Water Crystals not only store water for plants, but they also become a repository of nutrients. They will absorb any aqueous, non-petroleum-based solution.
If fertilizer is present and washed into the soil with water, the crystals will absorb the mixture. This prevents the nutrient from percolating through the soil, past the root system where it will do no good. When the crystals absorb the nutrient-laden solution, water-carrying capabilities are reduced, only absorbing half as much nutrient-laden solution. As the expansion and contraction of the crystals continue through subsequent waterings, the nutrient-laden water is slowly released into the soil.
As this "purging" takes place, the crystals return to their normal water-absorbing capacity. This process leads to better soil conditions and stronger, healthier root systems.