I repotted into slag soil

I just repotted my plants into 7 lt pots, Chose the soil i did because of the low nute values but it is too porus. When i water the plants it drains through way too quickly and it dry very quickly. I mixed about 20% perlite in aswell. I'm proper ragged off. It's been about 2 days now so i would guess the roots have spread well into the new soil. Any tips to help water retention @ this stage?p