged chicken, but it’s hard to gather, getting rare, and priced accordingly. Zoos need the money more than you need hippopotamus droppings, but if you have enough land to pile weird manure until it’s composted, the charity you spread will improve your soil.