For my second attempt I decided to measure the initial contents of the bottle more precisely. The diagram below shows how the bottle was put together. 

I dried the soil so that I could accurately control how much water the bottle contained at the outset. I didn’t want to overdo the water, partly because the CO2 sensor does not work if the humidity is too high (i.e. if there is condensation), and partly because I want to work out what the minimum water requirement is. It looks as though I haven’t used enough water, since the lower part of the soil still looks dry. I also suspect that the process of drying the soil and funnelling it into the bottle has resulted in soil that is too compacted and won’t allow gases to permeate it. Rather than stop the experiment and add more water, I want to continue it as-is just to see what happens. I used 5 carrot seeds, thinking that not all of them will germinate. They will be overcrowded if they grow large enough. I covered the seeds with a thin layer of damp soil from the garden - the reason for this was that I wanted to make sure that soil microbes were present in the bottle. Drying the soil may have killed the microbes it contained.

I decided to add some moss to the bottle for two reasons. Firstly to see whether it survives. Secondly because I wondered whether there was an overproduction of CO2 in my first attempt. Perhaps the moss will help to maintain an equilibrium in the bottle and consume excess CO2. Until I’ve measured CO2 levels, this is just speculation.

I started this bottle on 8th December, below is a photograph of it.

I noticed yesterday (18th December) that some of the carrot seeds have germinated. I can tell that they are carrot seedlings and not weeds because they still have the carrot seed case attached, as you can see in the photo below.

UPDATE 4th Jan 2018:

The seedlings in this experiment have gone droopy and they look like they will die. This can been seen by comparing this photo from 29th December, when the seedlings were still upright:

With this photo taken today (4th January):

As per my log entry from 2nd January, I believe that there is more CO2 in the bottle than the seedlings can tolerate (although I haven’t measured the CO2 level in this bottle).