It was calcium not sodium, sulfate
Albuquerque, N.M. — To the gardening editor: In the article on "gray water" by Peter Tonge (June 5), gypsum is explained as sodium sulfate. This is wrong. Gypsum is calcium sulfate.
The gypsum is recommended to reduce sodium buildup and excessive alkalinity in the soil. Gypsum, calcium sulfate, will help. Sodium sulfate will certainly not. [Editor's note: Thank you. The reference to gypsum as sodium sulfate was an error on my part and should have read calcium sulfate.]