Organic fertilizers enrich the soil and provide nutrients necessary for plant growth. They are made up of material that was once alive or the by-product of a living creature. Organic fertilizers are generally inexpensive and plentiful. The nutrients in them are released slowly as the fertilizer breaks down, decreasing the problem of fertilizer run-off sometimes found with inorganic fertilizers.
Alfalfa and cottonseed meal, corn gluten meal and soybean meal are processed plants and plant by-products. While they are often used as animal feed, they also can enrich the soil. They're a good source of nitrogen and trace minerals such as zinc and magnesium when tilled into the soil.
Horse, cow and chicken manure contain high levels of phosphorus and nitrogen. Also rich in these nutrients are bat guano (manure), bone meal from ground bones and fish emulsion from processed fish.
Naturally occurring minerals
Rock dust contains trace minerals essential to plant growth. Rock dust is made of crushed or ground volcanic rock.
Composted food wastes, yard clippings, manure and vermicompost from worm bins provide a nutrient-rich soil amendment high in both macro and micronutrients. Macronutrients is the term given to those nutrients plants use in large quantities. Nitrogen, phosphorus, potassium and magnesium are examples of macronutrients.
Kelp can be harvested from the ocean, or seaweed that washes ashore can be gathered and spread in the garden. Kelp and other kinds of seaweed are rich in micronutrients, which are trace elements plants use in small amounts. Manganese, zinc and iron are examples of micronutrients.