Using commercial fertilizers, particularly sustainable or non-synthetic ones, can get really expensive. Compost and manures add important organic matter to the soil, which helps to build the subterranean ecosystems that support plant health, but they don't add much in the way of major nutrients.
Fortunately, nature has provided a means to make our own sustainable fertilizers. Comfrey (Symphytum officianale), a plant that grows well in most gardens, provides nutrient boosts when added to compost or used in liquid fertilizers. And once you've bought the plants, it's free.
Comfrey is a borage relative. Hardy to Zone 3, it has large, broad leaves like those of Pulmonarias, to which they are related, and small clusters of blue or white pendulous flowers. Comfrey's fuzzy, sometimes prickly leaves discourage insect pests. It grows in a range of soils in sun to part shade, and uses its impressive root system, which may grow anywhere from 6 to 10 feet deep, to mine minerals from the subsoil, aerate its surrounding soil, and break up the heaviest clays. It will regrow from tiny slivers of root, though, so be sure to plant it where you want it. If you are as indecisive as I am, grow it in a container. You won't get quite the same level of benefits as compared to comfrey grown in the ground, but at least you won't have it permanently installed.
Comfrey has been used in the past in herbal medicine, but research proves comfrey contains poisonous chemicals called pyrrolizidine alkaloids (PAs). The roots contain up to 10 times the amount of PAs as the leaves. Its toxic chemicals can be absorbed through the skin, though, so it is advisable to wear gloves when handling comfrey. Do not take any part of the plant internally, as it is extremely toxic to the liver and may also be carcinogenic.
For plants, though, it's a wonder food.
Comfrey contains high levels of the macronutrients nitrogen (N), phosphorus (P), and potassium (K), as well as calcium and vitamin B12. Comfrey also contains allantoin, a chemical compound that stimulates cell growth and regeneration. It may be the allantoin that makes comfrey so effective as a fertilizer. Extracts from comfrey's leaves have antifungal properties, and have been shown to effectively combat powdery mildew.
It's good to know the specific epithet of the plant you're getting. Symphytum officianale, the species, will seed prolifically and may become problematic in the home garden. The Bocking 14 strain of comfrey (Symphytum x uplandicum) contains the high nutrient levels of the species, but it's also sterile, so it won't self-seed and become invasive. Bocking 14 is difficult to track down in the United States, but is available from a few Internet sources.
Comfrey leaves can be used as a mulch or buried in the garden bed at planting time. As they rot down, they'll enrich the soil. Take care when using them as a mulch, though: The decaying leaves can be attractive to slugs, so don't mulch with them around leafy greens or ornamentals like hostas.