Compost by definition is organic. It is the decayed

remainder of flora and fauna and thus is processed organic material.
That said, additives both known and unknown can add inorganic material
to the mix.

Some organic gardeners like to add certain mineral
additives to their compost such as dolomitic limestone or greensand to
lower pH and add calcium to the mix. It is doubtful if this is more
effective than adding these materials directly to the soil especially
in the case of pH control as compost piles tend to go neutral by the
nature of their composting action. Still, it means that both materials
can be added to the soil in one go.

On the other hand, one has to
be careful of the materials added to the compost pile. Grass clippings,
fall leaves and other supplies that may be picked up at the roadside as
others throw them out, can have pesticide and herbicide residue. Either
leave the offerings where they lie or be sure that the people disposing
of them have not used any chemical applications over the past month or
more.

It is easiest to ensure that compost is organic by making
one's own. There is no need for other than organic material in the
first place. A healthy mix of brown and green vegetable matter will
have almost all if not everyone of the minerals necessary for good
plant growth. Green or wet material will usually be damp, odored, and
some variation green in color. Examples are grass clippings and kitchen
waste. Brown or dry materials, on the other hand, will be dry, brown or
gray in color, and largely odorless. Autumn leaves, spoiled hay, wood
shavings, and dried weeds are examples.

When gathering materials,
a volume mix of about two parts brown to one part green matter is about
right for a good pile. If there is much newspaper or wood shavings in
the brown material, a ratio of one to one is good. You want enough
carbon from the dry material to feed the microorganisms in the pile and
enough nitrogen for them to produce protein. Wood shavings are not the
threat to compost piles that some composters believe them to be. In
fact, many wood turners and other wood workers seek spalted wood for
its color. Spalted wood is simply wood that has dark lines and tan
areas in it from the beginnings of rot. Composting is just a name for
controlled rotting so spalted wood is already underway. Look for
shavings of maple, birch, and poplar in particular as these woods are
very prone to spalt.

Mix the materials well and make sure that
they are sufficiently moist. Ideally they will feel like a wrung out
dishcloth. If the pile is well built it will heat in a couple of days
to where it is too hot to keep your hand in it. The more often it is
turned the faster it composts, but it will rot down for you into a rich
compost. When it is dark, crumbly and earthy smelling, your compost is
ready to use.

You can know the compost is organic easiest by
making your own. Like most things, honest materials and honest work
give honest results.