See the composition of this substrate mixture I've bought:

enter image description here

It only mentions organic stuff, like vegetal compost and composted pine bark. At the same time, it says that the organic matter content is 40-50%. So my question is: what would be the remainder 60-50% of the mixture that is not considered to be organic matter?

From the composition, I was expecting it to be 100% organic matter, since it's all composted organics.

This is what is looks like:

enter image description here

2 Answers 2


The numbers are confusing if you don't understand how they were measured, because the quoted "dry matter organic content" of 40%-50% does not account for all the organic material in the original compost.

Drying the compost in an oven to measure the "dry matter weight" causes chemical reactions in the organic material and the volatile products are "boiled off" along with the water content.

Based on a 40%-50% dry matter organic content, the actual organic content of the material in the bag measured by volume is probably around 85%-90%, and the other 10%-15% is mainly water.

Attempting to measure the "original water content" consistently is hard to do, because the compost in its natural state also contains air, and the amount of air obviously depends on how much you compress the material before you measure its volume. The "dry matter content" may not give the "correct" number, but it does give consistent numbers to compare different compost samples.


It probably just means the water content (by weight) is 60-50 per cent. The language is somewhat incoherent.

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