It is a common belief that most of the Turkish soils are rich in potassium (K) and magnesium (Mg) for crop production and that there is no crop response to fertilization of these nutrients. However, it is currently a common agricultural practice by farmers to use an excess amount of K and, in some instances, Mg fertilizers especially for horticultural cash crops. Two biological (pot and Neubauer experiments) and four chemical extraction methods (0.3 N HCl, 0.5 N HCl, 1 N NH4OAc, and 0.5 N NaHCO3) were employed to measure the amounts of extractable K and Mg in the selected ten soil series of Harran Plain (Fertile Crescent) in the upper part of the Mesopotamia area. Italian grass (Lolium italicum) and barley (Hordeum vulgare L.) were used as test plants in the pot and Neubauer experiments, respectively. The amount of slowly available K extracted using four chemical extraction methods were much higher in two soil series (Ekinyazi and Akcakale) than that of the other soil series. The Akcakale series had more slowly available Mg than the other soil series. Total amounts of slowly available K extracted with both of the biological, and 0.3 N HCl and 1 N NH4OAc chemical extraction methods were found statistically significant at 1% level. Correlation coefficients between the biological and chemical methods were calculated. As the number of grass harvests increased, percentage of K content decreased and Mg content increased in plant dry matter. At the last harvest, K content of the soils depleted, whereas Mg content nearly did not change. According to index grouping, Italian grass grown in the pots did not need K and Mg fertilization.