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Fertilizing Facts

By Dr. Van Cline,The Toro Company's Center for Advanced Turf Technology

Groundskeepers should aim to remove no more than one-third of turf growth in any single mowing. This mow looks to be slightly more than one-third. Over-fertilizing causes excess growth, which wastes time and money and can make turf more vulnerable to fungal disease. Photos courtesy of the Toro Company.

How Often Should a Lawn Be Fertilized? Frequency of fertilization depends on several factors, including turf species, soil conditions, intensity of management, intensity of lawn use, and type of fertilizer.

Some species, such as the fine fescues, require less fertilization. Sandy soils require more than naturally fertile loam or clay soils. Higher intensities of management and use require more fertilization for healthy growth. Slow-release fertilizers can be applied in fewer, but heavier, applications than soluble or quick- release fertilizers. However, as a general rule of thumb, most lawns should be fertilized two to three times a year.

Sixteen essential nutrients are required for healthy plant growth. Most of these nutrients are supplied naturally through the soil system. Lawn fertilizers typically supply the three nutrients consumed in the largest amounts: nitrogen, phosphorus and potassium. Nitrogen is the single most important nutrient, and as a result, fertilizer applications are usually referred to in terms of pounds of nitrogen applied per 1000 square feet of turf. Most turfgrasses require 1/2 to 1 pound of nitrogen per 1,000 square feet of lawn per month of active growth. Two to four pounds of nitrogen in northern or Midwestern climates is sufficient for an irrigated lawn. Southern climates with longer growing seasons will need more. Fertilizer applications on cool season lawns should be made in the spring and fall during active growth and avoided during the heat of summer, when turf is stressed. One scenario is to apply 1/3 of the total seasonal requirement in the spring in one application of a high-quality, slow-release fertilizer, and the remaining 2/3 in the late summer and fall in two applications. Late fall applications should include more soluble or quick-release nitrogen. Warm season grasses should be fertilized more in the mid-summer period, when the plants are most active.

Uneven fertilizer application can result in uneven growth--as seen here. Using a rotary spreader helps apply fertilizer more evenly, avoiding striped patterns on the grass.

Fertilizers should be applied responsibly, since they can be a significant source of pollutants in surface and ground water. Phosphorus-containing fertilizers actually have been banned in some states and municipalities, except for the establishment of new turf or in phosphorus-deficient soils. Label instructions for fertilizer applications should always be followed. Applying more than the suggested amount does not necessarily produce a healthier lawn. As a matter of fact, over-fertilizing can reduce the overall health and vigor of a turf and pollute the environment at the same time. Fertilizers should never be thrown onto hard surfaces, where they can end up in storm sewers. Over watering is detrimental, as well, because it flushes nutrients out of the soil system and into surface or ground waters. Collecting and disposing of grass clippings also is a potential source of pollutants and is not necessary to maintain a healthy lawn. In fact, collecting clippings removes up to 30 percent of the nutrients that you apply as fertilizer. The best advice is to apply only what you need to maintain your desired level of quality and to do it responsibly.

Another way to ensure even fertilizer application is to spread the granules in two directions--one half of the total from each end. Using a slow-release blend and watering well after application will also help avoid uneven growth patterns.

What Do the Three Numbers Represent on a Bag of Fertilizer?

Most lawn fertilizers contain a combination of nitrogen (N), phosphorus (P) and potassium (K) from different chemical sources. The three numbers on a fertilizer bag represent the concentration of each nutrient in that order (N-P-K) as a percent. For example, a 50 lb. bag of fertilizer labeled as 28-3-3 contains 28 % nitrogen (or 14 lbs. of the 50 total), 3% phosphorus (1.5 lbs. of the total) and 3% potassium (1.5 lbs.). If you know the square footage of a lawn, you easily can calculate the amount of a specific fertilizer product that you need to apply based on the desired application rate (guidelines above), the amount of nitrogen in the bag, and the area of the lawn.

What's the Best Way to Apply Fertilizer?

In general...

o Use only the amount of fertilizer called for, based on turf type and a lawn's square footage.

o Use a rotary spreader, which applies fertilizer more evenly, allows for a quicker application, and helps avoid a striped fertilizer pattern in the grass.

o Spread the fertilizer in at least two directions for each application (i.e. half the amount in one direction and half in the opposing direction).

o Water the turf well immediately after fertilizing.

o Sweep up any fertilizer spilled on paved areas and save for later use.

o Use slow-release fertilizers whenever possible, since they last longer and don't have to be applied as frequently as quick-release varieties.

Warm-season turfgrasses like this St. Augustine need more fertilizer than cool-season turf because they grow in warmer climates throughout the year. Peak fertilization for warm-season turf should take place in mid-summer, when the turf is growing rapidly.

Is There Such a Thing As Over-Fertilizing a Lawn?

A healthy lawn is supported by a healthy, deep root system. Applying too much nitrogen fertilizer stimulates excessive leaf growth at the expense of root growth. Since a grass plant has a limited amount of energy and resources to sustain itself, too much available nitrogen causes it to partition or shunt more energy to shoot growth which robs the roots of resources. The result can be an inadequate root system that, over time, results in a less vigorous and lower-quality turf. Excessive fertilization also causes what's called soft or succulent leaf tissue, which is more susceptible to fungal diseases and is less tolerant of traffic and wear.

Know Your Turf - Warm Season vs. Cool Season Grasses

The timing and intensity of any turf management practice should take into account the condition of each turf species in a particular stage of its annual growth cycle. The species are divided into two general categories--cool season grasses and warm season grasses -- and each grows according to a predictable growth cycle. The distinction between the two categories is based on physiological differences that have evolved through time in response to cool and warm climates. The effectiveness of any management practice--whether it's mowing, fertilizing, aerification, or watering--depends in large part on the condition of the plants.

Soft and pliable, zoysia turfgrass is a warm-season type that requires repeated fertilizer applications throughout the year.

The cool season grasses, such as Kentucky bluegrass, perennial ryegrass, creeping bentgrass and the fescues, are adapted to cool conditions, and, as a result, grow vigorously in the spring and fall when conditions are optimum. The cool season grasses tend to suffer through the heat of mid-summer, however. The warm season grasses, such as bermudagrass, zoysiagrass, centipedegrass, and St. Augustine, are adapted to hot climates and grow most vigorously in the high temperatures of summer. The warm season grasses are most delicate in periods of cool weather in the late fall, winter and early spring.

Cool-season grasses established from seed are frequently packaged as a mixture of two or more species. There are advantages to planting a mixture, since each species has slightly different tolerances for growth. Different species will exploit variations in conditions in a typical lawn. For example, fine fescues will do better in shady conditions, while Kentucky bluegrass and perennial ryegrass will thrive in full sun. Warm-season grasses tend to be planted as monostands since they are more commonly sodded or sprigged rather than seeded.

Using a mulching type mower can dramatically reduce the amount of fertilizer a lawn requires to stay green and healthy. Collecting and tossing clippings, in fact, can remove up to 30 percent of the nutrients you pay to apply.

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May 19, 2019, 8:29 am PDT

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