Investigation of the effect of plant density and humic acid on paddy yield, water use efficiency, and biochemical traits of direct-seeded rice cultivation (Oryza sativa L.) in northern Iran

The transition from transplanting rice cultivation to direct-seeding rice cultivation has recently begun in Iran, especially in Golestan province. No information is yet available on the effect of humic acid and plant density on paddy yield and some of its biochemical traits. For this purpose, an experiment was conducted in a research farm located in Gonbad city during May to September 2019 in the form of split plot-factorial based on randomized complete block design with three replications. Plant density (plant spacing per row) was considered as the main factor with 4 levels (5, 10, 15, and 20 cm) and humic acid consumption at 2 levels (humic acid consumption, no humic acid consumption) was considered as subplots. Humic acid fertilizer with a concentration of 250 ml per hectare was added in three stages including tillering (4-6 leaves), stalking (20-30 days after tillering), and before panicle initiation. Results showed that the effects of plant spacing on row and humic acid application on 1000-seed weight, paddy yield, biological yield, harvest index, grain protein, water use efficiency, and chlorophyll a and b and total chlorophyll were significant. Considering 5 and 10 cm plant spacing on rows along with humic acid application had the highest rice yield and water use efficiency. Also, chlorophyll content showed a significant change with increasing plant density and application of humic acid with a significant increase in chlorophyll a and b, which in turn was effective in increasing paddy yield. Rice protein also showed the highest level in 10 cm spacing between plants on each row along with the application of humic acid.