The most commonly available plant-based fertilizers include the following:
Alfalfa meal: Derived from alfalfa plants and pressed into a pellet form, alfalfa meal is beneficial for adding nitrogen and potassium (about 2 percent each), as well as trace minerals and growth stimulants. Roses, in particular, seem to like this fertilizer and benefit from up to 5 cups of alfalfa meal per plant every ten weeks, worked into the soil. Add it to your compost pile to speed up the process.
Compost: Compost is mostly beneficial for adding organic matter to the soil. It doesn't add much in the way of fertilizer nutrients itself, but it does enhance and help make available any nutrients in the soil.
Corn gluten meal: Derived from corn, this powder contains 10 percent nitrogen fertilizer. Apply it only to actively growing plants because it inhibits the growth of seeds. The manufacturer recommends allowing 1 to 4 months after using this product before planting seeds, depending on the soil and weather conditions. Use it on lawns in early spring to green up the grass and prevent annual weed seeds from sprouting.
Cottonseed meal: Derived from the seed in cotton bolls, this granular fertilizer is particularly good at supplying nitrogen (6 percent) and potassium (1.5 percent). Look for organic cottonseed meal because traditional cotton crops are heavily sprayed with pesticides, some of which can remain in the seed oils.
Kelp/seaweed: Derived from sea plants, you can find this product offered in liquid, powder, or pellet form. Although containing only small amounts of N-P-K fertilizer, kelp meal adds valuable micronutrients, growth hormones, and vitamins that can help increase yields, reduce the plant stress from drought, and increase frost tolerance. Apply it to the soil or as a foliar spray.
Soybean meal: Derived from soybeans and used in a pellet form, soybean meal is prized for its high nitrogen (7 percent) content and as a source of phosphorous (2 percent). Like alfalfa meal, it is particularly beneficial to nitrogen-loving plants, such as roses.
Humus: When looking at organic fertilizer products, you'll invariably come across those containing humus, humic acid, or humates. Some of these products have almost magical claims as to what they can do for your plants. Humus, humates, and humic acids are organic compounds often found in compost. Humus is touted to increase soil microbial activity, improve soil structure, and enhance root development of plants.
These products have no fertilizer value, but rather are used as stimulants to support soil microbial life that, in turn, support the plants. Use them as supplements, but not to replace proper soil building and nutrition.