I presume you are referring are planning to use these as a substrate?
Red modelling clay is very good as a base medium for growing plants. A thin layer can be put beneath your chosen aquarium gravel/sand, which will help to provide various minerals (mainly iron) to the roots.
Natural clumping cat litter can be used for this purpose too. It usually comes as bentonite clay (or similar). I'm unaware of the specific nutritional benefits of this type, but have had success with using a small amount under my regular substrate.
You can also use hard, baked clays. Many substrates produced specifically for aquarium plants are made from porous baked clay. The microporous nature of the material gives it a high cation exchange capacity, allowing it to absorb nutrients from the water column and store them to be used by plant roots. Moler clay is probably the best and most common example. It can be purchased from a number of bonsai websites in various grain sizes (smaller is usually better). A good rinse to remove dust, and it is ready for the aquarium.
Another source for moler clay is
Tesco Low Dust Lightweight Cat Litter 10 Litres - Groceries - Tesco Groceries. It is scented however, so will need a very thorough rinse before use.
Both of these materials can be used on their own, or used to cap other nutrient rich substrates. You can also scatter Osmocote beneath it as a cheap substitute to root tabs.
With regards to soil for the aquarium, there are many available. In the past, I have used B&Qs basics £1 a bag stuff. It worked very well, but did cause a small ammonia spike in the beginning. This was easily mopped up by rapid plant growth. Aquatics shops with a pond section tend to sell purpose made aquatic soil. I'm not sure what differences this soil has, but I presume it has a lower nutrient and peat content to reduce the possibility of water column contamination.