Like other byproducts of industrial farming, citrus wastes may contain significant amounts of pesticide residues.  The composting process will break down and eliminate most toxic organic residues, especially if the pile gets really hot through and through. (See also:  Leaves) The effect of such high levels of potassium on the nutritional qualities of my food would also concern me if the compost I was making from these wastes were used for vegetable gardening.

Coffee grounds are nutrient-rich like other seed meals.  Even after brewing they can contain up to 2 percent nitrogen, about 1/2 percent phosphorus and varying amounts of potassium usually well below 1 percent.  Its C/N runs around 12:1.  Coffee roasters and packers need to dispose of coffee chaff, similar in nutrient value to used grounds and may occasionally have a load of overly roasted beans.

Coffee grounds seem the earthworm’s food of choice.  In worm bins, used grounds are more vigorously devoured than any other substance.  If slight odor is a consideration, especially if doing in-the-home vermicomposting, coffee grounds should be incorporated promptly into a pile to avoid the souring that results from vinegar-producing bacteria.  Fermenting grounds may also attract harmless fruit flies.  Paper filters used to make drip coffee may be put into the heap or worm box where they contribute to the bedding.  See also:  Paper.

Corncobs are no longer available as an agricultural waste product because modern harvesting equipment shreds them and spits the residue right back into the field.  However, home gardeners who fancy sweet corn may produce large quantities of cobs.  Whole cobs will aerate compost heaps but are slow to decompose.  If you want your pile ready within one year, it is better to dry and then grind the cobs before composting them.

Cottonseed meal is one of this country’s major oil seed residues.  The seed is ginned out of the cotton fiber, ground, and then its oil content is chemically extracted.  The residue, sometimes called oil cake or seed cake, is very high in protein and rich in NPK.  Its C/N runs around 5:1, making it an excellent way to balance a compost pile containing a lot of carboniferous materials.

Most cottonseed meal is used as animal feed, especially for beef and dairy cattle.  Purchased in garden stores in small containers it is very expensive; bought by the 50-to 80-pound sack from feed stores or farm coops, cottonseed meal and other oil seed meals are quite inexpensive.  Though prices of these types of commodities vary from year to year, oil cakes of all kinds usually cost between $200 to $400 per ton and only slightly higher purchased sacked in less-than-ton lots.