Commercial fish emulsion is a by-product of the production of animal food. Gardeners like to use the product because it's high in nitrogen, which promotes leafy plants like tomatoes. You can make your own emulsion, which will be a different formula than the commercial product, or purchase a ready-made product that is neatly packaged so that you never have to handle fish. Both forms are excellent as fertilizers, especially for your tomato plants.
You might think fish emulsion is ground fish--and homemade formulas are pretty much just that. But the fish emulsions you buy as a fertilizer from garden centers are a by-product of the huge animal food production process. Smart manufacturers realized that after oily menhaden fish were ground up and the solids were removed for pet food, and the oils were removed for the fish oil vitamin market, the liquids that remained could be processed further to make a product they could sell to another market--gardeners. They cook the liquid down to concentrate it and then acidify it with phosphate to keep bacteria from causing it to spoil on the shelf.
Menhaden fish are collected in massive seine nets for this production process. School of millions of fish used to swarm the Atlantic ocean, and much of the sea life dined on the little fish. At the same time, the fish filtered the seawater, cleaning large bodies of water such as the Chesapeake Bay. Today, overfishing of the menhaden for animal food production has escalated into an environmental disaster. According to Susan Gottleib, a marine biologist, overfishing of menhaden is "just like removing your liver," she says, and "you can't survive without a liver."
Fish emulsion fertilizer applied twice a week to the soil causes tomato plants to grow faster and larger than plants not receiving the fertilizer. The high nitrogen content available immediately to the roots causes leafy growth but has no effect on germination rates, nor does foliar application have as good an effect as the soil application. The use of an organic fertilizer like fish emulsion reduces the dependence on chemical fertilizers with only a slight emission of methane gases. Tomatoes need the high nitrogen fertilizer to grow a thick cover of leaves to support the crop of fruit and the massive root system.
The fish emulsion as a high nitrogen source is especially beneficial in plants that have a heavy leaf growth like tomatoes, corn, spinach and beans. The nutrients are available immediately to the plant, unlike many soil applications of synthetic fertilizers. This immediate release can be seen as an equally immediate boost in growth of the tomato plants, without any buildup of salts in the soil.
You can make your own fish emulsion by taking some fish scraps and placing them in a couple of gallons of water. Place them in a sunny spot away from any homes and let them ferment for two or three months. It will be very rank, but when it is finished, you will have a layer of oil on the top. Remove it and dilute it with water to use as a fertilizer for your tomato plants and healthy, sweet and large tomato fruit.