What's the difference between chemical, organic fertilizers?

CORVALLIS - Ever wonder what the differences are between processed or "chemical" fertilizers and "organic" fertilizers?

Each differs in the degree of processing it goes through, according to the Oregon State University Master Gardener handbook, "Sustainable Gardening," and OSU soil scientist John Hart.

Processed fertilizers are manufactured or are refined from natural materials such as rock, animal or petroleum products. Nutrients are concentrated in industrial processes to make them more available to plants. Ammonium sulfate, ammonium phosphate and potassium sulfate are examples of processed fertilizers. The fertilizers that you buy for your garden or lawn, such as 10-10-10 are usually composed of mixtures of these types of fertilizers. Nutrients are usually immediately available to plants and are quite concentrated.

Organic fertilizers are composed of natural materials such as mined minerals, animal or plant materials, with little or no processing. Manure, cottonseed meal, rock phosphate, fish by-products, oyster shell and blood meal are examples of organic fertilizers. Nutrients are released slowly by natural biological and chemical processes in soil and are relatively low in concentration when compared to processed fertilizers.

Organics range from relatively cost free manure from a backyard horse, cow or rabbits to expensive packaged products such as bagged bat guano or blood meal.

Packaged processed fertilizers generally cost less per pound of nutrients than packaged organic fertilizers.

Plants can take up only nutrients that are in solution, dissolved in soil water, regardless of whether they come from organic or processed fertilizer sources.

Nutrients in processed fertilizers are usually immediately available to plants. Organic fertilizer nutrients are not all as quickly available; they release some of their nutrients more slowly, over the course of weeks or months.

Nutrients from either organic or processed fertilizer can be lost and run off to surface or ground water if plants do not use them. Most naturally occurring soil nutrients are tied up in insoluble forms. They become available to plants only after they are converted to soluble forms, through weathering or biological decomposition.

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