Daniel Reichenbach, a gardener in Chatillon-le-Duc (Doubs), started making his own fertiliser before you could readily buy them in the shops.
In summer he collects:
Nettles (Urtica species) for nitrogen
Confrey (Symphytum species) for potassium
Horsetail (Equisitum arvense) for silica, horsetail acts as a natural fungicide and in addition it helps against damping off of seedlings.
Ferns for its silica and to make the compost lighter.
He dries the herbs in the shade, shreds them and pounds them to a fine powder. Some stalks and harder bits may remain, they can be sifted out and put on the compost heap or can be used after maceration as a fertiliser to water the plants. He then stores the powder of the different plants individually in carton boxes or paper bags, in a dry, shady environment.
When he transfer his seedlings to a pot, he takes a pinch of each powdered plant adds it to the compost in the pot. He grows vegetables as well as flowers from seed each year with a very good result.
Interestingly all the plants in his natural fertiliser concoction are used or have been used in herbal medicine.
Nettle - Urtica dioica, more common in northern Europe, Urtica pilulifera (Roman nettle) and Urtica membranacea (Membranous Nettle) more common in southern Europe - the young leaves and shoots are rich in vitamines and minerals. An astringent, diuretic tonic herb that controls bleeding, clears toxins, and slightly reduces blood pressure and blood sugar levels. Once dried or cooked nettles do not sting.
Comfrey - Symphytum species, Symphytum bulbosum (Tuberous Comfrey) more common in southern Europe - contain allantoin, which promotes cell proliferation and is now synthesized for use in healing creams. It also contains pyrrolizidine alkaloids. These alkaloids have been shown to cause liver damage and tumours in laboratory animals. As a result, Comfrey now banned in the form of tables and capsules (made from roots or leaves) in several countries.
Preparations for external use are condered safe.
Horsetail (Equisetum arvense) - an astringent, healing herb that acts mainly on the genito-urinary system and controls both internal and external bleeding.
Ferns - for example Dryopteris filix-mas (European Male Fern). For centuries the roots were used to expel tapeworm and other internal parasites. Dryopteris borreri, a southern European fern is very like the common European Male Fern.
All the above plants like damp conditions. It may be more difficult to find Comfrey, but you can always grow it yourself in a shady, slightly damp spot in your garden. It's quite a lovely plant. Flower colour ranges from yellow, white to deep purple.
Bibliography: RHS Encyclopedia of Herbs