Sorry to break it down but you will probably get less than 2grams, probably the substrate from a public garden has a lot of dog piss and crap, most likely, if not dog's maybe it has human's piss and crap. The temperatures fluctuate too much... and after checking your plant i now know for sure the problem is that your pot is too small, it is root bound, wont grow anymore, and even if it could i dont know if ot would in that soil.
Try buying biobizz all mix and just place the seed in the soil and adjust temperatures, use a bigger pot and if possible get a ph meter, otherwise you can use bottled water, just look for one that says 6.0 to 6.5 ph on the lable.
Hope it helps and if it does please pick my answer as it would help out quite a lot on the grower of the month contest.
Thanks.