Many garden and home-improvement stores offer a seemingly endless selection of
fertilizers. What’s the best choice for your lawn or garden? Only your soil
knows for sure.
The easiest way to make sure your plants get the nutrients they need is by
having the soil tested, says David Trinklein, University of Missouri Extension
horticulture specialist.
Soil tests for lawns, gardens and farm fields are available through county
MU Extension centers for a nominal fee. A basic soil test report will include
information about soil pH, organic matter content and the availability of key
nutrients such as nitrogen, phosphorus and potassium.
Trinklein says laboratory tests are more accurate than over-the-counter
test kits and provide specific recommendations on fertilizer and other
amendments based on what you want to plant, whether it’s flowers, vegetables,
turf or trees.
“You are absolutely guessing unless you have the soil chemically analyzed,”
Trinklein says. “Not even the most astute soil scientist in the world can look
at or feel the soil and tell us the pH or the phosphorus content.”
The notion that “if a little is good, more is better” doesn’t apply to
fertilizer, he says. Overfertilizing contributes to water pollution when excess
nutrients in runoff find their way into streams, lakes and rivers. It can also
lead to brown or withered plants, or promote leaf growth at the expense of
blossoms or fruit.
Collecting the sample
While there are tools specifically designed for extracting soil samples,
Trinklein says an ordinary garden spade will do.
Remove a slice of soil, going about 7 inches deep. Remove the soil on
either side of the slice to leave a strip about an inch wide and 7 inches long.
Take about five samples from random spots in the garden, taking care to avoid
depressions and berms, which do not give good soil samples, Trinklein
said.
Mix the five samples thoroughly to create a composite. Collect about one
pint of the composite. You can get free sample boxes from your county MU
Extension center along with the form that should accompany your sample. You can
also download the form at www.extension.missouri.edu/MP555. Results will arrive by mail
in about a week or two.
Results and recommendations
The soil test report will include recommendations for adding specific
nutrients, measured in pounds per 1,000 square feet (or pounds per acre for
farms), based on the needs of the plants you specified in the form included with
your sample.
Commercial fertilizers generally contain a mix of three key
nutrients—nitrogen, phosphorus and potassium, and the label will list the
percentage of those nutrients in that order. For example, a 10-pound bag of
fertilizer with a grade of 5-10-5 would contain a half-pound of nitrogen, a
pound of phosphorus and a half-pound of potassium.
Another key part of the soil test report is the pH, a measure of how acid
or alkaline the soil is. The lower the pH, the more acidic the soil. Soil pH has
to fall within a fairly narrow range for plants to make full use of nutrients.
Most fruits and vegetables grow best in soil with a pH of 6.0-6.5, though some
plants prefer more acidic soil.
“The tendency in nature is for soils to become more and more acid as plants
use the nutrients,” Trinklein said.
Because of this, you may need to increase pH by adding lime—an
acid-neutralizing material typically made from crushed or ground limestone. If
the soil has an unusually high pH or you are growing plants that prefer acidic
soil, such as blueberries, you may need to apply sulfur to increase
acidity.
For more information on using your soil test report, see the MU Extension
publication “Lawn and Garden Soil Test Interpretations and Fertilizer
Recommendation Guide” (MP733), available for free download at www.extension.missouri.edu/MP733.
For more information about collecting and submitting soil samples, go to soilplantlab.missouri.edu/soil/soilsamples.aspx.
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