Horticulturist Explains Fertilizer Terminology

Thomas J. Koske  |  5/4/2006 11:35:54 PM

News You Can Use For May 2006

Horticultural articles and booklets often use special terminology in discussing fertilization practices. Knowing exactly what they are saying is essential for success with your cultural program.

"Let's define commonly used fertilizer terms," says LSU AgCenter horticulturist Dr. Tom Koske.

"Analysis" is the product's composition of nitrogen (N), phosphorus oxide (P2O5) and potash (K2O) as a percent by weight of the contents of the bag. A 10-pound bag of 12-4-8 contains 1.2 pounds N, 0.4 pounds P2O5, and 0.8 pounds K2O.

"Analysis ratio" is the relative comparison of the analysis values. A 12-4-8 fertilizer has a ratio of 3-1-2. Ratios are important in determining the fertilizer’s possible uses.

A "complete fertilizer" contains all three major nutrients to some extent, thus a 12-4-8 is complete, but 0-20-20 is not. Complete does not imply that the product has everything your plant needs.

"Blended fertilizer" contains more than one nutrient, such as 0-20-20.

"Nitrogen," also listed as N, is always the first number in the analysis and is given as the percent of elemental N by weight.

"Phosphorus" may be shown as P, is always the second number in the analysis and is given as percent of the oxide P2O5 equivalent by weight.

"Not to be concerned," Koske says, because except for some scientists, everyone uses the P2O5 form in giving recommendations – or they should be, because that is what's useful to you since that is the form on the bag. If you do get the value in pure P, multiply it by 2.3 to get the oxide or multiply the oxide equivalent by 0.44 to get the elemental amount.

"Potassium" may be shown as K, is always the third number in the analysis and is given as percent by weight of the oxide K2O. Here also you will normally be given a recommendation of x amount of K2O per acre. If you need to convert, multiply the oxide by 0.83 to get the elemental K or multiply the elemental K value by 1.2 to get the oxide value. Other fertilizer components are expressed as percent of the element such as S for Sulfur or Fe for iron, etc.

"Slow release" signifies a gradual availability of one or more nutrients. This is usually just the N component and usually just a small part of that N. Other similar terms are "controlled release" and "WIN" (water insoluble nitrogen).

"Soluble" fertilizer is the opposite of slow release. It goes into the soil solution or rainwater like salt dissolves in water. It is then immediately available for plant use. Solubles can be made to slow release by coating the granule prill with wax, plastic, sulfur, etc.

"Trace elements" are the micronutrients of iron (Fe), manganese (Mn), zinc (Zn), copper (Cu) and boron (B). Occasionally some even lesser micros are listed. "What we would really like to avoid is much sodium (Na) and chloride (Cl)," Koske says. Plants really don't need them applied, and they can ruin a soil if plentiful."

"Macronutrients," or major nutrients, are sometimes also listed as percent sulfur (S), magnesium (Mg) or calcium (Ca). These are in the same category of the big three: N, P and K, since larger amounts are present in soils.

"Organic" normally means that the fertilizer is developed from natural materials such as agricultural byproducts or sewage. It can simply mean, however, that it contains carbon (C). As such, there are natural organics and man-made synthetic organics like various urea-based nitrogen formulations. Nonorganic fertilizers are usually referred to as "inorganic" or "inorganic salts."

"Nonburing" refers to a material having a low salt index (burn potential) or a slow release of a fertilizer’s nutrient salt. Mature, seasoned or processed natural organic fertilizer is usually slow release and non-burning, but fresh organic manures may well produce a salt burn and rapid N release.

"Weed and feed" is a combination of an herbicide mixed with a fertilizer. Other pesticide and fertilizer combinations also are available.

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On the Internet: LSU AgCenter: www.lsuagcenter.com/
On the Internet: www.louisianalawnandgarden.org

Source: Tom Koske (225) 578-2222, or tkoske@agcenter.lsu.edu

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