As mentioned earlier, the vast majority
of the plants you will be growing in your landscape will
prefer a soil pH in the slightly acid range of pH 6.0 to
7.0. Those plants in the "acid loving" range will need a
pH in the range of 4.5 to 5.5 or so. If you have some of
these special needs plants, be sure to include that
information on your sample submission form. The
recommendations will be based on the proposed "crop".
A. Making the Soil Less Acid - If the soil pH is well below 6.0 and
you are growing the general range of landscape plants,
the soil test may recommend that you apply some garden
lime. This will react with the soil and raise the soil
pH (make it less acid) into the acceptable range.
There are a couple of things to remember about the use
of lime. One is that (as far as I know) the only thing
plane old lime does for your soil is to reduce the
amount of acidity. If you are around horticulture for
long enough, you will hear all kinds of claims for lime.
No, it does not kill beetle grubs or automatically cure
a moss problem. For the home gardener, lime only makes
the soil less acid.
There is a form of lime called
dolomitic limestone. Regular lime is calcium carbonate
while dolomitic lime has
magnesium combined with it.
This is great IF you need more magnesium (Mg) in your
soil. However, the calcium carbonate part still just
raises the pH like regular lime.
The second rule about lime is even more important. It is
that you should only apply lime in response to a soil
test from a professional soil laboratory. Some
people follow old garden tales and just routinely apply
lime to their gardens and landscapes each spring. In
some areas of the country where the soils may be very
acid to begin with, this may work out. However,
in most regions, this is a big gamble because you are
raising the pH level higher and may push it out of the
desired range for your plants.
Also, even if you get an accurate pH reading from a home
kit or other source, you won't know how much lime you
need to apply. To raise the pH from 5.0 to 6.5 will
require differing amounts of lime depending on the soil
type, the chemical buffering property of the soil and
other chemistry. Again, soil test results from a
professional lab will calculate how many pounds of lime
you need to apply per 1,000 square feet of bed or border
to make the appropriate change. Remember that it may
take a full season for that change to actually occur in
the soil.
B. Making the Soil More Acid - If the pH is too high (above 7.0 i.e.
alkaline), you may need to attempt to make the soil more
acid. I say, "attempt" because this is generally a less
precise process than raising the pH with lime. The most
commonly used products for lowering the pH include those
containing sulfur. Garden sulfur and aluminum sulfate
are commonly used products that react with the soil to
form more acid. Depending on the buffering factors in
your soil, things may or may not change even if you
apply the recommended amounts. Also, it may take a long
time to see the expected results.
Certain types of organic matter can also contribute to a
lowering of the pH in a soil. Compost that has been
totally decomposed is generally neutral in pH. However,
such materials as oak leaves, pine needles and certain
peat moss products create an acid environment as they
decompose. Incorporating large amounts of these into the
soil when preparing beds or borders for rhododendrons or
boxwood can help or to just help lower the pH should
help.
Be aware of the potential impact of
your irrigation water on the pH of your soil. Many
municipal water systems add alkalinity to their waters
to "protect the pipes". I know of one local water supply
that delivered water with a pH of around 9.0. Using this
water consistently for irrigation of your beds and
borders may have an effect on your soils. Have your
water tested for pH or give your municipal water system
a call to find out their general pH level. This will
help you know if you need to make extra efforts to keep
your soils in the proper acidity range.
C. When to Apply - If (and only if) your soil test
recommends applications to either raise or lower your
soil pH, this should be done during the soil preparation
process. After you have turned your soil and worked in
the organic matter, spread the materials (lime or
sulfur) on top of the soil and mix it in to the top 6
inches or so. This can be done at the same time as the
initial application of fertilizer.
Related Articles: Soil pH
- Acid Soils -
Alkaline Soils - Lime -
Soil Test
