Soil with Clay pellets?

Hi, I'm pretty new to the planted aquarium community and have been playing with this hobby for slightly over 2 years only.

I currently have a 50g tank with lighting and yeast C02. I'm thinking of redoing the whole tank - again. It seems like I've done all sorts of wrong things with my substrate. My plants are all alive but their growth are almost nothing. I would like some healthy and fast plant growth. Unfortunately, I cannot afford to spend money on commercial substrate or soil. I have to go natural and I need some help here.

Would red coloured soil / clay be good for my tank? I thought of using it for the bottom. Do I need to add sand to it? Its the safest I can find from a garden supply shop since it doesn't have additives.

What about clay pellets? They are also relatively quite cheap here and easy to obtain. Could they these be used at all as a substrate?

I'm thinking of replacing the gravel with these pallets to cover the soil.