JoAnne Skelly: Organic fertilizers

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What are the advantages and disadvantages of applying organic or natural fertilizers? While we may decide to choose organic fertilizer because we are environmentally conscientious, plants do not know the difference. The nutrients supplied via a fertilizer are the chemical elements or combination of elements the plant uses for all its processes and elements stay the same. It’s just the sources of the nutrients that are organic or inorganic.
Nutrient availability and how long the nutrients last do differ between organic and inorganic fertilizers. They also differ on how they affect soils and the environment. According to Utah State University Extension, “Virtually any organic material can be used as a fertilizer; however, materials vary considerably in the concentration of plant nutrients they contain and the rate at which these nutrients are released for plant use” (
With that in mind, organic fertilizers must be selected based on plant requirements. “Different materials need to be applied at different rates to supply the correct amount of plant nutrients” (USUE).
Some examples of naturally occurring organic fertilizers are worm castings, composts and compost teas, alfalfa hay or pellets, bone meal, blood meal, cottonseed meal, feather meal, fish meal, kelp, manures and seaweed extract. Grass clippings, pine needles, leaves, sawdust and straw release very low amounts of nutrients very slowly. Sewage sludge is sometimes listed as organic, but sometimes contains heavy metals, which are harmful to plants and soil. There is an excellent chart delineating organic fertilizers, their nutrient amounts and release rate at the site I listed above.
Whether organic or inorganic, misuse can still cause problems for plants and the environment with potential ground water pollution, salt burn, nutrient deficiency or even toxicity. As with inorganic fertilizers, the analysis numbers on the package tell us the percent of the macronutrients: nitrogen (N), phosphorus (usually as phosphate - P2O5), and potassium (potash or K2O). Some products contain micronutrients such as iron, sulfur and zinc, but these may not be listed on the analysis because they are present in such low quantities. See the photo for a product label, which lists 7% nitrogen, 2% phosphate, 1 percent soluble potash “derived from fermentation of soybean meal, cottonseed meal.” Notice that it is “sterilized and free of weed seeds”, which is very important.
A distinct advantage to organic fertilizers is that they can increase soil organic matter a, the black gold of a good soil, as well as soil health. More organic matter increases water holding capacity, drainage and overall soil workability.
JoAnne Skelly is Associate Professor & Extension Educator, Emerita, University of Nevada Cooperative Extension. She can be reached at


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