The objective of this study was determining the most limiting plant growth factor in the wheat root zone dominated by illite in clay fraction and a high specific surface with ample ammonium acetate extractible potassium. A completely randomized block design with 4 replicates was used in Seyed Miran Research Farm (Gorgan) during 2009-2010 growing season. Treatments were mineral fertilizers (to achieve different levels of yields), gypsum (1000 Kg/ha calcium), calcium chloride (1000 Kg ha-1 Ca), urea (93 Kg ha-1 N) and potassium chloride (105 Kg ha-1 K) combined, gypsum (1000 Kg ha-1 Ca) and potassium chloride (105 Kg ha-1 K) combined, calcium chloride (1000 Kg ha-1 Ca) and potassium chloride (105 Kg ha-1 K) combined and control. Wheat cultivar (N-80-19) was planted in experimental site at 2009/12/9. The results showed that potassium is the most limiting plant growth factor in the site of the experiment. Electric diffuse double layer is expected to be truncated with a high specific surface soil in this area minimizing the soil solution-diffuse double layer interface for rapid potassium diffusion. The highest yield grain and straw with urea and potassium chloride showed a greater effect on plant and soil potassium concentrations. A greater potassium diffusion rate may be achieved as a result of greater concentration gradients between the exchange sites and soil solution by potassium fertilization and more potassium excess. Ammonium from urea diminished potassium fixation with illite and increased potassium root uptake.

Keywords: High specific surface, Potassium diffusion, Wheat.

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