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Organic vs. manufactured fertilizers

Renewed concern about the environment has stimulated interest in the use of organic fertilizers. As a result, there is a debate about the qualities of organic versus manufactured fertilizers. Primary considerations in selection include release rate, cost and soil-building effects.

Organic fertilizers are commonly processed from various animal and plant by-products, like poultry feathers, manures and treated sewage sludge. These materials have slow release rates, requiring soil microorganisms to break down the material into chemical forms plants can use. Approximately half or less of the nitrogen is released for crop use the first year. A significant advantage of organic products is improvement in soil organic content, which is key to successful gardening in Colorado's soils.

Organic fertilizers are comparatively low in nutrient content so they require significantly larger volumes. Nutrient release rates are slower in cool or dry soils. If you rely solely on organic materials and your soil is low in organic matter, lawns and gardens often will appear pale green. This indicates the nitrogen level is low.

On the other hand, manufactured nitrogen fertilizers are normally made from petroleum or natural gas. Typical release rates vary from a few days to a few weeks, giving a quick response after fertilizer application. They also may require some processing by soil bacteria. The phosphate and potash in manufactured fertilizers generally are processed from rock deposits. Costs are comparatively low to moderate.

Because manufactured fertilizers are relatively high in nutrient content, only small amounts are required. Overapplication causes leaves to become yellow or brown, which is sometimes known as "burning." On sandy soils, an overapplication of nitrogen can leach into groundwater.

Both types of fertilizer will pollute our groundwater, lakes and streams when spilled or spread onto streets, sidewalks and driveways. Neither will pollute when correctly applied to lawns and garden soils.

For more information, see the following Colorado State University Extension fact sheet(s).

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Updated Tuesday, July 22, 2014