How to use it
- Enter the weight of each material as you would add it, wet. Set the weight of an unused slot to 0.
- Check the moisture, the nitrogen share (on a dry basis) and the C:N ratio of each material. Change the defaults if you know better values.
- Read the C:N ratio of the mix and its moisture. Put the material you would add more of (usually the brown one) in slot 2 to see how much brings the pile to the target.
How it works
For each material the calculator works out the dry matter, then the nitrogen and carbon in it:
- dry matter = weight × (1 − moisture)
- nitrogen = dry matter × N%
- carbon = nitrogen × C:N
C:N of the mix = total carbon ÷ total nitrogen. The weight of material 2 for a target ratio R makes the carbon left over by the other materials balance out: the sum of nitrogen × (C:N − R) over all materials equals zero. If no positive amount does that, the line is not shown.
Example
10 kg of grass clippings and 10 kg of dry leaves, with the defaults:
- grass: 10 × 0.18 × 0.024 = 0.0432 kg of nitrogen and 0.0432 × 17 = 0.734 kg of carbon;
- leaves: 10 × 0.62 × 0.009 = 0.0558 kg of nitrogen and 3.013 kg of carbon.
The mix is (0.734 + 3.013) ÷ (0.0432 + 0.0558) = 37.9 : 1, with a moisture of 60%. To reach 30 : 1 you need only 4.2 kg of leaves per 10 kg of grass, because the wet grass has so little nitrogen per kilogram of fresh weight.
Tips and common mistakes
- Moisture matters as much as the ratio: a squeezed handful should feel like a wrung-out sponge.
- Shred woody and leafy browns; a good ratio does not help much if the pieces are large.
- Food scraps and fresh grass are greens; dry leaves, straw, cardboard and wood chips are browns.