Zinc Nutrition of Onion: Proposed Diagnostic Criteria

ABSTRACT

Zinc (Zn) deficiency is a global nutritional problem in crops grown in calcareous soils. However, plant analysis criteria, a good tool for interpreting crop Zn requirement, is scarcely reported in literature for onion (Allium cepa L.). In a greenhouse experiment, Zn requirement, critical concentrations in diagnostic parts and genotypic variation were assessed using four onion cultivars (‘Swat-1’, ‘Phulkara,’ ‘Sariab Red,’ and ‘Chilton-89’) grown in a Zn-deficient (AB-DTPA extractable, 0.44 Zn mg kg⁻¹), calcareous soil of Gujranwala series (Typic Hapludalf). Five rates of Zn, ranging from 0 to 16 mg Zn kg⁻¹ soil, were applied as zinc sulphate (ZnSO₄·7H₂O) along with adequate basal fertilization of nitrogen (N), phosphorus (P), potassium (K), and boron (B). Four onion seedlings were transplanted in each pot. Whole shoots of two plants and recently matured leaves of other two plants were sampled. Zinc application significantly increased dry bulb yield and maximum yield was produced with 8 mg Zn kg⁻¹. Application of higher rates did not improve yield further. The cultivars differed significantly in Zn efficiency and cv. ‘Swat-1’ was most Zn-efficient. Fertilizer requirement for near-maximum dry bulb yield was 2.5 mg Zn kg⁻¹. Plant tissue critical Zn concentrations were 30 mg kg⁻¹ in young whole shoots, 25 mg kg⁻¹ in matured leaves, 16 mg kg⁻¹ in tops and 14 mg Zn kg⁻¹ in bulb. Zinc content in mature bulb also appeared to be a good indicator of soil Zn availability status.

Keywords:

• Allium cepa
• calcareous soils
• diagnostic norms
• onion
• plant analysis
• zinc efficiency
• zinc requirement