The addition of organic materials in arid and semi-arid regions not only improves the physicochemical and biological characteristics of the soil but also increases plant growth. In this study, the effect of individual and combined application of humic acid (HA) (150 mg kg(-1) soil), wood vinegar (WV) (75-fold dilution), and biochars (corn straw biochar (CSB) and cotton residue biochar (CRB) each at 2% wt.) on the soil Zn chemical fractions and its uptake by Sour Orange (Citrus aurantium L.) seedlings were investigated in a calcareous soil. The results showed the CRB was more effective in increasing the Zn concentration in fractions with high bioavailability (soluble+exchangeable (WsEx) and organic (OM) forms) than the CSB. This may be due to its higher organic carbon content (74%) and electrical conductivity (3.52 dS m(-1)) than those for the CSB. There was no significant difference between HA and WV in terms of the effect on soil Zn chemical fractions. The CRB+WV and CRB+HA were the best treatments for enhancing the soil Zn bioavailability. The greatest increase in the shoot dry matter yield (2.5-fold) and Zn uptake (29%) was associated with the combined treatment of CRB+HA compared to the control. Furthermore, the shoot dry matter yield of Sour Orange seedlings had a significant and positive correlation with the Zn-WsEx fraction and Zn uptake. It could be concluded that among the treatments, the combined application of CRB+HA was the best in enhancing soil Zn bioavailability, Zn uptake and shoot dry matter yield of Sour Orange seedlings in the calcareous soil.
