Dynamics of alkali and alkaline-earth cations in semi-arid environment of northern Spain

The dynamics of calcium (Ca2+), magnesium (Mg2+), sodium (Na+), and potassi um (K+) from their unavailable to their plant-available forms was examined. Soil samples from an area where materials of volcanic origin coexist with saline sediments were studied. The soils belonged to diapiric formations, w hich are uncommon in the Mediterranean basin but relatively frequent in the Iberian Peninsula, where they occur in highly complex heterolytic structur es encompassing a wide variety of soils. Based on the analyses, the base co ntents in fresh rocks of ophitic texture and in the resulting solids are ve ry similar and vary in the same direction as the mobility sequence: Ca2+ > Na+ > Mg2+ > K+. On the other hand, the alkali cation contents in Keuper ro cks and soils vary over wide ranges owing to the diverse lithology of the a rea and its chaotic distribution. Regarding the cation exchange complex, th e whole diapiric formation exhibits strong retention of exchangeable Ca. Be cause of the high prevailing exchangeable calcium-to-magnesium ratio (> 10) , the soils may be Mg-deficient. The low contents in exchangeable K found a re duc both to its ability to occupy lattice positions and to the antagonis tic effect of Ca. The high exchangeable Na contents may be toxic and marked ly antagonistic to Mg.