Grow in polymers?

[Ray]

The acrylates used for such water-retention products do degrade over time, but that would likely be fairly slow.

However, as they absorb they swell, then shrink as the give up the water, so the medium would constantly be "repacking" itself, leading to a rot-suffocating mess in short order.

And, as Pierangelo related, the fertilizer minerals will affect the polymer.