I was wondering about this because that seems like a good idea upfront, but apparently:

• The extra minerals and stuff is a very minor benefit for plants and will not substitute for fertilizer.
• If you put salt or seasoning in the boiling water, don't use it for plants.
• The starch might also promote mold / bacteria growth.

Pasta water is in fact an excellent thing to save... As a cooking ingredient.

Specifically, what you can do is freeze it in an ice cube tray, and then store the cubes in a Ziploc. A handful of cubes added to a soup, stew, stock or sauce will give it a smoother, more silky texture thanks to all the starch.

Satan, please...