There are so many fertilizers on the market that it is sometimes hard to choose between them. Considering that most are chemicals rather than natural and can be quite expensive, finding cheap alternatives that work well is a blessing. One of these is homemade fish fertilizer.
Start with a large pot half full of fish. These can be trash fish that have been caught, they can be the fish leavings after filleting and cleaning them, they can be fish that was thawed out but never used. They can even be fish parts from the local fish store since many such stores will give the scraps away. Try to stay away from fish that came from polluted waters, especially if the pollution is in the form of heavy metals like lead or mercury.
Note that the emulsion can be made from saltwater fish, too. The fish don't have to come exclusively from fresh water sources. In fact, saltwater fish often contain iodine, which can be useful for garden plants.
Add water until the water level is above the level of the fish. Bring the water to a slow boil, stirring often. If necessary, more water can be added as needed to maintain the same level in the pot. Using a lid also helps to keep the water level up and keeps the volatile oils in the water.
Boil the fish or fish parts for an hour or more, if whole fish are being used. Whole fish are actually preferable, because the innards contain more fish oil than the flesh does. The idea is to completely cook the fish while additionally getting the juices and oils that they contain.
Separating the fish broth
Allow the fish 'tea' to cool completely, then using a ladle or strainer, remove the fish parts and meat from the water. The parts and meat can be buried where you are planning on planting trees, near rose bushes, next to vegetables and fruits like tomatoes, flowers and so forth. The fish meat can also be a wonderful treat for cats, packed with protein, vitamins and minerals. Not all cats will eat the fish, so don't be discouraged if some don't. The bones from the fish, if there are any and if they are cooked properly, should be soft and easy for an animal to digest.
Using the emulsion
Pour the fish juice around perennials and trees, especially roses, fruit trees, strawberries, vegetables or showy flowers. Since the boiling partly breaks down the fish juice, it doesn't burn the plant by putting too much raw nitrogen in the soil, so this fertilizer can be poured right around the base of these plants.
It can also be diluted, which helps stretch it out so it can help more plants. Some sources recommend dilution.
The juice will last well for about a week in the fridge, in a container with a tight fitting lid, but it really should be used quickly. It will nearly always have a strong fishy aroma, which isn't as noticeable outside. Also, once it is in the soil, the odor fades fairly rapidly.
As a side benefit, the smell tends to discourage animals like deer and rabbits from the garden.
This fertilizer is better than most commercial preparations because it doesn't need to break down extensively before the plants can use it. Be prepared though, because using this can result in bumper crops and larger than normal plants and flowers.