Use of organic materials helps in reducing the consumption of chemical fertilizers, contributing to the sustainability of agricultural ecosystems. Humic acid can improve soil structure, especially in soils with low clay content, aiding in water retention and enhancing water absorption by plants. This characteristic has led to the attention of humic acid application in drought stress, particularly in arid soil regions. The present study aimed to investigate the simultaneous effects of humic acid and plant water requirements on the physical and biological properties of soil under bell pepper cultivation. The factorial experiment was conducted in a completely randomized block design with three levels of humic acid: 0 (HA0), 2 (HA2), and 4 (HA4) g/Kg of soil and four levels of plant water requirements: 60 (L60), 80 (L80), 100 (L100), and 120 % (L120), with four replications under greenhouse conditions. Bell pepper seedlings were transferred to 10 Kg pots after applying humic acid treatments and after the establishing of seedlings, plant water requirement treatments were applied for 80 days. During this period, plants were maintained at a temperature of 25 degrees Celsius during the day, 18 degrees Celsius at night, and a relative humidity of 75%, with a carbon dioxide concentration of 80 milligrams per liter. The results of this study showed that treatments L80HA4, L100HA4, and L120HA4 had the most significant impact on improving soil physical properties, such as normal stability index, total soil stability index, mean weight diameter of soil particles, soil particle stability, geometric mean diameter of soil particles, and reduction in soil particle destruction percentage. The highest values of wet and dry weight diameter of soil particles were observed in treatment L100HA4, which were 42.92%, 74%, and 67.01% higher, respectively, compared to the control (L60HA0). In contrast, the highest percentage of soil particle destruction was observed in treatment L60HA0, showing a 32.90% increase compared to treatment L100HA4. Furthermore, the best performance of bell pepper yield was observed in treatments L80HA4, L100H4, and L120HA4, although the differences between treatments were not statistically significant. The application of humic acid, in addition to improving bell pepper yield, increased water use efficiency and reduced water use. The lowest yield (3283 Kg/ha) and the lowest water consumption (3584 cubic meters/ha) were observed in treatments L60HA0 and L60HA4, respectively. Additionally, the results showed that the highest water use efficiency observed in L80HA4 treatment (7.45 Kg/m³), while the lowest was in L60HA0 treatment (0.80 Kg/m³). Moreover, humic acid significantly influenced the activity of soil enzymes, including soil phosphatase and urease. With more application of humic acid, soil moisture retention, microbial biomass carbon, urease, phosphatases activities increased. However, in treatments with full water requirements and 20 percent more than plant water requirements, enzyme activities decreased.

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