University of Vermont Extension
Department of Plant and Soil Science
Spring News
Article
FERTILIZING
LAWNS
Dr. Leonard Perry, Extension
Professor
University of Vermont
Fertilizer
is essential for good lawn growth, yet with good lawn management and soil
testing you often can reduce the amount needed.
Soil test kits are available from state Extension Services, and include
instructions for proper sampling.
Testing
your soils every three years or so is a good practice. A key component of a
soil test is the soil acidity, or pH. If
this is not between 6.0 and 7.0 (the latter being neutral, between acid and
alkaline), certain nutrients won’t be available to the lawn, no matter how much
you apply.
If
available, have your soil organic matter tested too. It should be above
three percent. If lower, you should sprinkle compost heavily
(up to half inch) over the lawn a few times during the season. Make sure
the
compost is aged, and free of weed seeds.
Mow
more frequently to avoid having an excess of grass clippings so you can leave
them on the lawn. This simple practice
recycles nutrients back into the soil, and has been shown in research at the
University of Connecticut to reduce fertilizer needs by 50 percent or more.
In Vermont and similar cold climates with
shorter growing seasons, fertilize lawns in late April, early summer, and then
late summer. If you leave grass
clippings, and have good organic matter in the soil, you may need to only
fertilize twice—early spring and late summer.
At each
application time, you should apply one pound of actual nitrogen for every 1000
square feet of lawn. The key words here
are “actual nitrogen”, not pounds of fertilizer. Of the three numbers on
a fertilizer bag, the first is percent nitrogen. So a 5-3-4 fertilizer has five percent
nitrogen. To figure how much fertilizer
will give you one pound of nitrogen, divide one by the first number (one
divided by five is 0.2), then multiply by 100.
So 20 pounds of 5-3-4 will yield one pound of actual nitrogen. If you have 5000 square feet for instance,
you’d need five
times that, or
100 pounds of fertilizer, evenly spread over the area.
Apply
fertilizer with a fertilizer spreader. Spreading fertilizer by hand will always
cause some spots to be over-fertilized and others to have none. When using a
spreader, be sure to get complete coverage of the lawn. Keep moving at a
constant speed to prevent uneven spreading. Any missed spots will appear quite
yellow. Do not fill the spreader when it is sitting on the lawn. Fertilizer
spills are inevitable and may cause a large dead spot that persists for weeks.
Use
caution when applying fertilizer combined with herbicide, especially with
broadcast spreaders. These spreaders can throw the material into flowerbeds
where the herbicide can injure desirable ornamental plants, or tree and shrub
roots can pick these up from under lawns.
Lawn
fertilizers vary in composition and price. An ideal composition for a
chemical lawn
fertilizer is a 4-1-2 ratio of nitrogen, phosphorus, and potassium, for
example
20-5-10. Many others created for lawns, that may not be this exact
ratio, will
work fine too. For a natural fertilizer,
a 3-1-2 ratio is good, but anything close such as a 5-2-4 fertilizer
works fine. Some areas where phosphorus (P, the middle number)
may be carried with rain into watersheds, causing potential pollution
and algae blooms, restrict P usage. Check with your local
supplier on such rules. If your soil has sufficient P already, as
many do, none may be needed. In fact, some lawn fertilizers no
longer contain P.
The
price of the fertilizer relates somewhat to the analysis and the nutrient
carriers used
in the
fertilizer. Less expensive chemical fertilizers are usually water
soluble, thus
have a high potential to burn the grass if applied when hot, at high
rates, and
not watered in after spreading. Being
water soluble they are easily leached out of soil with rain and
watering, so may
pollute water sources. Yet water-soluble
fertilizers will give a response for four to six weeks, become available
in the
spring when temperatures are still cool, and result in more and darker
growth sooner. If overfed with such fertilizers, succulent
grass growth may be more susceptible to insects and diseases.
More
expensive fertilizers are not water-soluble (often seen as WIN for water
insoluble nitrogen), have low burn potential, and give a response for up to
eight weeks, or longer. These fertilizers rely on microorganisms in the soil to
release the nutrients. Since the microorganisms are not active when the soil is
cool, the fertilizers will not become available early in the spring. Not being
water soluble is the reason they stay around longer, and so have less potential
for pollution from runoff. Many good
lawn fertilizers will have a mix of both water soluble, and water insoluble
nitrogen. Where lawns are watered
regularly, especially on sandy soil, these water insoluble fertilizers should
be used.
Many
natural-based fertilizers are water insoluble, so last for a long period. Such natural-based or organic fertilizers
have other benefits, such as aiding soil microorganisms needed for healthy
soil. These various forms of soil life
provide many benefits, including converting nitrogen from the air for use by
plants, producing carbon dioxide which plants need, dissolving nutrients from
minerals in rocks, decomposing thatch, aerating the soil, and helping to reduce
pests and diseases. So in spite of lower numbers (percent of nutrients) on the
bag than with chemical fertilizers, these natural fertilizers enrich the soil
in many more ways than the chemical fertilizers.
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