Animals, fish, and birds all provide organic fertilizers that can help plants grow. Animal-based fertilizers contain nitrogen, phosphorous, and potassium ¯ the primary nutrients plants need to grow. They each play a critical role in plant growth and development.
If your soil is deficient or if you're growing vegetables, fruits, or other demanding crops, you may want to supplement the soil's nutrients with these animal-based fertilizers:
Manures: Animal manures provide lots of organic matter to the soil, but most have low nutrient value. A few, such as chicken manure, do have high available nitrogen content. In general, use only composted manures, because fresh manures can burn tender roots.
Bat/seabird guano: Yes, this product is what it sounds like ¯ the poop of bats and seabirds. Guano comes in powdered or pellet form and is high in nitrogen (10 to 12 percent).
Bat guano provides only about 2 percent phosphorous and no potassium, but seabird guano contains 10 to 12 percent P, plus 2 percent K. The concentrated nitrogen in these products can burn roots if they're not used carefully.
Blood meal: Blood meal is the powdered blood of slaughtered animals. It contains about 14 percent nitrogen and many micronutrients. Leafy, nitrogen-loving plants such as lettuce grow well with this fertilizer. Reportedly, blood meal also repels deer (but may attract dogs and cats).
Bonemeal: A popular source of phosphorous (11 percent) and calcium (22 percent), bonemeal is derived from animal or fish bones and is commonly used in powdered form on root crops and bulbs. It also contains 2 percent nitrogen and many micronutrients. It may attract rodents.
Fish products: Fish by-products make excellent fertilizers, and you can buy them in several forms:
Fish emulsion is derived from the fermented remains of fish. This liquid product can have a fishy smell (even the deodorized version), but it's a great complete fertilizer (5-2-2) and adds trace elements to the soil. When mixed with water, it's gentle yet effective for stimulating the growth of young seedlings.
Hydrolyzed fish powder has higher nitrogen content (12 percent) than fish emulsion; it's mixed with water and sprayed on plants.
Fish meal, which is high in nitrogen and phosphorus, is applied to the soil.
Other animal-based organic fertilizers include crab meal, dried whey, earthworm castings, feather meal, and leather meal.
Using composted human waste (euphemistically called night soil) to fertilize gardens has been a common practice in countries such as China for generations. In many Western countries, our version of this practice is using composted sewage sludge as fertilizer. Modern sewage treatment plants, however, receive waste from many sources, including industries. Although sludge has fertilizer and soil-building value, sewage sludge is not generally considered to be an organic fertilizer, because it may contain toxic heavy metals that accumulate in the soil. Although you can buy granular fertilizers made from sludge, organic gardeners generally avoid them.