Keywords: 
• soil science /
• growth medium /
• water loss /
• volumetric water content

Abstract: With spray sowing，water loss in the growth medium directly affects seed germination and seedling growth. Using time－domain reflectometry（TDR），the volumetric water content（VWC） of the growth medium which composed of the same materials in a greenhouse pot culture was measured. Four plant species were cultured with three repeats for each species. Regression equations were established for the progress of water loss to show the regulation and key drought time points of the water loss in growth medium. After seeding with Trifolium repens and Cosmos bipinnatus（44 d），Amorpha fruticosa（45 d），and Lolium perenne（43 d），evaporation was near 0 with 90% of the seedlings having withered. Water content in the growth medium decreased the most between 12 : 00 － 13 : 00 followed by a strong decrease from 13 : 00 － 15 : 00. Also，water condensation was minimal at night. Regression equations composed of temperature，humidity，time，and VWC showed highly significant correlation（R2＞0.92）.Water content in the 0 － 5 cm layer was always lower than the 5 － 10 and 10 － 15 cm layers. In addition，the rate of water loss in layer 10 － 15 cm was slow in earlier and later stages，but fast in the middle stage. After about 19 d，water content of the three layers was about the same.［Ch，4 fig. 2 tab. 19 ref.］