A surfactant (in BAS's case, plant-derived saponin) added to water to break surface tension so moisture can penetrate hydrophobic peat moss evenly. Peat naturally repels water until it is saturated; the wetting agent forces even distribution and prevents dry pockets.
Jeremy uses thermex 70 saponin extract. He also notes you don't need that specific brand — any agricultural saponin product or even local garden store wetting agent works and they're usually cheaper than consumer options.
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