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.