Constructing a map of potassium inputs to paddy fields from irrigation

Geographical and geological parameters, including land use, lithology, and water quality, and volume of irrigation were investigated to map K inputs into paddy fields from irrigation in the eastern part of Fukushima, Japan. This area has had one of the greatest needs for K maintenance worldwide due to the control of radiocesium uptake by plants after the Fukushima nuclear accident in 2011. The K+ concentration in irrigation water and its sources was in the range of 0.2–5.2 mg L−1, which was probably affected by both the natural conditions (e.g., amount of silicate mineral dissolution and presence of hot springs and coal seams) and human activities (e.g., agriculture and urbanization). Irrigation volume was estimated to range from 306 to 2300 mm in response to daily water requirement rates. K inputs were computed by multiplying the concentrations and volumes, giving estimated inputs of 2.5–133 kg K2O ha−1 crop−1. The inputs proved spatially variable and were high in the western and southeastern parts of the study area. Irrigation was estimated to supply more than half of the K application rate in the rice cultivation guidelines to 12% of the paddy field area. This map should provide useful information for devising K maintenance plans.