Today’s daf mentions that maaser is given on the bulb of the onion that was planted. That bulb was planted to start new growth, i.e. more onions. Even though the bulb grew last year and maaser was given from it then, when it become part of this year’s crop, it is again subject to maaser.
I once learned that when Esav asked how much maaser is there on straw and salt, it wasn’t just a simple question. He was asking like this. For other crops we give maaser even on the part that we planted, which means we give more than 10%. For straw, which comes from the stem of the wheat plant, we should give more. And on salt, which takes no investment of seeds, we should give even more.
Esav’s question was not just for acting pious but also for acting lomdish.
I have Rabbeinu Nissim on Nedarim. If you want, after Tisha B’av I can check for you. Shabbos for me is starting soon. I’ll check if the Master a Mesechta also speaks of it.