Water transfer from soil to seed: the role of vapor transport

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 study tests the hypothesis that vapor is the major source of water for imbibition under normal soil moisture conditions. Water uptake was measured under varying degrees of seed-soil contact. 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 [seed.sup.-1] 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. Reevaluation of the role of vapor transport should result in improved multiphase transport models in soil and other media.