by Pat Vance
Volunteers for the Philadelphia Master Gardener Hort Line
have researched and answered some interesting questions. From time to time,
we'd like to share some of those questions with you.
If you have a question about gardening, call us at 215 314 8711 or send an email to philadelphiamg@psu.edu
If you have a question about gardening, call us at 215 314 8711 or send an email to philadelphiamg@psu.edu
There are more questions about soil testing than any other
topic on the Hort Line. We get questions about how to have testing done as well
as how to interpret the results.
Soil Test Kits
Getting a soil test is simple. Stop by any Penn State
Extension office to pick up a kit. The address for the Philadelphia office is
675 Sansom Street, Philadelphia, PA and the phone number is (215) 471-2200. Call
ahead to make sure someone will be there. The cost for a basic soil fertility
test is $10. The kit consists of a sample bag, envelope, and form to be
completed for your garden.
You can also print a pdf of the form, collect samples in
your own plastic bag, and mail it in your own envelope. This is the ink to
instructions on submitting samples:
To print the form, go to this url:
Open and print the pdf for “Individual Submission Form for
Turf, Home Garden, Noncommercial Fruit, Flower, Woodlot, Christmas Trees and
Landscape Plants.”
Include a check payable to “The Pennsylvania State
University” along with your sample and completed form.
Complete the entire
form as instructed.
Different plants have different nutritional requirements. Read
the list on the second page of the form and choose the one that most closely
describes your garden. Include the serial number from the bag if using a kit
from the office. You can leave that space blank if you are using your own bag.
Any clean garden trowel will work to collect samples. Collect
soil from the top 6 to 12 inches from 5 to 10 different spots in the garden to
get a good representative sample. Remove plant debris and stones. Dry the soil
on clean newspaper and then place it in your sample bag.
Be sure to label the
bag with your name! And mail it to the address on the form.
If you have multiple beds, each with a different use, you
may want to submit more than one sample, with a completed form and fee for each
one.
Test Results
Your report will arrive in about 2 weeks. There will be a
text box labeled “Soil Nutrient Levels” that will list pH as well as
phosphorous, potassium, magnesium, and calcium levels. Look at the bar graph to
see whether those levels are below optimum, optimum or above optimum. The lab
will adjust this for your stated garden use. The actual values are listed in a
box at the bottom of the page. If any item is not within the optimum range,
there will be suggestions in the text box below.
Choosing a Fertilizer
To address nutrient levels, there will be a recommendation
for fertilizer. Fertilizers available in garden centers will have an N-P-K
value consisting of 3 numbers. The first
number is the total percentage of nitrogen (chemical symbol N), the second for
phosphate (chemical formula P205), and the third for potassium (chemical formula
K2O), usually in the form of potash.
For example, a fertilizer with an N-P-K value of 5-10-50
would consist of 5% nitrogen, 10%
phosphate and 50% potassium. Fertilizers come in a variety of compositions
of these chemicals. Typical combinations include: 5-10-50, 5-10-10, 10-10-10,
8-0-24, and 6-6-18. You can see that these fertilizers would give very
different results in the garden. Your soil test will help you decide which of
these would be best for your garden.
Keep in mind, though, that chemical fertilizers must be
applied correctly. Too little will have minimal effect, and too much can harm
rather than help. Follow directions on the package carefully.
Also, chemical fertilizers don’t have a long-term effect on
the soil. The addition of compost and/or composted manure can make more
permanent improvements. You can add bone meal to increase phosphates and kelp
to increase potassium. These are available at garden centers. Compost will take
longer to improve your soil than chemical fertilizers, so you may want to add
fertilizer for a year or two while the compost does it’s magic.
pH Analysis
Another important part of your soil test is the pH analysis.
The lists on the back of the submission form shows that plants vary widely in
their optimum pH level. pH also affects how well your plants take up nutrients.
pH is a measure from 1 to 14 that indicates acidity or
alkalinity. Low pH is acidic and high pH is alkaline or basic. Neutral pH is
7.0, where a substance is neither acid nor basic. pH units change by an order
of 10, so a pH measurement of 6.0 is 10 times more acidic than a pH of 7.0.
Generally, vegetable gardens should be about pH 6.5, or a
little lower than neutral. But some plants, such as blueberries like to grow in
acidic soil, while clematis, for example, thrives in slightly alkaline soil.
You will need to do a little research on your plants to be sure you are working
toward the correct pH.
As with nutrients, there are actions you can take to change
pH. Your soil report will recommend an amount of lime to increase the pH of
your garden. The addition of sulfur will decrease pH but this has the same
short-term effect as chemical fertilizers. Compost and other organic matter
will decrease the pH of soil more gradually, but will be a more long-term fix.
More Information
For more information on building healthy soil, go to these
links:
https://extension.psu.edu/gardening-from-the-ground-up
http://agsci.psu.edu/aasl/soil-testing
http://agsci.psu.edu/aasl/soil-testing/soil-fertility-testing/handbooks
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