Agricultural management of alkaline calcareous soils is considered challenging issues. For alkaline calcareous soil fertility, it is important to understand the action mode of organic amendments such as acidic biochar (ABC). Pot experiment was carried out under the greenhouse condition on 15^th of October 2020, at the Experimental Farm, Faculty of Agriculture, Al-Azhar University, Assuit, Egypt, to investigate the effect of ABC application rates on calcareous sandy soil properties and Fennel-Baladi plant growth. The soil treatments were control without any addition of biochar or fertilizers (CK), the recommended dose of granular superphosphate 12% (CF), and 1%, 2% and 3% of ABC per kg of soil, respectively. The acidified biochar enhances soil chemical properties (soil reaction pH, soil salinity EC and organic matter content). Available NPK were significantly increased by increasing ABC doses. The treatment of 3% acidic biochar (ABC3) recorded the highest value of available N (38.1 mg kg^-1), P (24.0 mg kg^-1) and K (112 mg kg^-1). The highest value of N, P and K uptake (8.34, 6.83 and 9.53 g pot^-1) was recorded with the treatment ABC3.The addition of acidified biochar enhanced the growth parameters (fresh and dry weights, plant height, stem diameter, branch number and chlorophyll A and B) of the fennel plant. Increasing biochar application rate caused an increase in NPK use efficiencies since they increased by 275, 275 and 275%, respectively, compared to the control treatment.