Potassium is an important macronutrient for plants. Soil microbial biomass contains considerable amounts of potassium which can be taken as an important source of potassium for plants. We conducted a study in two distinct parts. In the first part of the study, an standard methodology was used to measure the range of microbial biomass potassium in some acid soil.A comparison was also perfomed between the results of the standard procedure and those of the two alternative methods including direct liquid chloroform and microwave irradiation-extraction.It was proposed that substitution of chloroform by microwave irradiation can be either faster or less harmful. Nine acid soils of Guilan province were collected. The soil samples belonged to the natural forests and rice fields. Microbial biomass potassium was measured under glucose unamended and glucose amended conditions. Soil microbial biomass potassium ranged from 44 to 149 mg kg -1 under glucose-unammended condition and between 44 to163 mg kg -1 under glucose-amended conditionThe values of soil microbial biomass potassium were negative whe direct liquid chloroform extraction was used and therefore the method was found to be unsuitable for determining microbial biomass potassium in acidic soils. The range of microbial biomass potassium in microwave radiation method was between 74-284 mg kg -1 . Microbial biomass potassium in glucose induced soils were between 150-315 mg kg -1 . Despite the significant correlation, values obtained from the microwave irradiation method were larger. In the second part of the study, the response of microbial biomass potassium, soluble potassium, exchangeable and non-exchangeable potassium to calcium carbonate addition was investigated. In this study, 7 acidic soils of Guilan province were analyzed. First, soils were treated with three levels of calcium carbonate, i.e. 0, 2 and 16 g kg -1 . The calcium carbonate-treated samples were incubated for 30 days at 25 ° C and optimum humidity. Soluble potassium at three levels of calcium carbonate (0, 2 and 16) ranged between 38-123, 38-132 and 38-133 mg kg -1 respectively. Exchangeable potassium at three levels of 0, 2 and 16 g kg -1 were 74-330, 74-332and 74-333 mg kg -1 respectively. Non-exchangeable potassium were at the level of 0 and 2 g kg -1 were 111-741, 111-740 mg kg -1 and at the level of 16 g kg -1 was obtained 111-592 mg kg -1 . Microbial biomass potassium at three levels of 0, 2 and 16 g kg -1 were 40-173, 40-172 and 40-80 mg kg -1 respectively. Simple linear regression showed that microbial biomass potassium had a significant relationship with other forms of potassium. Decreased potassium in microbial biomass by increasing calcium carbonate levels may be due to shrinkage of microbial biomass. In order to investigate the decrease of microbial biomass potassium, Substrate induced respiration of studied soils were measured. The mean of Substrate induced respiration at the three levels of calcium carbonate were 330, 244 and 126 mg-CO 2 kg -1 h -1 respectively Overall, it can be concluded that the microbial biomass potassium of the studied acidic soils is not limited by carbon and energy. Microwave irradiation energy, provided an appropriate alternative method for the fumigation -extraction method. The suitability of the microwave irradiation technique for assessing plant available potassium should be investigated. We obviousely showed that application of calcium carbonate in acid soils can adversely influence the native microbial populations due to pH changes and thereby, transfer the microbial potassium to other sources like soluble and exchangeable potassium. Key Words Microbial biomass potassium, Fumigation- extraction, Direct liquid chloroform extraction, Microwave irradiation, Substrate induced respiration, Calcium carbonat.