potassium supplying power of selected alkaline-calcareous soils in the north-west of iran

solution potassium (so-k) and exchangeable k (ex-k) extracted by nh4oac are the major sources of potassium used by plants, but initially non-exchangeable k (nex-k) can also contribute significantly to plant uptake. perennial ryegrass (lolium perenne, cv. roper) was successively planted in ten surface soil samples with different clay mineralogical composition belonging to five soil types to determine the contribution of ex-k and nex-k to k availability in the greenhouse during 260 d (five cuttings). before planting, soil samples were leached to remove so-k and ex-k so that the plants would use more k from the nex-k pool. the leaching was performed using 0.1 m chloride solution containing ca and mg in the molar ratio of 4:1. the soil types having initially high nex-k contents showed a high k uptake. a highly significant positive correlation was observed between total k uptake and illite content for the unleached soils (r = 0.81, p