Background: Humic acid improves soil fertility, nutrient availability and microbial activity. This study evaluated the effects of different humic acid levels on nitrogen availability and nitrifying bacteria in rhizosphere and outside of rhizosphere soils of sorghum. 
 
Methods: A pot experiment was conducted at the College of Agriculture, University of Al-Qadisiyah, Iraq, during the autumn season of 2025. were planted with Sorghum bicolor (L.) Moench on 15 July 2025. Six levels of humic acid (0, 2, 4, 6, 8 and 10 ml L-1) were applied using a completely randomized design (CRD) with three replicates. Sorghum plants were grown in plastic pots containing 20 kg of soil. Available ammonium (NH4+), nitrate (NO3-), numbers of nitrosomonas and nitrobacter, chlorophyll content and plant height were evaluated after 60 and 90 days of planting. 
 
Result: The highest humic acid level (10 ml L-1) significantly increased nitrogen availability and nitrifying bacterial numbers in both rhizosphere and outside of rhizosphere soils. The highest concentrations of ammonium and nitrate reached 49.81 and 55.13 mg kg-1&nbsp;soil, respectively, in the rhizosphere after 60 days of planting. The same treatment recorded the highest numbers of nitrosomonas and nitrobacter, reaching 19.03 &times; 105&nbsp;and 11.26 &times; 105&nbsp;cells g-1&nbsp;soil, respectively. Chlorophyll content and plant height were also significantly improved, reaching 55.7 SPAD units and 96.2 cm after 90 days of planting.

Effect of Soil Application of Humic Acid on Nitrogen Availability and Nitrifying Bacteria in the Rhizosphere of Sorghum (Sorghum bicolor L.)