THE CALCAREOUS SOILS OF AZERBAIJAN II. PHOSPHATE STATUS

The phosphate status, before and after P treatment in the laboratory, of surface soils from thirteen profiles of an Iranian catena, described in Part I, was investigated by determining the distribution of labile phosphate in solution and external and internal surfaces, and by examining their calcium phosphate solubility relationships. The labile P distribution of added P suggests generally that soils with most carbonate in the silt fraction (Part I) hold added P mostly on external surfaces, and those with increasing carbonate in the clay fraction, less so. Recovery of 200 and 400 mg/kg soil of added P in the labile phosphate was related to soil phosphate status directly and to the total carbonate area of the soil inversely. Solubility relationships indicated saturation with CaCO₃ and a compound approaching octacalcium phosphate in composition. The two saline-alkaline end-members of the catena from the Low Land area differed somewhat in having more of their P content in soluble and labile forms, and in storing added P uniformly on external and internal surfaces.