What is the best particle size for bulking agents?

14 January 2013  |  HOTBIN Composting

Choosing the right bulking agent is a balancing act. Larger chips are stronger and create big gaps, but large chips are undesirable in the final compost. Small irregular particles dramatically increase the surface area (hence FAS) and will largely compost down to form part of the final product. Very small particles (fines) tend to compress and gaps are lost.

A range of particles sizes provides the best balance.

Our recommended distribution range is:

  • 0% >2cm
  • 15% 1-2cm
  • 40% 0.5-1cm
  • 40% 0.2-0.5
  • 5% fines (<0.2cm)

What is the correct water content of bulking agents?

Bulking agent should be dry, the dryer the better.

Remember - the bulking agent retains airflow. It is not meant to compost down.

Do not add soaking wet bulking agent - it will hinder your HOTBIN achieving high temperature. Avoid storing outdoors in containers without lids that are open to rain and damp. In winter, it is preferable to keep the bulking agents indoors (e.g. garage or shed).