I've heard some people talk about using modeling clay after they dry it, then break it up and put it in the substrate but I'm a chicken, if I didn't know I'd be afraid to use it. Try doing an on-line search. I have powdered laterite, which I mix in alittle water and roll clay balls, let them dry, then insert them close to plant roots. Messy but it didn't come in till after the tank was up and planted.