Estimation of Plant‐Unavailable Iodine Concentrations in Agricultural Fields

In this study, a method for the estimation of plant‐unavailable I concentrations in soils was developed. Concentrations of total I and potentially plant‐available I (i.e., water‐soluble and organically bound I) were measured in 141 agricultural soil samples. For total I measurements, pressed powder pellets were prepared from soil samples, and concentrations of I were measured by energy dispersive x‐ray fluorescence. Potentially plant‐available I contents were measured by inductively coupled plasma mass spectrometry after the soil samples had been extracted with tetramethyl ammonium hydroxide and their soluble and organically bound I concentrations had been measured (TMAH‐extractable I). The results showed a high correlation between the total and TMAH‐extractable I concentrations. The geometric means of total and TMAH‐extractable I in the paddy field samples were 1.9 and 1.1 mg kg −1 dry weight, respectively, while those in the upland field soil samples were 5.7 and 4.2 mg kg −1 dry weight, respectively. For the agricultural soils having <5 mg kg −1 total I, it was estimated that the average concentrations of plant‐unavailable I in paddy fields and upland fields were 0.7 ± 0.2 and 0.7 ± 0.4 mg kg −1 dry weight, respectively. The different total I concentrations for the fields from the two agricultural uses could be explained by the differences in their redox conditions; I is mobile under reducing conditions so that it was removed from the paddy fields by leaching from the plowed layer or by I volatilization from the soil.