Investigating the application of humic acid and irrigation levels on some physical, chemical, and biological properties of the soil under bell pepper cultivation

Humic acid can improve water absorption by plants by improving soil structure and water retention in soil. These characteristics have caused the use of humic acid in the soils of dry areas to be considered. This study aimed to investigate the simultaneous effect of humic acid and deficit irrigation on soil physical and biological properties under the cultivation of bell pepper. A factorial-based experiment in a randomized complete design was carried out with three levels of humic-acid (0 (HA0), 2 (HA20), and 4 (HA40) g per pot) and four irrigation levels (60 (L60), 80 (L80), 100 (L100), 120 (L120), (percent of soil moisture depletion) with three replications in the research greenhouse of Payam Noor Bojnourd University. The results showed that the best treatments for improving soil physical properties, AS (aggregate stability), and decreasing PAD (percentage of aggregate destruction)), were L80HA40, L100HA40, and L120HA40 treatments. Also, by increasing the dose of HA and increasing the irrigation level up to L80, the activity of urease, alkaline phosphatase, and acid phosphatase enzymes increased and then decreased in L100 and L120.Therefor the highest amount of urease, alkaline phosphatase, and acid phosphatase enzymes (μg NH4+-N g-1 dry soil 2h-1) 678.98, 845.54 671.30 (μg PNP g−1 dry soil h -) were observed in L80L40, which were to be 4.6, 5.9, 2.8 and 0.6 times more than L60HA0 treatment, respectively. By increasing the amount of humic acid and increasing the irrigation levels up to L100, respiration rates increased and then decreased at L120, however, microbial activity followed by respiration at L80 and L100 were not significantly different.