The three main nutrients that plants get from soil are nitrogen, phosphorous and potassium. In addition, secondary nutrients and micro-nutrients are also obtained from the soil plants grow in. As plants use the nutrients in the soil, they must be replenished with soil additions or fertilizers, or the nutrient content will eventually be diminished.
Plant Matter (Humus)
Plant matter can be added directly to the soil. It will decompose naturally, adding the three main nutrients, as well as significant sulfur content to the soil.This process happens over time and is best done in the fall, to prepare soil for the following season. It can either be turned into the soil or laid over the top and left to decompose over the winter.
Compost is probably the most nutrient rich organic fertilizer available. You can purchase compost or make your own compost pile. Turn compost directly onto the soil in the spring, about two weeks before planting, to start with a garden bed that is rich in nutrients. Top dressings of compost can also be added throughout the growing season.
Organic gardening expert Steve Solomon recommends a special homemade fertilizing mixture containing seed meal, agriculture lime, gypsum, and dolomitic lime, with optional additions of bone meal, rock phosphate, or high-phosphate guano, and/or kelp meal. He recommends use of this mixture, along with regular applications of compost for best results.
To fertilize seedlings started indoors or for additional fertilization throughout the season, applications of fish emulsion or compost tea are excellent sources of nutrients. Compost tea is made by placing one quart of compost in a cloth bag and steeping in a gallon of water for several days.
Other Soil Amendments
When soil is found to be deficient in any one of the three basic nutrients, it can be amended by adding materials that contain them individually. Nitrogen is found in bat guano, blood meal, fish meal and soybean meal. Phosphorous sources are bone meal, colloidal phosphorous and rock phosphate. Potassium can be found in granite dust, green sand and ground kelp.