Particle Size Diameter of Number of Surface Area
Particles in mm Particles per gm per square cm
Very Coarse Sand 2.00-1.00 90 11
Coarse Sand 1.00-0.50 720 23
Medium Sand 0.50-0.25 5,700 45
Find Sand 0.25-0.10 46,000 91
Very Fine Sand 0.10-005 772,000 227
Silt 0.05-0.002 5,776,000 454
Composters use several strategies to maintain airflow. The most
basic one is to blend an assortment of components so that coarse,
stiff materials maintain a loose texture while soft, flexible stuff
tends to partially fill in the spaces. However, even if the heap
starts out fluffy enough to permit adequate airflow, as the
materials decompose they soften and tend to slump together into an
airless mass.
Periodically turning the pile, tearing it apart with a fork and
restacking it, will reestablish a looser texture and temporarily
recharge the pore spaces with fresh air. Since the outer surfaces of
a compost pile do not get hot, tend to completely dry out, and fail
to decompose, turning the pile also rotates the unrotted skin to the
core and then insulates it with more-decomposed material taken from
the center of the original pile. A heap that has cooled because it
has gone anaerobic can be quickly remedied by turning.
Piles can also be constructed with a base layer of fine sticks,
smaller tree prunings, and dry brushy material. This porous base
tends to enhance the inflow of air from beneath the pile. One
powerful aeration technique is to build the pile atop a low platform
made of slats or strong hardware cloth.
Larger piles can have air channels built into them much as light
wells and courtyards illuminate inner rooms of tall buildings. As
the pile is being constructed, vertical heavy wooden fence posts, 4
x 4's, or large-diameter plastic pipes with numerous quarter-inch
holes drilled in them are spaced every three or four feet. Once the
pile has been formed and begins to heat, the wooden posts are
wiggled around and then lifted out, making a slightly conical airway
from top to bottom. Perforated plastic vent pipes can be left in the
heap. With the help of these airways, no part of the pile is more
than a couple of feet from oxygen
_Moisture. _A dry pile is a cold pile. Microorganisms live in thin
films of water that adhere to organic matter whereas fungi only grow
in humid conditions; if the pile becomes dry, both bacteria and
fungi die off. The upwelling of heated air exiting the pile tends to
rapidly dehydrate the compost heap. It usually is necessary to
periodically add water to a hot working heap. Unfortunately,
remoistening a pile is not always simple. The nature of the
materials tends to cause water to be shed and run off much like a
thatched roof protects a cottage.
Since piles tend to compact and dry out at the same time, when they
are turned they can simultaneously be rehydrated. When I fork over a
heap I take brief breaks and spray water over the new pile, layer by
layer. Two or three such turnings and waterings will result in
finished compost.
The other extreme can also be an obstacle to efficient composting.
Making a pile too wet can encourage soft materials to lose all
mechanical strength, the pile immediately slumps into a chilled,
airless mass. Having large quantities of water pass through a pile
can also leach out vital nutrients that feed organisms of
decomposition and later on, feed the garden itself. I cover my heaps
with old plastic sheeting from November through March to protect
them from Oregon's rainy winter climate.
Understanding how much moisture to put into a pile soon becomes an
intuitive certainty. Beginners can gauge moisture content by
squeezing a handful of material very hard. It should feel very damp
but only a few drops of moisture should be extractable. Industrial
composters, who can afford scientific guidance to optimize their
activities, try to establish and maintain a laboratory-measured
moisture content of 50 to 60 percent by weight. When building a
pile, keep in mind that certain materials like fresh grass clippings
and vegetable trimmings already contain close to 90 percent moisture
while dry components such as sawdust and straw may contain only 10
percent and resist absorbing water at that. But, by thoroughly
mixing wet and dry materials the overall moisture content will
quickly equalize.
_Size of the pile._ It is much harder to keep a small object hot
than a large one. That's because the ratio of surface area to volume
goes down as volume goes up. No matter how well other factors
encourage thermophiles, it is still difficult to make a pile heat up
that is less than three feet high and three feet in diameter. And a
tiny pile like that one tends to heat only for a short time and then
cool off rapidly. Larger piles tend to heat much faster and remain
hot long enough to allow significant decomposition to occur. Most
composters consider a four foot cube to be a minimum practical size.
Industrial or municipal composters build windrows up to ten feet at
the base, seven feet high, and as long as they want.
However, even if you have unlimited material there is still a limit
to the heap's size and that limiting factor is air supply. The
bigger the compost pile the harder it becomes to get oxygen into the
center. Industrial composters may have power equipment that
simultaneously turns and sprays water, mechanically oxygenating and
remoistening a massive windrow every few days. Even poorly-financed
municipal composting systems have tractors with scoop loaders to
turn their piles frequently. At home the practical limit is probably
a heap six or seven feet wide at the base, initially about five feet
high (it will rapidly slump a foot or so once heating begins), and
as long as one has material for.
Though we might like to make our compost piles so large that
maintaining sufficient airflow becomes the major problem we face,
the home composter rarely has enough materials on hand to build a
huge heap all at once. A single lawn mowing doesn't supply that many
clippings; my own kitchen compost bucket is larger and fills faster
than anyone else's I know of but still only amounts to a few gallons
a week except during August when we're making jam, canning
vegetables, and juicing. Garden weeds are collected a wheelbarrow at
a time. Leaves are seasonal. In the East the annual vegetable garden
clean-up happens after the fall frost. So almost inevitably, you
will be building a heap gradually.
That's probably why most garden books illustrate compost heaps as
though they were layer cakes: a base layer of brush, twigs, and
coarse stuff to allow air to enter, then alternating thin layers of
grass clippings, leaves, weeds, garbage, grass, weeds, garbage, and
a sprinkling of soil, repeated until the heap is five feet tall. It
can take months to build a compost pile this way because heating and
decomposition begin before the pile is finished and it sags as it is
built. I recommend several practices when gradually forming a heap.
Keep a large stack of dry, coarse vegetation next to a building
pile. As kitchen garbage, grass clippings, fresh manure or other wet
materials come available the can be covered with and mixed into this
dry material. The wetter, greener items will rehydrate the dry
vegetation and usually contain more nitrogen that balances out the
higher carbon of dried grass, tall weeds, and hay.
If building the heap has taken several months, the lower central
area will probably be well on its way to becoming compost and much
of the pile may have already dried out by the time it is fully
formed. So the best time make the first turn and remoisten a
long-building pile is right after it has been completed.
Instead of picturing a layer cake, you will be better off comparing
composting to making bread. Flour, yeast, water, molasses, sunflower
seeds, and oil aren't layered, they're thoroughly blended and then
kneaded and worked together so that the yeast can interact with the
other materials and bring about a miraculous chemistry that we call
dough.
_Carbon to nitrogen ratio._ C/N is the most important single aspect
that controls both the heap's ability to heat up and the quality of
the compost that results. Piles composed primarily of materials with
a high ratio of carbon to nitrogen do not get very hot or stay hot
long enough. Piles made from materials with too low a C/N get too
hot, lose a great deal of nitrogen and may "burn out."
The compost process generally works best when the heap's starting
C/N is around 25:1. If sawdust, straw, or woody hay form the bulk of
the pile, it is hard to bring the C/N down enough with just grass
clippings and kitchen garbage. Heaps made essentially of high C/N
materials need significant additions of the most potent manures
and/or highly concentrated organic nitrogen sources like seed meals
or slaughterhouse concentrates. The next chapter discusses the
nature and properties of materials used for composting in great
detail.
I have already stressed that filling this book with tables listing
so-called precise amounts of C/N for compostable materials would be
foolish. Even more wasteful of energy would be the composter's
attempt to compute the ratio of carbon to nitrogen resulting from
any mixture of materials. For those who are interested, the sidebar
provides an illustration of how that might be done.