DATE: 02/22/06

CONTACT: Clemson Extension Home & Garden Information Center, 1-888-656-9988

WRITERS: Trent C. Hale, Bob Polomski, Debbie Shaughnessy

FERTILIZING LAWNS – WHAT DO THE NUMBERS MEAN?

CLEMSON – For lawns, the best yearly fertilization program is based on soil analysis results and usually includes a combination of one or two applications of a complete fertilizer and one or two supplemental applications of a nitrogen fertilizer. The complete fertilizer supplies nitrogen, phosphorus, and potassium, while the nitrogen material supplies mainly nitrogen. While nitrogen fertilization is based on the desired growth rate and type of turfgrass being grown, the phosphorus and potassium fertilization rate should be based on the analysis of a soil sample and the recommendations obtained from it.

When a soil test from the lawn is not available there are general guidelines to follow. Call Clemson’s Home and Garden Information Center at 1-888-656-9988 for guidelines specific to your type of turfgrass and location.

All plants require certain chemical elements for proper growth and appearance. Of these nutrients at least 16 are known to be essential elements. All essential elements except carbon, hydrogen and oxygen are obtained from the soil and absorbed by plant roots. If limited nutrients are available in the soil, lawn growth and quality may be limited. However, essential elements can be added to the soil through fertilizer applications.

Lawns require the macronutrients nitrogen (N), phosphorus (P) and potassium (K) in the greatest quantities. Calcium, magnesium and sulfur are required less frequently and in smaller quantities. The micronutrients iron, manganese, zinc, copper, chlorine, molybdenum and boron are required in very minute quantities and less often than the macronutrients. Micronutrients are as essential as the macronutrients but are required in smaller amounts.

Fertilizers are identified by analysis and/or brand name. Many common commercial fertilizers are known by their grade such as 16–4–8, 12–4–8 or 10–10–10. The numbers indicate the percentage of each of these nutrients. A

16–4–8 grade, for example, contains 16% nitrogen, 4% available phosphate and 8% soluble potash. Thus, a 100-pound bag of 16–4–8 would contain 16 pounds of nitrogen, 4 pounds of phosphate, and 8 pounds of potash. These three constituents – nitrogen, phosphorus, and potassium – are called the primary plant foods; if all three are present, the fertilizer is a complete fertilizer. Complete fertilizers like 16–4–8, 12–4–8 and 10–10–10 are commonly recommended for lawn fertilization.

Besides the primary elements (N, P and K) the fertilizer may contain secondary plant foods, such as calcium, magnesium, sulfur, manganese, zinc, copper, iron and molybdenum. Both primary and secondary elements, if present, are listed on the fertilizer label.

Most fertilizers are applied at a rate determined by the type and amount of nitrogen present in the material. Nitrogen is the nutrient most used by the grass, and often is the material that burns the lawn when applied at excessive rates.

The pounds of actual N in every fertilizer can be determined by dividing the percent N listed on the label into 100. For example, in applying soluble nitrogen from ammonium sulfate, divide 20% (the N content of ammonium sulfate) into 100 to find out the number of pounds of fertilizer that will supply 1 pound of N. Since 100 divided by 20 equals five, apply 5 pounds of ammonium sulfate per 1000 square feet of lawn to supply 1 pound of actual nitrogen per 1000 square feet of lawn. If applying N in a 16-4-8 fertilizer and the nitrogen in the product is all slow-release organic nitrogen, one could apply 2 pounds of actual nitrogen. The calculation is the same as the first sample. Divide 100 by 16 (16 is the percent N in the fertilizer). The answer is about six, so 12 pounds of the 16-4-8 would supply 2 pounds of nitrogen.

An alternative method of calculating this rate is to divide the amount of nitrogen desired per 1,000 square feet of lawn by the percentage of nitrogen contained by the fertilizer source being used. For example, if you wish to apply 1 pound of actual nitrogen per 1000 square feet of lawn using a 16-4-8 fertilizer source, divide 1 pound nitrogen desired per 1000 square feet of lawn by 0.16 (or 16% N from the 16-4-8 fertilizer). Therefore, 6.25 pounds of a 16-4-8 fertilizer should be applied per 1000 square feet of lawn to supply 1 pound of actual N per 1000 square feet of lawn.

In addition to complete fertilizers, some materials are used almost exclusively to supply nitrogen to the lawn for rapid growth and dark green color. These materials include ammonium sulfate (20% N), IBDU (31% N), urea (45% N), calcium nitrate (15.5% N) and ureaform (38% N). Nitrogen fertilizers are used as frequently as, or more frequently than complete fertilizers.

There is much confusion over whether to use organic or inorganic fertilizers on lawns. Both types have advantages and disadvantages; however, the type of fertilizer makes no difference to the grass. Grasses absorb nitrogen only as nitrate (NO 3 -) or ammoniacal-nitrogen (NH 4 +). Organic nitrogen is not used directly by the plant but must first be converted to one of these chemical forms by soil microorganisms before plants can use them. The advantages and disadvantages of organic or chemical fertilizers relate to the consumer, not the lawn grass.

Many times turfgrasses, such as centipedegrass, bahiagrass, zoysiagrass and St. Augustinegrass, turn yellow during the spring due to a lack of iron or nitrogen. However, fertilization with nitrogen is not always desirable since this often encourages disease and insect problems. Many times the addition of iron (Fe) to these grasses provides the desirable dark green color, but does not stimulate excessive grass growth, which follows nitrogen fertilization. Usually iron sulfate (2 ounces per 3 to 5 gallons of water per 1000 square feet) or a chelated iron source is used to provide this greening effect. The effect from supplemental iron application is only temporary (about 2 to 4 weeks); therefore, repeat applications are necessary for summer-long color. Do not apply iron when air temperatures are greater than 80º F or onto wet grass, and water-in immediately after application to minimize turf burn.

All fertilizers may burn lawn grasses if improperly applied. Never exceed the recommended rate, or the lawn may be damaged. Always apply fertilizers when temperatures are cool and the grass leaves are dry and water thoroughly after application.

Excerpted with permission from Southern Lawns, Bert McCarty, Editor, Clemson University Public Service Publishing, 2003.

For more information on lawn care and for answers to your other gardening, pest and food safety questions, visit the Clemson Extension Home & Garden Information Center website at http://hgic.clemson.edu or call us toll-free at 1-888-656-9988.

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