Changes in Different Forms of Soil Potassium at Various Growth Stages of Wheat

Changes in soil potassium pools including soluble, exchangeable and non-exchangeable forms at various growth stages of wheat (Triticum aestivum L.) may be important in management of this macronutrient and knowledge of its dynamic process. To study that issue, an experiment was done in a completely randomized design on a 9-ha field during 2012-2013. Soil samples were taken from depths of 0-15 and 15-30 cm before sowing wheat, and at its various growth stages until ripening. Also, at various growth stages, plant samples were collected and different K forms in soil including soluble, exchangeable and non-exchangeable, and total plant K were determined. Results indicated that soluble K in surface soil decreased severely during early growth stages until stem elongation, and thereafter continued constantly. But, changes in subsurface soil were slower. Significant decreases in exchangeable K in surface soil occurred at stem elongation and ripening stages, but this reduction in subsurface soil occurred at grain milk stage. Non-exchangeable K changes in surface soil were observed at ripening stage, while changes in subsurface soil were not significant. Sum of soluble, exchangeable and non-exchangeable decreased from759 mg kg^-1 to 720 and 680 mg kg^-1 at stem elongation and ripening stages, respectively. Generally, HNO₃-extractable K decreased in surface soil five times more than subsurface soil. Potassium concentration in shoots increased at tillering stage and reached 6.8 %, followed by a  decrease to 1.1% and 0.8 % in shoot and seed, respectively, at ripening stage. Generally, results indicated a maximum K uptake by wheat at stem elongation stage that may be considered as a key tool in K fertilizer management. On the other hand, the role of non-exchangeable K and K in subsurface soil may be considered in available K estimation.