Organic fertilizers are made from naturally occurring substances that can be derived from a wide variety of sources. Including animal manures and by-products, plant and seed meals, natural minerals, fish and seaweed products.
Many organic fertilizer products are slow release, meaning they release their nutrient over a period of time rather being instants as are most synthetic fertilizers. Organic fertilizers usually require active soil bacterial to help make the nutrients plant ready. There are a few that are considered fast release and much more plant nutrient ready.
They can range considerably in available macro nutrients, micro nutrients and trace elements, with some being high in a single nutrient and usually need to be combined with other amendments to create the right balance, other sources are complete fertilizers with a good balance of nutrients for plant growth.
Animal by-products are products of animal origin not intended for human consumption, they are derived from carcasses and parts of carcasses, include meat, bone, blood, offal and feathers, mostly from slaughterhouses. These products go through a process known as “rendering” where high temperature are used to cook and sterilize the product. Like animal manures they are utilized by being dug into the topsoil, and although they are less bulky and don't contain much organic fibrous material they are still a very good source of plant nutrients.
Plant and seed meals are made from different plants containing as the name implies plant material. There are no animal products in plant and seed meal fertilizers making them acceptable for vegan gardening. Many are rich sources of mineral and trace elements as well as the better known macro nutrients.
Mineral based fertilizers are a natural mineral that are found in rock. Mostly these are slow release fertilizers that will increase the fertility of the soil over a long duration of time. Some of the most common types of mineral based fertilizers used by organic gardeners are: Limestone, Greensand, Rock Phosphate and Potash Salts.
Nutrients from the sea are probably the richest source of plant nutrients and minerals available for the vegetable garden. Ocean based fertilizer products increasing crop yields, drought resistance, frost protection, and stress recovery. Apart from their value in supplying the basic macro-nutrients, their main value is in their growth hormones which stimulate cell division and larger root systems. Most can be applied as a foliage spray or as a soil soak and are excellent as a root dip for reducing transplant shock.
Compost comes in several form, from the traditional compost heap made from basic plant material like yard trimmings, food wastes, leaves, manure from cows, horses, sheep and poultry. To other types of composting like vermi-composting, can all be considered organic fertilizers too.