Why finished-looking compost burns seedlings
A heap can look done long before it is. The outside goes dark and crumbly first and smells like a wood in autumn, while the middle is still working. Sow into that and two things go wrong. Microbes still feeding on woody scraps take nitrogen out of the mix faster than the seedlings can, so the leaves pale within a fortnight. And the same breakdown gives off ammonium and short organic acids that scorch fine root tips before the first true leaf.
Salt does the same damage from a different direction. Plenty of kitchen scraps, fresh manure or seaweed raise the conductivity, and a seedling standing in salty water cannot drink. None of this shows to the eye or the nose, which is why I measure it rather than guess.
How the respiration jar works
Living compost breathes. Young compost breathes hard because there is still plenty for the microbes to eat, and mature compost has slowed right down. So I bring a weighed portion to a set moisture, stand it in a glass jar beside a small cup of alkali, screw the lid down and leave it in the incubator at 25 degrees for 72 hours.
Everything the microbes breathe out in those three days is carbon dioxide, and the alkali soaks it up. Then I open the jar and titrate the alkali from a burette to see how much it caught. That figure, set against the organic matter in the jar, goes on the report as a breathing rate. Slow and steady is what you want. A high rate means the heap needs more time, whatever it looks like.
How the cress test is laid out
Cress is quick, cheap and fussy, which makes it a good witness. I shake the compost with clean water, let it settle and filter it to a clear amber tea. Six dishes each get a disc of filter paper and 20 seeds, spaced so no two roots tangle. Three dishes are wetted with the compost extract and three with clean water from the same tap. Lids on, into a dark cupboard.
On day 7 I count every seed that has germinated in all six dishes and measure each root against a steel rule to the nearest millimetre. That is 120 seeds and usually more than a hundred roots, done by eye and by hand. The water dishes show what a good week looks like for that packet of seed, so a cold lab or a slow batch of seed never counts against your compost.
The sum on the bench sheet
GI = (extract seeds germinated × extract mean root length) ÷ (water seeds germinated × water mean root length) × 100
Compost exactly as kind as clean water scores 100.
What a germination index under 80 means
At 80 or above, the compost is fit to sow and pot into. Under 80 something is holding the roots back, usually ammonium, salt or the acids of a heap still working, and the report says which of those the other figures point to.
- Between 50 and 80, use it as a mulch or dig it into a bed four weeks or more before planting. Keep it out of seed trays and modules.
- Under 50, it goes back on the heap. Turn it, wet it if the moisture figure came back low, and give it another six to eight weeks before you test again.
- Plastic or glass in the sieve count is worth knowing about before it goes on a bed you will hand weed for years. I give the count per litre, so you can see whether it was one stray wrapper or a habit.
Most heaps I see are closer than their owners fear. A low score usually means one more month, not a failed year.
Femke