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.)

<div>Green revolution dramatically change the nitrogen application in paddy cultivation and day by day its demand increased but excessive and imbalanced use of nitrogen fertilizer has raised certain global concerns, also its low nitrogen use efficiency. Nitrogen is required in huge amounts for rice and supply of N in the right amount, at the right rate and at right time throughout the growing season is most important to increase the yield. Approximately 90% of the N-fertilizer applied worldwide is in the NH4+ form, which is rapidly oxidized to NO3- by soil nitrifier bacteria. Whereas, NCU temporarily delays the bacterial oxidation of the ammonium-nitrogen by depressing over a certain period of time the activity of Nitrosomonas bacteria in the soil. So far more than 75 studies have been conducted to study the performance of NCU in increasing the yield of rice and several other crops. In rice more than 30% of the urea consumed in India is applied, the mean increase in grain yield by replacing urea with NCU is 5 to 6%. NCU has been observed to improve nitrogen use efficiency and subsequently grain yield of rice. Possibly, applying NCU following the site-specific nutrient management principles will lead to paddy production of higher levels as observed with ordinary urea but with lower fertilizer application rates.</div>

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

Neem Coated Urea (NCU), An Efficient Nitrogen Source for Paddy Cultivation: A Review