Technical Abstract: Gypsum (CaSO4.2H2O) is used in agriculture both as a source of calcium (Ca) and sulphate (SO4) and as an amendment to improve soil structure. The effect of gypsum on the adsorption of SO4 in irrigated and nonirrigated soils was examined. Almost all of the indigenous sulphate (SO4) in a range of Golesthan and North Khorasan soils with moderate pH values (>6) was found to be present in the soil solution and, as a consequence, was highly susceptible to leaching. The adsorption of sulphate to the soils receiving no gypsum was greater with correlation coefficient of r=0.91 at 0 kg S ha-1 as compared to the soils received 40 kg ha-1 of gypsum as fertilizer with the value of r=0.88 in Golesthan Province. The same trend was observed in Khorasan Province with r=0.79 and r=0.75 with soils receiving 0 and 40 kg S ha-1, respectively. The results were more pronounced in irrigated field for both provinces. The amount of sulphate adsorption in Golesthan Province soils was comparatively greater than soils of Khorasan Province. The results raise questions regarding the efficiency of SO4 –containing fertilizers in correcting and preventing S deficiency in situations where leaching is a concern.