A few growers place charcoal in the bottom of containers to absorb excess salts and maintain sweet soil, if plants are grown more than a few months.
Charcoal has the unique ability to take in salt
It was used to absorb access salt 
( z1 possible :ARG1 ( z2 take-in :ARG0 ( z3 coal ) :ARG1 ( z4 salt ) ) :ARG1-of ( z5 unique ) )