Potassium is one of the most essential macro elements from the viewpoint of uptake rate, as well as physiological and biochemical processes in plant rice. This experiment was carried out to evaluate the relative K extractability of 16 chemical extractants for selection of the best extractor or extractors. A determination of the critical K level in Khazar cultivar in 21 paddy fields of the Guilan province was also investigated. Fields selected were of a wide range of clay content, cation exchangable capacity, organic matter and ammonium acetate extractable K. The treatments included 6 K levels (0, 100, 200, 300, 400 and 500 kg/ha as K2O) supplied from KCl (1/2 at transplanting, 1/2 max. tillering) in a randomized complete block design with three replications. Results indicated at that K application increased grain and straw yield, as well as K uptake in straw. Mean maximum and minimum K extracted, were through 1N HNO3 (239mgkg-1) and 0.05 M HCl (11mgkg-1) extraction respectively. High correlations were observed among 1M, 0.5 M and 0.25 M NH4OAc and 1 N HNO3 extractants with relative grain yield of: 0.80**, 0.79**, 0.81** and 0.87** respectively, and K uptake by straw of: 0.73**, 0.73**, 0.75** and 0.81** respectively. Moreover, high correlations were observed among water, Morgan, and Kelovana extractants with K concentration in straw of: 0.75**, 0.72**, and 0.71** respectively. The critical K concentrations were evaluated for extractants that had the highest correlation with plant yield. The critical K concentration based on 90 percentage of relative yield by 1M NH4OAc, 0.5 M NH4OAc and 0.25 M NH4OAc was 120mg kg-1 and for 1N HNO3 was 210 mg kg-1.
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