Fertilizers made from plants generally have low to moderate N-P-K (Nitrogen-phosphorous-potassium) values, but their nutrients quickly become available in the soil for your plants to use. Some of them even provide an extra dose of trace minerals and micronutrients. The most commonly available plant-based fertilizers include the following:
Nitrogen, phosphorous, and potassium are the three nutrients plants need in the largest quantities; they're sometimes referred to as the primary nutrients. They each play a critical role in plant growth. Healthy, fertile soil contains these three elements. But if your soil is deficient or if you're growing vegetables, fruits, or other demanding crops, you may want to supplement the soil's nutrients with fertilizers.
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Alfalfa meal: Derived from alfalfa plants and pressed into pellet form, alfalfa meal is beneficial for adding nitrogen and potassium (about 2 percent each), as well as trace minerals and natural growth stimulants. Roses in particular seem to like this fertilizer; they benefit from up to 5 cups of alfalfa meal per plant every ten weeks, worked into the surface of the soil. Add some to your compost pile, too, to speed the decomposition process.
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Compost: Compost is beneficial mostly for adding organic matter to the soil. Although it supplies some nutrients, its most important roles are enhancing soil life and helping make nutrients available to plants.
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Corn gluten meal: Derived from corn, this powder contains 10 percent nitrogen fertilizer. Apply it only to growing plants, because it inhibits the growth of seeds. The manufacturer recommends allowing one to four 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 perennial grasses and prevent annual weeds like crabgrass from sprouting.
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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; traditional cotton crops are sprayed heavily with pesticides, some of which can remain in the seed oils.
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Kelp/seaweed: Derived from sea plants, this product comes in liquid, powder, and pellet forms. Although kelp/seaweed fertilizer contains only small amounts of N, P, and K, kelp adds valuable micronutrients, growth hormones, and vitamins that help increase yields, reduce plant stress from drought, and increase frost tolerance. Apply it to the soil or as a foliar spray.
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Soybean meal: Derived from soybeans and used in pellet form, soybean meal is prized for its high nitrogen (7 percent) content and is used as a source of phosphorous (2 percent). Like alfalfa meal, it is particularly beneficial to nitrogen-loving plants, such as roses.
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Humus: Humus is a stable end product of organic-matter decomposition that's believed to increase microbial activity in soil, improve soil structure, and enhance the root development of plants. Although it doesn't necessarily add nutrients directly, humus may help plants take up the fertilizers you apply. If you add compost to your garden regularly, you're already adding humus.