The practice of simultaneous biofortification (enrichment) of plants with iodine (I) and selenium (Se) is based on solid grounds. Their low content in soils is the cause of an endemic deficiency of I and Se in several billion people worldwide. There is still no objective information as to the impact of I and Se interactions on mineral nutrition of plants. The study (conducted in 2012–2014), included soil fertilization of the lettuce cv. ‘Valeska’ in the following combinations: control, KI, KIO₃, Na₂SeO₄, Na₂SeO₃, KI + Na₂SeO₄, KIO₃ + Na₂SeO₄, KI + Na₂SeO₃, KIO₃ + Na₂SeO₃. I and Se were applied twice: before sowing and as top-dressing (each 2.5 kg I·ha^-1 + 0.5 kg Se·ha^-1) – a total dose of 5 kg I·ha^-1 and 1 kg Se·ha^-1 was used. Fertilization with Na₂SeO₄, KI + Na₂SeO₄ and KIO₃ + Na₂SeO₄ considerably reduced the dry matter yield of the plants – it also lowered the content of P, K, Mg, Ca, B, Zn and Cd in the lettuce. Fertilization with Na₂SeO₃, KI + Na₂SeO₃ and KIO₃ + Na₂SeO₃ had a less negative impact on dry matter yield than the use of Na₂SeO₄ – every year it affected the mineral content in the lettuce in a highly varied manner. Dry matter productivity of the plants after fertilization with KI and KIO₃ varied between the research years – in those plots, I content in lettuce was negatively correlated with the content of K, Mg, Ca, S, Na, B, Cu, Fe, Mn, Zn, Cd and Pb. Combined fertilization with KI and KIO₃ with Na₂SeO₄, and with Na₂SeO₃ reduced the negative correlation between I content (in the KI and KIO₃ plots) and the content of K, Mg, Ca, S, Na, B, Cu, Fe, Mn, Zn, Cd and Pb. After fertilization with Na₂SeO₄, Se content was positively correlated with Na content and negatively correlated with the content of Mg, Ca, Fe, Mn and Cd. Se content in the lettuce after fertilizing exclusively with Na₂SeO₃ was positively correlated with the content of P, K, Na, Mn, Mo and Zn. The changeable climatic conditions “disguised” the influence of fertilization with I and Se on the mineral composition of the lettuce plants.

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