Vapour is the principal source of water imbibed by seeds in unsaturated soils

The assumption that seeds imbibe most of the water required for germination as liquid through seed-to-soil contact has been a dominant theme in germination research and seeding technology. Under most conditions, seeds are also exposed to water vapour during imbibition, but the relative contributions of liquid and vapour are difficult to assess. In water uptake models that include vapour, procedures used to estimate potential vapour imbibition have underappreciated the effect of distance on diffusion rate. At the same time, the amount of seed-to-soil contact and the liquid bridge from soil water to the seed tend to be greatly overestimated, considering the soil water contents often found in the field. Most researchers have recorded an approximately equal time to germination at soil water contents ranging from field capacity to nearly permanent wilting point, and little response to bulk density, soil type or seed–soil contact. While hydraulic conductivity decreases by several orders of magnitude as soil water content, bulk density and seed-contact decrease, relative humidity remains near 100%. There are several experiments demonstrating timely germination in water vapour alone. The combined evidence contradicts the assumption that seed–soil contact is important for imbibition of water by seeds. Water vapour should be considered the primary source of water for seeds in unsaturated soils.