The influence of crop nutrition on the quality of onion bulbs destined for export markets

The key export onion markets demand consistent supply of high-quality bulbs with long storage life from Tasmania. Supplying these market requirements challenges our body of knowledge relating to bulb quality. This study sought to address the potential links between crop nutrition and bulb quality by surveying thirty-four commercial onion crops across seven soil types. Plant tissue concentrations were recorded through key growth and development stages from two true leaf to harvest. These results were then related to yield and bulb quality attributes through multivariate analyses. The findings were compared with existing literature on onion bulb production and in many cases, provides new information on elemental tissue concentrations at a number of growth stages not previously reported. The survey established linkages between element concentration in plant tissue and bulb quality, particularly apropos to skin loss. Here a relationship of bulb moisture content below 87.6% together with tissue concentrations of molybdenum lower than 0.047 ppm was associated with a decrease in skin loss. In contrast, nitrate levels greater than 20 ppm were associated with higher levels of skin loss and this effect was exacerbated if bulb tissue sulphur concentrations also exceeded 0.34%. Expanding on the quality linkages established from the initial survey, factorial experiments were then undertaken across four sites with two cultivars to explore the interaction of applied sulphur, molybdenum and nitrogen on onion plant elemental composition and bulb skin loss. Bulb robustness was assessed by subjecting harvested bulbs to multiple handling assessments over a five-month period. Amending the base fertiliser programme with ammonium sulphate increased sulphur concentrations in the bulb tissue of both cultivars, and nitrogen levels in Regular Creamgold. Supplementation with foliar applied molybdenum also increased concentrations of this element within this cultivar. This study has complemented existing knowledge and added new data for some onion growth stages not previously reported. This improves scientific understanding of the range of nutritional element concentrations found in high yielding onion crops and has provided evidence that consideration of plant nutrition not only applies to crop yield, but also to the quality of the onion bulbs produced.