Weathering of clay minerals is a source of nutrients to plants. Palygorskite is a Mg-rich fibrous clay mineral that commonly occurs in the soils and sediments of arid regions.Although many studies have examined the environmental conditions required for the formation and stability of palygorskite,information on the transformation of this mineral in the root zone (or rhizosphere) of agricultural crops is limited.This study explored the possibility of palygorskite transformation in the rhizosphere of wheat and oat and compared the ability of these crops to extract structural Mg from palygorskite. The crops were cultivated in pots consisting of a mixture of sand from Hamadan region,Iran and Florida palygorskite,irrigated with distilled water,and treated with either complete or Mg-free nutrient solutions.After 100 d,Mg uptake by the crops was measured.Clay-sized particles in each pot were also separated from the sand and were analyzed through X-ray diffraction (XRD).The X-ray pattern of the particles was compared with that of the pure palygorskite particles (before cultivation).The results showed that palygorskite could provide sufficient Mg for the growth of wheat in the pots supplied with Mg-free nutrient solution.In spite of the magnesium uptake by both plants from the palygorskite structure,no detectable peaks indicating the transformation of palygorskite were recognized by XRD.A decrease of 1.05 nm in peak intensity was more obvious for the pots containing palygorskite and irrigated with Mg-free nutrient solution than for the pots irrigated with complete nutrient solution.Although the decrease in peak intensity was greater for oat than wheat,indicating greater palygorskite weathering by oat,Mg uptake by oat was low,as evidenced by the deficiency symptoms observed.