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