To date, most studies on biological nitrification inhibition (BNI) in sorghum have been performed with plants grown in hydroponic systems. However, the current study was conducted to determine whether or not sorghum inhibits nitrification in fields of Alfisols, and clarify the mechanism that results in inhibition of soil nitrification in the field. Nitrification activity in the rhizosphere of sorghum (Sorghum bicolor (L.) Moench) i.e. soil attached to its roots within a few millimeters was measured and compared with those in adjacent bulk soil. Sweet sorghum (6 varieties) and grain sorghum (3 varieties) were cultivated in 4 Alfisol fields in a semi-arid tropical region of India during the 2010 or 2011 rainy seasons. Soil samples were collected three times during the growing season. Nitrification activity in the rhizospheric soil was significantly lower than that in the bulk soil during 8 out of 12 samplings while the pH (H₂O, 1:2) of the rhizospheric soil was significantly lower than that of the bulk soil in 10 out of 12 samplings. Acidification of the soil by sulfuric acid decreased the nitrification activity to a comparable extent, as emerged in the rhizospheric soils. Our results indicate that acidification of soil around roots would be one of the main causes of nitrification inhibition by sorghum in the field.