Hey blink,

To get the soil to hold more water you've got to let the soil soak up the water more slowly. What I do from time to time when the soil gets very dry is fill the bath with a couple inches of water and let the plants sit in it for about 15 mins, then I water them from the top slowly, alternating between plants to give them a minute or two before I water each plant a little more. It takes a while to do it but its the only way of doing it properly and like I said it's only from time to time when the soil gets really dry, not every watering.