Water soluble potassium sulfate. $10 for a pound $15 for 5 pounds. Will last you forever and is what Big Bud is made from. 1/4 tsp per gallon weeks 4 - 6 of flower with you're normal regime. Keep an eye on ppm's when you start adding other stuff. The nutrient company feed charts will have you frying your plants. You are better off cutting back than using full strength.
Amazon.com: 1 Pound - Potassium Sulfate - Sulfate of Potash: Garden & Outdoor
www.amazon.com
I saw this and it looks interesting.
Potassium magnesium sulfate. Only $6.60 a pound.
Custom Hydro : KMS, Diamond-K brand - Water Soluble Fertilizers Plant Growth Lighting Lab Analysis Hydroponic systems pH and EC testing Environment Liquid Fertilizers Natural and Organic Fertilizers Foliar Fertilizer Supplies BioStimulants Pest and Disease Controls Amendments Harvest Irrigation...
customhydronutrients.com
"Source of Sulfur, Potassium & Magnesium. KMS is a natural source of readily available 22% K,11% Mg, and 22% S. One gram in one gallon of water yields 48 ppm Potassium, 29ppm Magnesium, and 58ppm Sulfur."
I'm ordering some right now. I don't remember seeing it before.