The pungent smell is mainly caused by compounds of sulphur so can be influenced by the sulphur content of any fertilisers that are applied.
In the experiments reported here, onion seeds were sown in beds of a sandy loam and the seedlings were then transplanted into pots of the same soil when they had reached the twoto three-leaf stage. The treatments consisted of four levels of ammonium nitrate, ranging from 50 to 200 milligrams per kilogram, and three of potassium sulphate - zero, 175 and 350 milligrams per kilogram.
One half of the ammonium nitrate dose and all of the potassium sulphate was applied to pots at the time of transplanting. The remainder of the ammonium nitrate was applied 15 days later. The plants were grown in a greenhouse and the crop was harvested 150 days after transplanting.
Pungency increased as the potassium sulphate concentration was increased from zero to 350 milligrams per kilogram and as the ammonium nitrate concentration rose from 50 to 150 milligrams per kilogram.
On the other hand, although the vitamin C content of onion bulbs also increased with increasing concentrations of potassium sulphate, it was reduced by concentrations of ammonium nitrate above 50 milligrams per kilogram.
Dr Ken Cockshull, Associate Fellow, Warwick Crop Centre, University of Warwick
Influence of NH4NO3 & K2SO4 on Qualitative Characteristics of Onion by Bolandnazar, Mollavali and Tabatabaei (2012). Scientia Horticulturae 136: 24-28. The contents of issues of Scientia Horticulturae and abstracts of papers are provided at www.elsevier.com/locate/scihorti.