I am a chemical engineer and find this an interesting read. I would drop into the pharmacy and grab up some P.H. strip. Just fold it in a ball of that clay. Black should not be sulphur. More like nitric or phosphorus. It could be so green it looks black.

A cheap very fine highly available lime and is a waste product is kiln dust from a cement plant. Should get it for about $5 per tone and it has a P.H. of 11. You should get calcium nitrate, potassium nitrate, calcium phosphate, and potassium phosphate titrate out and sink to the bottom.

Ag lime calcium carbonate is the best for maintaining P.H. not dolomite lime with high magnesium content.

Keep us posted on clay content. You could have a high P.H. in the clay with a stronger covalent bond than the acid in the water and making the water P.H. spike. This would mean the P.H. rise is a gas and can be evaporated off.

Cheers Don.

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7/8th of an acre, Perch only pond, Ontario, Canada.