Do I have humus in my compost?

Wednesday, 5 February 2014  |  HOTBIN Composting

Compost contains humus, but there is no standard set for how much (the %). We believe most mature composts have around 10% (+/- 5%) humus. If you compared this to a commercial 'humic fertiliser' derived from ligninite, this would have >65% humic content. The exact amount of humus is likely to vary with each batch of compost.

Compost is not routinely tested for humus/humic content. The key reason for this is due to the complexity and cost of testing for fulvic acid, humic acid and humin - they need complex laboratory tests to determine its presence.

We have found the following indicator useful (based on a the key property of humus - ie it is a colloidal material):

Take a handful of compost and squeeze it - it should ooze moisture between your fingers. Now roll between your palms into a ball. Leave the ball for a couple of hours in the sun (or on a dry surface). Come back and try rubbing the dried ball between your fingers.

  • If the ball crumbles into a neat pile of small flakes – our view is it will be low in humus
  • If the ball splays between your finger and thumb into a flat sheet, and you can roll it for ages before it breaks down into crumbs, our view is this indicates high humus content. (Soil scientists call the sheet a 'thread'. Colloidal materials behave this way.

(NB: this method is used in soil science to indicate the presence of colloidal clays. So, if you have added a lot of soil or turf sods to your compost you will probably just be noticing the effect of clay rather than humus!