According to the U.S. Department of Agriculture, "The principal guidelines for organic production are to use materials and practices that enhance the ecological balance of natural systems and that integrate the parts of the farming system into an ecological whole." This philosophy also underlies the definition of organic fertilizer as a substance added to soil to improve fertility, often using methods that mimic natural processes, with minimal negative impact on the environment and focusing on long-term sustainability in addition to increased yield.
What It Isn't
Organic fertilizers are so diverse that, at times, the easiest way to understand them is to begin by understanding what they are not. According to the USDA, "Organic food is produced without using … fertilizers made with synthetic ingredients or sewage sludge." Prior to the early 20th century, fertilizers--particularly nitrogen-based ingredients--came primarily from natural sources. Just prior to World War I, German scientists discovered a process to synthetically "fix" atmospheric nitrogen so that it could be used in soil. This process increased the availability of fertilizers, resulting in higher crop yields. For example, according to Raymond Zmaczynski of Princeton University, chemical fertilizer use has increased 400 percent since the 1940s. However, these chemically produced fertilizers have come at a price: surface and ground water pollution, aquatic dead zones from chemical runoff, soil depletion and harmful chemical residues in food.
Purpose of Fertilizers
Both chemical and organic fertilizers share the purpose of improving plant growth and crop yields by making essential nutrients available to the growing plants. Plants require many nutrients to live, the primary of which are nitrogen, phosphorus and potassium. In nature, these elements are being constantly recycled into the soil through decaying plants and animals and helpful bacteria found in certain plant roots that convert atmospheric nitrogen into a form that can be used in the soil.
Organic fertilizers tend to mimic natural processes, restoring nutrients to the soil in a way that parallels how soil fertility develops in nature. It is therefore less harmful to the environment and the consumer. For example, farmer Joel Salatin, owner of Polyface Farm and a leader in the sustainable agriculture movement, restores soil fertility using manure from his livestock and composted byproducts from animals killed for meat. This imitates natural processes, where grazing animals fertilize the soil with their manure and their decaying bodies after death.
Types of Organic Fertilizers
Organic fertilizers are as diverse as the natural environment that inspires them. Salatin's approach describes a method used by farms that raise pastured animals for meat. For the gardener, similar results can be achieved with manure, bone meal and blood meal, all of which can be purchased from garden supply stores or acquired as byproducts from farms that raise livestock. Gardeners can compost materials typically discarded as garbage: food scraps, paper shreds and grass clippings. If these items are mixed together in the correct ratio, they are broken down by helpful bacteria into nutrient-rich soil that can be applied to the garden as fertilizer. Finally, gardeners can capitalize on plants' natural abilities to restore nutrients to the soil by rotating crops or planting cover crops during the off-season that restore nitrogen used by food plants during the growing season.
One of the defining traits of organic agriculture is its focus on sustainability--on maintaining soil and plant health over the long term rather than simply seeking short-term increases in yield. Christos Vasilikiotis, of University of California Berkeley, reviews several studies that show that organic fertilizers not only produced and sustained high crop yields, equivalent or higher than those achieved using chemical fertilizers, but did so without the loss of soil fertility that is common when relying upon chemical fertilizers.