The one-atom difference
Both are cross-linked polyacrylates. The difference is the counter-ion held on the chain: sodium (Na⁺) or potassium (K⁺). That choice changes little about absorption in pure water, but a lot about what the polymer leaves behind.
Why soil needs potassium
Sodium released into soil disperses clay particles, which damages soil structure and drainage over time — the problem known as sodicity. Potassium is a plant nutrient and does not cause that damage. For repeated agricultural use, potassium polyacrylate is the appropriate choice.
Why industry uses sodium
Sodium polyacrylate is produced at very large scale for hygiene products, which makes it cost-effective and available in tightly specified grades. In a diaper core, gel pack or solidified waste drum, the sodium never meets a field.