I'm thinking of experimenting with garden soil as well, but don't have a courage doing it right now.
And I have a question about water change.
I have 10 gallon planted tank with regular gravel, liquid fertilizer and tabs.
The first 6 month my tank started i did 50% water change every 2 weeks, my nitrate always around 5ppm and the plants are doing OK (growing slow and have some deficiency).
And now I'm doing a experiment by not to change my water and monitoring my nitrate every week, it's been 3 month since my latest water change, and my nitrate stays at around 10ppm, and my plant doing great (growing faster and less deficiency).
I'm kind of newbie for planted tank, does it mean, if the nitrate stay under control, do i need a water change?