If you want to go organic and steer clear of chemical fertilisers in the garden then there are a few natural alternatives. Common nettles make a good, if smelly alternative to commercially prepared chemical versions (another good reason to leave a patch of your garden to run wild).
For obvious reasons, wear some rubber or gardening gloves to harvest, tear and scrunch up enough nettle stems and leaves to loosely fill a watertight container such as a bucket. Then weight them down, say with an old plate. Fill the container with enough water to cover the crushed greenery and then leave to rot down. (This is a bit smelly so you may want to place this somewhere away from the house)!
The brew should be ready to use in 3 to 4 weeks and needs to be diluted for use – usually in a ratio of roughly 1 part nettle liquid to 10 parts water (the resulting diluted liquid should look the colour of tea). You can keep topping up your supply bucket with nettles and water as the season progresses. Once your flowers have finished flowering and you no longer have use for the homemade fertiliser, just tip what remains onto the compost heap.
Another alternative is coffee grinds. Sprinkle them around plants before you water or before rain and the grinds will slowly release nitrogen into the soil.
Crushed eggshells are a well-known old-fashioned fertiliser and work particularly well scattered around roses because of their calcium carbonate content. (An added bonus is that their sharp edges also help to deter slugs).
If you’re lucky enough to live by the seashore then some of the best fertiliser is freely available in the form of seaweed. You can either treat it in the same way as the nettles above and make a ‘tea’ out of it (which again needs to be diluted for use) or, if it is winter time, dig the seaweed directly into plant borders to feed and condition the soil.
Lastly, but by no means least, consider making either a compost heap or set up a worm composting bin. In my experience worm bins don’t smell (I kept mine in the garage) and given time they produce wonderful, fine compost and the ’run-off’ is a good liquid fertiliser for the garden (use diluted as above).