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Composite soil samples from 0-25 cm depth of 51 randomly selected paddy fields in North of Iran were taken in two consecutive years (25 samples in the first year, 26 samples in the second year). The selected rice fields were well distributed all over the region. Some general soil properties such as pH, SP, %CaCO3 and CEC were measured. In addition, the following methods were used to measure the extractable soil K: IN ammonium acetate (two
methods), 0.5M magnesium nitrate, cold sulfuric acid (two methods) and boiling nitric acid. Growth and potassium uptake were studied in the greenhouse with two high yielding varieties of rice, i.e. Amol-3 and Khazar. The first variety, cultivated on the soils sampled in the first year, was irrigated with distilled water during the growing season, while the other variety (Khazar), grown on the soils sampled in the second year, was irrigated with distilled water containing 3 mgKIL. The mean of extractable K by the ammonium acetate, magnesium nitrate, sulfuric acid and nitric acid methods
were 133, 115, 153 and 305 mg kg-! , respectively. The standard deviations of these data were relatively high, indicating a wide difference in K release properties of the soils. In this study, K extracted by the ammonium acetate, magnesium nitrate, and modified sulfuric acid methods were commonly highly correlated with the plant parameters (rough rice yield, aboveground yield, and total K uptake) in both years, whereas, poor correlations were obtained for
the nitric acid method. Comparison of the extractants on the basis of these correlation coefficients together with the ease and speed of extraction, indicated the following order of suitability of these extractants for the
estimation of available soil K: IN ammonium acetate> 0.5 M magnesium
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nitrate> modified sulfuric acid> nitric acid. However, if the cost and speed of analysis is considered, the modified 1 N ammonium acetate method is preferable, especially where a large number of soil samples have to be analysed in a short period of time. The critical levels of 142 and 135 mgK kg-1 of soil were obtained for the ammonium acetate method by the statistical and the graphical methods of Cate and Nelson, respectively. The same values for
the modified ammonium acetate and the magnesium nitrate extactants were 127 and 122 mgK kg-1 , respectively, when the graphical method of Cate and
Nelson was employed. In this study, it was indicated that when Hunter and Pratt method of cold sulfuric acid was modified for the effect of soil
carbonates, a significant improvment in the correlations between the extractable K and the plant responses was obtained.