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.
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.
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.
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.