Soil-solution cations in competition for exchange sites
Only half as many Ca2+ are needed as Na+ to neutralize the charge of the ion exchanger (here: clay).  

The affinity of clay for bivalent ions is larger than that for monovalent ions:
Ca2+ > Na+

For most clays, the affinity for equally charged ions increases with the nonhydrated radius of the ions:
Cs+ > K+ > Na+ > Li+
Ba2+ > Sr2+ > Ca2+ > Mg2+