Iodine (I) biofortification of vegetables by means of soil and foliar applications was investigated in field experiments on a sandy loam soil. Supply of iodine to the soil in trial plots fertilized with potassium iodide (KI) and potassium iodate directly before planting (0, 1.0, 2.5, 7.5, and 15 kg I ha-1) increased the iodine concentration in the edible plant parts. The highest iodine accumulation levels were observed in the first growing season: In butterhead lettuce and kohlrabi the desired iodine content [50–100 μg I (100 g FM)-1] was obtained or exceeded at a fertilizer rate of 7.5 kg IO3--I ha-1 without a significant yield reduction or impairment of the marketable quality. In contrast, supplying KI at the same rate resulted in a much lower iodine enrichment and clearly visible growth impairment. Soil applied iodine was phytoavailable only for a short period of time as indicated by a rapid decline of CaCl2-extractable iodine in the top soil. Consequently, long-term effects of a one-time iodine soil fertilization could not be observed. A comparison between the soil and the foliar fertilization revealed a better performance of iodine applied aerially to butterhead lettuce, which reached the desired iodine accumulation in edible plant parts at a fertilizer rate of 0.5 kg I--I ha-1. In contrast, the iodine content in the tuber of sprayed kohlrabi remained far below the targeted range. The results indicate that a sufficient spreading of iodine applied on the edible plant parts is crucial for the efficiency of the foliar approach and leafy vegetables are the more suitable target crops. The low iodine doses needed as well as the easy and inexpensive application may favor the implementation of foliar sprays as the preferred iodine biofortification strategy in practice.

Iodine biofortification of butterhead lettuce (Lactuca sativa) via foliar sprays was investigated in field trials, focusing on assessing the influence of the time and application method. The iodine (I) concentrations in the edible plant parts increased when potassium iodide (KI) and potassium iodate (KIO3) solutions were sprayed at doses up to 0.25 kg I ha−1 on different dates close to harvest. Crop yield and marketable quality were not significantly affected by I treatments. A greater efficacy of KI was frequently observed and probably related to its lower point of deliquescence and smaller anion size in comparison with KIO3. KI sprays on butterhead lettuce at different times of the day resulted in a higher I enrichment when applied at 11:00 and 15:00 h. The diurnal variation in I uptake may reflect the impact of fluctuating climatic conditions at the time of application. Iodine treatments at different application dates near harvest led to an increasing I concentration in the vegetable produce that could be related to the rising shoot fresh mass and leaf area. When KI and KIO3 were sprayed simultaneously with commercial calcium fertilizers, fungicides or insecticides, I accumulation in butterhead lettuce was not negatively affected or in some cases even significantly enhanced. The results show that foliar sprays of KI and KIO3 are an effective method to biofortify butterhead lettuce with I and this approach may easily be implemented as a routine method in commercial cultivation.