Marijuana Horticulture: The Indoor/Outdoor Medical Grower's Bible

in a soil or soilless mix with a pH below 7 such as Peat-Lite, incorporating one cup of fine (flour) dolomite lime per gallon of medium ensures adequate supplies of calcium and magnesium.
    Macronutrients are the elements that the plants use most. The fertilizers usually show nitrogen (N), potassium (P), phosphorous (K) as (N-P-K) percentages in big numbers on the front of the package. They are always listed in the same N-P-K order. These nutrients must always be in an available form to supply marijuana with the building blocks for rapid growth.
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 stabilize 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 than 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 the 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 upward before dying. The entire plant could discolor in a few weeks, and if severe, turn a yellow-whitish tinge before browning and dying. A minor deficiency will cause little or no problem with growth. However, minor deficiencies escalate and cause 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, ammonia (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 evaporation and will also diminish magnesium availability.
    Progression of deficiency symptoms at a glance:
    No deficiency symptoms are visible during the first three to four weeks.
    In the 4 th to the 6 th week of growth, the first signs of deficiency appear. 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.

Early stage of Mg deficiency.

Progression of Mg deficiency.

Later stage of Mg deficiency.
    Younger leaves develop rust-colored spots and interveinal yellowing.
    The leaves dry and die in extreme cases.
    Treat deficiency by watering with two teaspoons of Epsom salts (magnesium sulfate) per gallon of water. For fast results, spray the foliage with a 2 percent solution of Epsom salts. If the deficiency progresses to the top of the plant, it will turn green there first. In four to six days, it will start moving 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 available.
    Control the room and root-zone temperatures, humidity, pH, and EC of the nutrient solution. Keep root zone and nutrient solution at 70 to 75°F (21-24°C). Keep ambient air temperature at 75°F (21°C) day and 65°F (18°C) 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