Abstract: Drought seriously limits seedling emergence of crops, resulting in low and unstable crop yield in arid and semiarid areas in northern China. As a soil amendment, humic acid could resist drought and protect seedlings. Based on a field experiment, we investigated the effects of humic acid applications with duration of 1 to 5 years on foxtail millet emergence rate, dry matter accumulation, root length, antioxidant enzyme activity, and soil moisture at the seeding stage. Results showed that humic acid application increased soil moisture and soil storage water in the 0-60 cm layer at the seedling stage. All the treatments significantly increased emergence rate, above and belowg round dry matter accumulation, root shoot ratio, and root length at the seeding, with the increments ranging from 2.62% to 16.53%, 3.70% to 111.11%, 8.21% to 181.33%, 4.22% to 33.25%, and 2.00% to 23.87% respectively. Humic acid application significantly increased SOD, POD and CAT activities. Above ground biomass and grain yield increments ranged from 4.06% to 20.65% and 0.67% to 16.25%, respectively. The treatment with continuous application of humic acid for five years had the greatest effect, but there was no significant difference between the humic acid treatments continuously applied for four and five years. It is recommended to apply humic acid continuously for 4 years considering the actual production cost. Our results provide technique supports for combating drought and protecting the seedlings in dryland farming in the loess hilly-gully regions along the Great Wall in northern China.

Key words: humic acid amendment, foxtail millet, emergence rate, antioxidant enzyme activity.