You harvested them while they were sprouts. All that growth came from the energy and nutrients stored in the seed. You can grow similar sprouts on wet pater towels.,(reply>), I think that's why they had the control of a paper towel but I totally agree you definitely can and it's probably just using nutrients in the seed not the environment around it. If you use completely unfertilized soil the plant will die about two weeks after that stage from my experience. The nutrients don't look available to me at all.