There are many types of organic gardening fertilizer which can be either 100% organic or organic-based, which will be less than 100%. The organic-based fertilizers are generally hormone and vitamin supplements that should not harm other life in your garden but are not entirely natural. If you want to run a truly organic garden you will probably want to go for one of the 100% organic options.

Seaweed extract

Seaweed, or kelp, has long been known to be a wonderful natural fertilizer. There are many different varieties but they are all vegetable-based rather than animal products like the other 100% organic alternatives that we consider below.

Kelp fertilizer is particularly effective for leafy plants such as tomatoes, corn and peppers. It can help with seed germination, development of healthy roots, and resistance to disease. It contains many of the nutrients that are healthy for plants such as potassium as well as many amino acids and trace minerals that other fertilizers lack. Some varieties of kelp can contain up to 60 naturally occurring nutrients in a form that is easily absorbed by the plants. No chemically-based fertilizer can boast anything like this.

Seaweed extract in its pure form can also be used in a hydroponics garden - that is, a water-based rather than soil-based garden. Being naturally soluble in water, many varieties of kelp are ideal for hydroponics and can produce noticeably stronger plants.

Bat guano

Bat guano is probably the richest animal-based fertilizer known to man. It has been used for centuries and was highly prized by farmers and vegetable gardeners in many different societies. It includes nitrogen (to strengthen stalks and shoot growth) and phosphorus (to promote flowering) in high concentrations. There are also many other minerals and nutrients which can vary depending on the bats' diet.

Bat guano is particularly useful if you are in the process of converting your garden from chemicals over to organic. This is because it contains particular microorganisms and trace elements that work together as a bioremedy to remove toxins from the soil.

Worm castings

Every gardener knows that worms are one of the gardener's best friends. They aerate and break down the soil as they work their way through it, and the castings that they leave behind are one of the best fertilizers for your garden. They are particularly helpful for building strong plant roots, especially for perennials, lawns and root vegetables.

If you have a standard backyard garden you probably already have a lot of worms in your soil but you can either add to them and increase the variety of worm species by buying worms, or you can simply buy worm castings to spread over your garden.

If your garden has been created from bought-in soil or compost in raised beds or pots then the soil probably has a low worm population and you should definitely consider increasing it with live worms or bought castings.

Be careful when buying worm castings to always read the label. Sometimes the castings are mixed with other fertilizers and some of these may be chemically based. An organic gardener will want either pure worm castings or at least a mix with other compounds that are 100% organic.

Fertilizer is required in any soil that grows plants. In a wild environment, fertilizer is constantly produced by decaying animal and vegetable matter that falls on the soil. In our gardens where we clear away a lot of this matter, and where there are not many wild animals living, excreting and dying, we need to add back the nutrients if we want to keep growing strong and healthy plants.

Chemical fertilizers may help our plants grow strong but they can damage the balance of life forms in the garden. The whole ecosystem of the garden, including insects, worms and microorganisms, can flourish if the soil is kept rich with organic gardening fertilizer.