A hygroscopic material absorbs and holds water vapor from the air. In gardening and indoor plant care, hygroscopic substances change moisture content as humidity changes, which can affect soil structure, water retention, and storage stability.
Many organic materials used in potting mixes, including peat moss, coco coir, bark, and wood products, are hygroscopic to some degree. Fertilizers and salts can also be strongly hygroscopic, causing them to clump or dissolve when exposed to humid air. Seeds stored in humid conditions may absorb moisture, increasing the risk of mold, premature germination, or reduced viability.
Hygroscopic behavior differs from capillary action. Hygroscopic materials absorb water into their structure from humid air, while capillary action moves liquid water through pores or spaces between particles.
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