Potassium fixation in northern Iranian paddy soils

•Potassium fixation in northern Iranian paddy soils was studied.•The methodology for evaluating fixation was adapted to simulate flooded conditions.•NH4OAc-extractable K decreased in air-dried soil after flooding.•K deficiency is indicated for ~50% of northern Iranian paddy soils.•Adapted K fertilization is needed to restore and sustain paddy soil K fertility. Reliable data on K fixation in soils are of agronomic interest in paddy rice production. Considering the factors flooding time and K concentration, we evaluated fixation characteristics of K-depleted paddy soils of northern Iran using an adopted method to assess K fixation in flooded conditions. Potassium fixation in 68 topsoils (i.e. 0–30 cm) from paddy fields of the Mazandaran and Guilan provinces ranged between ~48 and ~700 mg K kg−1 soil with the maximum occurring within 30 days of flooding. These data indicate that NH4OAc-extractable, readily-available K may be below the critical level in ~220,000 ha of the paddy rice cultivated areas in the two provinces. Clay content, CEC and NH4OAc-extractable K in air-dried soil can predict 73.2% of the variation in K fixation, with K fixation increasing with clay content and CEC. The fact that the K fixation percentage remained unchanged in the range of 39.2–314 mg K added kg−1 soil shows, that large amounts of K are likely be needed to restore K fertility. These may be added in portions over several years. After restoring the K fertility in the soils, K outputs have to be balanced continuously by K fertilization.