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Turf Fertilization: You've Got Questions. We've got Answers

By J. Mike Henry, University of California Cooperative Extension

To find out how much of a particular fertilizer is needed to supply 1 pound of actual nitrogen, simply divide 100 by the first number of the analysis shown on the bag. This will give you the number of pounds of the fertilizer you need to apply to 1,000 square feet of lawn area to supply 1 pound of actual nitrogen to the turf. For example, if the fertilizer analysis is 21-0-0, 100 divided by 21 = 4.76 pounds of fertilizer needed to apply 1 pound of actual nitrogen. (A similar calculation is performed for metric measurements.)

Proper maintenance is the prerequisite to having an attractive landscape. One of the major requirements of proper maintenance is adequate fertilization to insure optimal growth and development of leaves, roots, and the other parts of the plant. A well-planned and executed maintenance program-which includes mowing, irrigation, and thatch and soil compaction control as well as fertilization-will produce good looking, green turfgrass that will quickly recover from wear, pest damage, or mechanical injury.

To further your knowledge on the subject, Mike Henry from the University of California answered some frequently asked questions about lawn fertilization

Q. Will a lawn fertilizer that also kills weeds damage trees in a lawn area?

A. Most such products are safe as long as they do not contact tree foliage. Products containing dicamba may damage shrubs and trees if applied in their root zone. Check the label for use recommendations.

Q. Is fertilizing once a year adequate?

A. Fertilizing a lawn once a year is generally inadequate. However, if only one application is made, it should be in the fall on cool-season grasses and in late spring on warm-season grasses; a slow-release one.

Equal amounts of nitrogen should be applied yearly to both sandy and clay soils. However, on sandy surfaces, it should be applied at lower rates at a higher frequency, or in a slow-release form-to prevent nutrient leaching out of the root zone.

Q. How can I tell if my fertilizer program is adequate?

A. Your fertilizer program is adequate if your lawn is green, is dense and uniform, and has a good overall appearance. Yellowing grass blades, weed invasion, and lack of growth may indicate a need for increased amounts or more frequent application of fertilizer, assuming that poor irrigation (excess or lack of water) is not the cause.

There are a number of universities across the U.S. that have programs devoted to the study of turfgrass. One such example is the Virginia Tech Turfgrass Culture and Physiology Program. By conducting research on cultivar adaptation, cultural practices (mowing, fertilization, cultivation, irrigation) they hope to improve turfgrass quality and persistence and implement improved management practices.

Q. When does your turfgrass need fertilizer?

A. Warm-season grasses make most of their growth in the warmest months and are often dormant (brown) in winter. Warm-season species include hybrid and common bermudagrass, zoysiagrass, St. Augustinegrass, buffalograss, seashore paspalum, and kikuyugrass. Warm-season grasses are best adapted to the Central Valley and Southern California, including the inland deserts. Fertilize when grass is green, but avoid excessive nitrogen in the summer when the grass is already growing fast. Many warm-season grasses developed for Southern California can be coaxed to stay green in the winter by regular nitrogen fertilizer applications into the late fall and winter, except where frosts and colder temperatures are common.

Cool-season grasses grow well in spring, fall, and winter where the climate is moderate to mild. In areas having summer temperatures of 80 degrees F (26 degrees C) and higher, these grasses often come under heat stress, which results in reduced growth, increased disease potential, and poor appearance. Avoid nitrogen fertilizer applications in hot times of the year on cool-season grasses. Commonly used cool-season species include tall fescue, Kentucky bluegrass, perennial ryegrass, and red fescue. They are best adapted to coastal areas, Northern California (except the Central Valley), and mountainous regions of the state.

Q. How much fertilizer should you apply?

A. The amount of fertilizer to apply depends on the fertilizer product (percent nitrogen and release rate), the square footage (area) of lawn, and the purpose the lawn serves (athletic field or low-traffic lawn).

At planting--Fertilize soil before planting seed, sod, plugs, or stolons. A general recommendation for a pre-plant fertilizer for most California soils is to apply 12 pounds of ammonium phosphate-sulfate (16-20-0) per 1,000 square feet (6kg per 100 sq m), rototilled into the top 4 to 6 inches (10 to 15 cm) of soil.

Existing lawns--Most mature lawns benefit from about 4 pounds of actual nitrogen per 1,000 square feet (2 kg per 100 sq m) per year. Recent University of California research on grasses suited to low nitrogen and water applications (e.g., zoysiagrass and buffalograss) found that they could perform adequately with only 2 pounds per 1,000 square feet (1 kg per 100 sq m) of actual nitrogen per year. Grass growing in light shade requires less fertilizer than grass growing in full sun. Turfgrasses under a grasscycling program need slightly less nitrogen; turfgrasses under heavy wear from foot traffic or sports require more nitrogen to encourage faster growth to repair damage.

Irrigate your lawn deeply a few days before you apply fertilizer. The grass blades should be dry if applying a dry fertilizer, especially soluble types. Follow the fertilizer application with a thorough watering to move fertilizer off the grass blades and down into the soil.

Generally, a maximum of 1 pound of actual nitrogen per 1,000 square feet (0.5 kg per 100 sq m) should be applied at one time when using a soluble chemical fertilizer. Nitrogen is the major element, so it is the element that the application rate is based on. Also, nitrogen is the most soluble element and has the most potential for burning the grass if applied too heavily. Often, less than 1 pound of actual nitrogen can be applied, but 1/2 to 1 pound of actual nitrogen per 1,000 square feet (0.25 to 0.5 kg sq m) at a time is the usual recommendation. Slow-release fertilizers can be safely applied at higher rates. See the product label for specific recommendations on rates an frequency of application; the frequency can range from every 6 to 8 weeks to as long as every 6 months.

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October 15, 2019, 10:20 pm PDT

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