pH is the measure of the acidity or alkalinity
of soil, with 7 considered “neutral” and
numbers below acidic and above alkaline.
Composting feedstocks have a pH, which will
fluctuate during the composting process.
The initial pH of garbage, yard clippings,
manure, and other compostable material is
likely between 5.0 and 7.0 unless it contains
ash or other highly alkaline materials. If
the material has begun putrefying before
being received for composting, the pH will
be near the lower value, since anaerobic
organisms produce acids. When the initial
pH is between 6.0 and 7.0, the pH of the
composting material may drop a little during
the first two or three days of aerobic composting,
also due to the formation of acids. If the
pH is 5.0 or 5.5, there will be little change
during this period.
After two to four days the pH usually begins
to rise and will level off at between 8.0
and 9.0 towards the end of the process. The
control of the pH in composting is seldom
a problem requiring attention if the material
is kept aerobic, but large amounts of organic
acids are often produced during anaerobic
decomposition on a batch basis. Ash, carbonates,
lime or other alkaline substance will act
as a buffer and keep the pH from becoming
too low. Adding alkaline material is rarely
necessary in aerobic decomposition. In fact,
it may do more harm than good because the
loss of nitrogen by the release of ammonia
as a gas will be greater at a higher pH.
Since the optimum pH for most organisms is
around 6.5 to 7.5, it would probably be beneficial
if the pH could be maintained in that range.
However, since composting is necessarily
a batch-process operation, minor changes
in the pH are normal.
Apparently, initial pH values of 5.0 to 6.0
do not seriously retard initial biological
activity since active decomposition and high
temperatures develop rapidly after material
is placed in the stack. Temperatures do appear
to increase a little more rapidly when the
pH is in the range around 7.0 and above.
The usual feedstocks available for composting
present no problem of pH control.