It sounds more like salt build up in the soil. If your water has a high salt/mineral level this is a common problem. If you have any of these plants in clay pots and unglazed you may notice a whitish hue on the pots, if so that is probably what is wrong. Before I go on let me know if this is the case as the remedy is easy to fix if so.

Scott

Thanks very much for your help! I don't have them in clay pots, and I'd never considered salt damage before. I live in South Korea, and while I'm not sure what the mineral content of the water is, it certainly does leave behind quite a lot of mineral residue when it dries.

Ok usually you see the salt/mineral deposits better on unglazed clay/terra cotta pots as they leach into the porous material used to make the pots. Since you can see the residue then my opinion is damage from salt/mineral residue. Plants as they take up water with the minerals deposit the salts in the tissue toward the end of the leaves. A big problem where I live here in NE Pennsylvania USA. The easiest way to fix the problem is to repot the plants first if possible if not soak the entire pot in water for 24 hours-this will not harm the plant. This will help dissolve the salts then thoroughly water the plant as in a shower if it is cold outside where you are in South Korea or if warm outside you can water with a garden hose to flush the dissolved salts out of the soil. If possible due this once every 6 months to keep the salt levels down. Again if you can repot them that would be the best and most effective option.

Scott