Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/7/2002
Publication Date: 11/1/2002
Interpretive Summary: Predictive models have found that, at normal field soil moistures, seed germination seems to defy established principles of liquid water flow through soil. My research indicates that most of the water transferred from soil to seed travels as vapor, not liquid. Wheat seed supplied only with vapor absorbed 85% as much water in 24 hours as seed planted in soil. The seed only took 40% longer to germinate. These results demonstrate that vapor must be considered a major component of water transport to seed except when soils are very wet. Examining old assumptions concerning the relative importance of liquid and vapor flow in soil and other air-liquid-solid media should improve our understanding and modeling efforts.
Technical Abstract: Mixed solid-water-gas media such as soil can supply water to a dry object as both liquid and vapor. Modelers and agricultural engineers have commonly assumed liquid transport dominates imbibition by seed. This experiment tested the alternative hypothesis that water vapor is the major transport mechanism for imbibition by seed under normal soil moisture conditions. Wheat (Triticum aestivum L.) seeds were suspended in holes in blocks of moist soil (average water potential -0.16 MPa). Hole diameter varied from 13 to 0 mm (no hole), so that the distance between the moist soil and the seed varied from a maximum of 5 mm to intimate seed-soil contact. The amount of water absorbed by seed in 24 h increased from 15 to 17 mg as seed-soil contact increased from none to maximum contact, indicating that at least 85% of the water absorbed by seed in intimate seed-soil contact could be attributed to vapor alone. Re-evaluation of the role of vapor transport should result in improved multi-phase transport models in soil and other media.