Cardboard in the compost heap
Cardboard will rot down well in the compost heap providing a good source of carbon as is particularly useful when other sources such as dried autumn leaves and straw or hay is not available . Cardboard will also introduce air pockets providing oxygen to the aerobic composting microbes.
In the past, some types of cardboard packaging where not suitable for composting because the inks, glosses and glues were petroleum-based and contained metallic pigments but now most are water-based and made from organic materials such as vegetable oil, soybean and kaolin (clay).
Corrugated cardboard is usually used for packing It composts well and has the advantage of containing air in the corrugations. Corrugated cardboard boxes may have been sealed with sticky tape, which will need removing before being added to the compost bin as they do not degrade will only result in compost contamination from micro-plastics.
Corrugated cardboard can be used to insulate a compost bin with slated sides in winter.
A 4-inch (10 cm.) layer of shredded corrugated cardboard can also be used as a base layer of a heap where twigs, straw, or brassica stalks are not available.
Flat cardboard is used for cereal boxes, shoe boxes. This lacks the corrugations which makes it easier to shred using a shredder. Paper towel and toilet rolls and cardboard egg cartons can be easily torn into small pieces by hand
Wax-coated cardboard is not compostable and can be distinguished from shiny cardboard, which can be composted by means of the Scratch Test which simply involves scratching the cardboard and see if the wax comes off
Shred or tear?
Shredded cardboard decomposes more quickly as it will have a higher surface area exposes to the composting microbes, but a heavy-duty shredder will be required for corrugated cardboard and one may not be available.
If the cardboard cannot be shredded cutting it with a box cutter is quite effective but is time consuming and runs the risk of self-inflicted injury it is probably easier to just tear it. Large sheets of cardboard can be soaked by using them as a cover on the heap in rainy weather. The cardboard will also help retain warmth and help keep the heap dry. The soaked cardboard will be easier to tear and any tape will be easier to remove.
Alternatively soaking the cardboard in a container of water with a few drops of liquid detergent added will make it easier to tear and will speed the decomposition process. Soaking also makes it easier to remove the parcel tape