Sulfur and Soil pH
A soil pH in the range of 6 to 7 provides the best chemical
environment for growing plants.
When pH values fall outside of this range, there are potential
problems with the availability of nutrients and there may be
detrimental shifts on microbial populations. If the pH is excessively
low (acidic), the soil can be “limed”, and the pH
increased to the proper level. Reliable tests are available
to determine how much lime is needed in any given situation,
and the process of raising pH is relatively easy and inexpensive
in most soils.
Often the problem is not one of an excessively low pH, but
actually one of excessively high pH. In this area of Texas,
pH values of 7.5 to 8.3 are the rule rather than the exception.
The lowering of soil pH (acidification) is generally not as
easy as raising pH. Sulfur is the element usually recommended
for acidification. Theoretically, this is a sound practice.
Sulfur applied to the soil is converted to sulfuric acid by
soil microbes that lower pH. The problem is that in many soils,
the amount of sulfur required to lower pH is so large that the
process is impractical.
The problem soils are the calcareous soils—those that
contain lime (calcium carbonate). This is the same lime used
to raise the pH of acidic soils. In many soils, calcium carbonate
(CaCO3) is a part of the parent material from which the soil
was formed. Soil scientists have found that as long as solid
lime is present in the soil, it is not possible to decrease
the pH. Remember that statement, because it is the key to the
decision on whether the use of sulfur will be practical for
the lowering of pH. When sulfur is added to a calcareous soil,
sulfuric acid is formed and some of the CaCO3 dissolves, but
as long as CaCO3 remains in the soil, the pH cannot be permanently
lowered. This resistance to change in pH is known as "buffering."
It is very difficult to change the pH of a highly buffered soil,
whereas the pH of a soil with a lower buffering capacity can
be changed more easily. An analogy used by soil scientists to
explain this phenomenon is to compare pH testing to pressure
testing of tires. Both a bicycle tire and a tractor tire could
be measured at 50 pounds of pressure. When air is released for
10 seconds from both tires, the bicycle tire's pressure will
drop several pounds, but the tractor tire will remain very close
to 50 pounds of pressure. The tractor tire is "buffered"
against pressure change, but the bicycle tire is not.
The pH of soils is similar. Two soils may both have a pH of
7.5. One may be readily decreased to pH 7.0 by the addition
of sulfur, whereas the other, a highly buffered soil with large
amounts of CaCO3, could be treated with large amounts of sulfur
and show little decrease in pH. Agronomists have calculated
how much sulfuric acid would be required to dissolve CaCO3 in
the soil. They have determined that if sulfuric acid costing
$1.45 per pound were to be added to a calcareous soil, 68 tons
of acid per acre at a cost of $198,000 would be required to
dissolve just one percent of CaCO3 in the upper 7 inches of
Temporary decreases of pH in highly buffered soils treated
with sulfur are not unusual. However, the detrimental effects
of excess sulfur on the plants usually outweigh any benefit
from the slight pH decrease. Some sulfur is beneficial for plants,
but if used in excess the sulfur will form excessive salts that
can easily kill the plants that you are trying to help. Excessive
salt accumulation can be tolerated if mixed in a volume of soil
such as in a garden area being tilled 8 to 10 inches deep. However,
when too much sulfur is applied as a surface application and
excess salts are formed and concentrated in a shallow soil zone,
grass or any other plants can be killed or severely damaged.
Another analogy to explain this phenomenon of temporary pH decrease
is that of a coffee pot with a glass stem along its side to
show the coffee level in the tank. When coffee is released,
the level in the glass stem drops sharply, but as soon as the
flow is stopped it quickly returns to the level in the tank.
Likewise, a highly buffered soil will show a temporary drop
in pH when sulfur is applied, but it will quickly return to
the higher, buffered pH when sulfur applications is stopped.
Situations exist in which the use of sulfur to lower pH may
be practical. A soil with pH normally in the range of 6.9 to
7.0 that has been raised by the application of irrigation water
high in calcium to a moderately high pH of 7.3 to 7.4 could
potentially be lowered by sulfur.
The main concern when the pH is high is that some plant essential
nutrients will not be available for uptake by the root system.
Rather than trying to lower pH in these calcareous soils, a
better approach may be to apply micro?nutrients to the foliage
in the liquid form. This approach has been recommended to avoid
zinc deficiencies in pecan trees by spraying the foliage with
NZN or zinc sulfate periodically.
The banding (placement under the plant in a narrow band) of
minor elements such as iron to prevent iron chlorosis has been
recommended for vegetable crops before planting occurs. This
procedure involves acidifying only a small portion of soil in
the root zone, and will increase the availability of phosphorus,
iron, zinc and manganese in the area of the band. The banding
procedure begins after planting beds have been formed in the
garden area. Simply split open the planting bed so that after
application and recovering with soil, the band of sulfur will
be 2 inches below the growing plant. For example, if a corn
seed is planted 1 inch deep, then the sulfur band should be
3 inches deep. Distribute ½ pound (½ cup) of sulfur
or iron sulfate per 10 linear feet in the split planting bed.
Iron sulfate is a better source of iron than it is acidifier
and is sometimes recommended as a "cure" for iron
chlorosis. However, in an alkaline soil, the iron of iron sulfate
(copperas) rapidly becomes unavailable for plant use. In a severely
iron deficient soil, iron sulfate should be substituted for
sulfur in the banding procedure described above.
For plants, such as blueberries, azaleas, camellias and gardenias,
which are severely sensitive to high pH soil conditions and
the subsequent lack of minor elements, an artificial growing
media must be created if successful growth is expected. The
best technique is to excavate a 4x4x4 foot hole, dispose of
alkaline soil and rock, and refill the hole with a mix of 2/3
spaghum peat moss and 1/3 washed sand. This, too, may not always
be practical, but it would be preferable to trying to lower
pH with sulfur.