Iodine biofortification of spinach by soil fertigation with additional application of humic and fulvic acids

The aim of this research was to determine the interaction between fertigation with KIO3 and pre-sowing application of exogenous humic and fulvic acids (used in the form of the commercial concentrate Humistar, HU) on yield, iodine biofortification and chemical composition of spinach cv. ‘Olbrzym zimowy’. The plants were cultivated in a pot experiment with the following treatments: control, pre-sowing HU application at doses of 0.0125, 0.05 and 0.2 cm3 HU dm−3 of soil, KIO3 fertigation and KIO3 fertigation with pre-sowing HU application at doses of 0.0125, 0.05 and 0.2 cm3 HU dm−3 of soil, fertigation at a dose of 0.0004% I. The use of HU and fertigation with KIO3 caused an increase of iodine content and iodine transfer factor (TF) from the soil to the plants; with the highest extent in the pot treated with KIO3 + 0.2 cm3 HU dm−3 of soil.