In its natural environment alocasia grows in the humid, tropical climate of South Asia and on the islands of the Pacific, so the substrate should be comparable to the natural one in its home conditions. A mixture of universal soil with coconut chips and pumex, expanded clay or perlite as airy grit will be suitable. Big chunks of expanded clay at the bottom of the pot will create good drainage. Mix everything in the given proportions: universal soil pH 5.5-6.5 40%, coconut chips 30-40%, airy grit 30-40% plus expanded clay at the bottom. You can also add crushed activated carbon.