I used promix BX for a while and kept running into the same exact problems. By like the 5th week into flowering all my fan leaves looked like that and it was due to a mag lockout. People just don't realize how important magnesium is..
Magnesium (Mg)- mobile
Practical Information: Marijuana uses a lot of magnesium, and deficiencies are common, especially in acidic(pH below 7) soils. Adding dolomite lime to acidic potting soils before planting will stabalize the pH, plus it will add magnesium and calcium to the soil. Add Epsom salts with each watering to correct magnesium deficiencies, if no dolomite was added when planting. Use Epsom salts designed specifically for plants rather then the supermarket-type.
Technical Information: Magnesium is found as a central atom in every chlorophyll molecule, and it is essential to the absorption of light energy. It aids in the utilization of nutrients. Magnesium helps enzymes make carbohydrates and sugars that are later transformed into flowers. It also neutralizes the soil acids and toxic compounds produced by the plant.
Deficiency: Magnesium deficiency is common indoors. The lower leaves, and later middle leaves, develop yellow patches between dark, green veins. Rusty-brown spots appear on the leaf margins, tips, and between the veins, as the deficiency progresses. The brownish leaf tips usually curl upwards before dying. The entire plant could discolour in a few weeks, and if severe, turn a yellowish-whitish tinge before browning and dying. A minor deficiency will cause little or no problem with growth. However, minor deficiencies escalate and causes a diminished harvest as flowering progresses. Most often, magnesium is in the soil but is unavailable to the plant because the root environment is too wet and cold or acidic and cold. Magnesium is also bound in the soil if there is an excess of potassium, ammonium(nitrogen), and calcium(carbonate). Small root systems are also unable to take in enough magnesium to supply heavy demand. A high EC slows the water evapouration and will also diminish magnesium availabilty.
Progression of deficiency:
- No deficiency symptoms are visible during the first 3-4 weeks.
- In the 4th-6th week of growth, the first signs of deficiency appears. Interveinal yellowing and irregular rust-brown spots appear on older and middle-aged leaves. Younger leaves remain healthy.
- Leaf tips turn brown and curl upward as the deficiency progresses.
- Rust-brown spots multiply and interveinal yellowing increases.
- Rust-brown spots and yellowing progress, starting at the bottom and advancing to the top of the entire plant.
- Younger leaves develop rust-coloured spots and interveinal yellowing.
- The leaves dry and die in extreme cases.
Treat deficiency by watering with 2 teaspoons of Epsom salts(mangesium sulfate) per gallon of water. For fast results, spray the foliage with a 2% solution of Epsom salts. If the deficiency progresses to the top of the plant, it will turn green there first. In 4-6 days, it will start to move down the plant, turning lower leaves progressively more green. Continue a regular watering schedule with Epsom salts until the symptoms totally disappear. Adding Epsom salts regularly is not necessary when the fertilizer contains available magnesium. Use a foliar spray of Epsom salts for a fast cure. Another option is to apply magnesium sulfate monohydrate in place of Epsom salts. Add fine dolomite lime to soil and soilless mix to add a consistent supply of both calcium and magnesium over the long term. Always use the finest dolomite lime available.
Control the room and root-zone temperatures, humidity, pH, and EC of the nutrient solution. Keep root-zone and nutrient solution at 70-75F(21-24C). Keep ambient air temperature at 75F(21C) day and 65F(18C) night. Use a complete fertilizer with an adequate amount of magnesium. Keep the soil pH above 6.5, the hydroponic pH above 5.5, and reduce high EC for a week. The extra magnesium in the soil is generally not harmful, but it can inhibit calcium uptake.
Toxicity: Magnesium toxicity is rare and difficult to discern with the naked eye. If extremely toxic, the magnesium develops a conflict with other fertilizer ions, usually calcium, especially in hydroponic nutrient solutions. The toxic buildup of magnesium in soil that is able to grow marijuana is uncommon.
MJ gobbles TONS of magnesium during flowering