Upper critical level of zinc in tops of toria (Brassica campestris L.) in a Typic Haplustept

A pot experiment was conducted during 2008 using a Typic Haplustept loamy sand soil to study the effect of Zn fertilization (0, 2.5, 5.0, 7.5, 10.0, 12.5, 15.0, 17.5, 20.0, 22.5, 25.0, 27.5, 30.0, 32.5 and 35.0 mg Zn kg⁻¹ soil) on yield and mineral composition of toria (Brassica compestris L.). Dry matter yield (DMY) of 45 days old toria increased significantly with graded levels of Zn application up to 12.5 mg kg⁻¹ soil and thereafter decreased by 16 and 41% with application of 15 and 35 mg Zn kg⁻¹ soil, respectively. 5 mg DTPA-Zn kg⁻¹ soil at harvest and 100 μg Zn g⁻¹ in dry matter of toria produced the maximum dry matter yield. The concentration of P, K, Ca, Cu and Mn in dry matter decreased while that of S increased with Zn application. P: Zn, K: Zn, Ca: Zn, Mg: Zn, S: Zn, Cu: Zn, Fe: Zn and Mn: Zn ratios of 20, 10, 12, 70, 5, 0.05, 2 and 0.4 in dry matter produced 70% of the maximum DMY of toria, respectively. A level of >7 mg DTPA-Zn kg¹ soil at harvest and >140 μg Zn g⁻¹ dry matter greatly reduced the DMY of toria which may be considered as the upper critical level of soil and plant Zn for toria, respectively.