The mud cubes idea I got several years ago from a Kiwi.
Works quite well.

If you have a chance to do a new tank, boiling the mud for 10 min works well to leach out/oxidize any Urea/NH4 into NO3.

Freezing will not do that, it just helps adding soil to a preexisting tank.

You sort of ride the balance between adding too much/too rich nutrients and too little, but you can always add more/less later.

The problem is that it is very difficult for folks to test their substrate for nutrients.
They rely on the plants rather than any test kit or quanatification.

You can allow your soil/mud to dissolve in a DI water solution, say 5 grams in 100mls of of water and wait 24 hours. Take a small sample and measure for NO3/NH4/PO4.

Add 10 mls of Acetic acid, wait for 30 minutes, swirl etc good, then measure again.


Regards,
Tom Barr