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

boron quickly becomes toxic if it is concentrated in the solution.
    Toxicity: Leaf tips yellow first, and as the toxic conditions progress, leaf margins become necrotic toward the center of the leaf. After the leaves yellow, they fall off. Avoid using excessive amounts of boric acid-based insecticides.
Chlorine (Chloride) (Cl) - immobile
    Practical Information: Chloride is found in many municipal water systems. Cannabis tolerates low levels of chlorine. It is usually not a component of fertilizers and is almost never deficient in gardens that grow cannabis.
    Technical Information: Chlorine, in the form of chloride, is fundamental to photosynthesis and cell division in the roots and the foliage. It also increases osmotic pressure in the cells, which open and close the stomata to regulate moisture flow within the plant tissue.
    Deficiency: It is uncommon to be deficient. Young leaves pale and wilt, and roots become stubby. As the deficiency progresses, leaves become chlorotic and develop a characteristic bronze color. The roots develop thick tips and become stunted. NOTE: Both severe deficiency and excess of chloride have the same symptoms: bronze-colored leaves.
    Treat deficiency: Add chlorinated water.
    Toxicity: Young leaves develop burned leaf tips and margins. Very young seedlings and clones are the most susceptible to damage. Later, the symptoms progress throughout the plant. Characteristic yellowish-bronze leaves are smaller and slower to develop.
    Treat toxicity: Let heavily chlorinated water sit out overnight, stirring occasionally. Chlorine will volatize and disappear into the atmosphere. Use this water to mix the nutrient solution or to irrigate the garden.
Cobalt (Co) - immobile
    Practical Information: This nutrient is seldom mentioned as necessary for plant growth, and most fertilizer labels do not include cobalt. Cobalt is virtually never deficient in indoor cannabis gardens.
    Technical Information: Cobalt is necessaryfor countless beneficial bacteria to grow and flourish. It is also vital for nitrogen absorption. Scientific evidence suggests this element is linked to enzymes needed to form aromatic compounds.
    Deficiency: When deficient, the problems with nitrogen availability occur.
Copper (Cu) - immobile
    Practical Information: Copper is concentrated in the roots. It is also used as a fungicide.
    Technical Information: Copper is a component of numerous enzymes and proteins. Necessary in minute amounts, copper helps with carbohydrate metabolism, nitrogen fixation, and the process of oxygen reduction. It also helps with the making of proteins and sugars.
    Deficiency: Copper deficiencies are not rare. Young leaves and growing shoots wilt, leaf tips and margins develop necrosis and turn a dark, copper-gray. Occasionally, an entire copper-deficient plant wilts, drooping even when adequately watered. Growth is slow and the yield decreases. A small deficiency can cause new shoots to die back.
    Treat deficiency: Apply a copper-based fungicide such as copper sulfate. Do not apply if the temperature is above 75°F (24°C) to avoid burning the foliage. Apply a complete hydroponic nutrient that contains copper. Cannabis plants seldom develop a copper deficiency.
    Toxicity: Copper, although essential, is extremely toxic to the plant even in minor excess. Toxic levels slow the overall plant growth. As the toxic level climbs, symptoms include interveinal iron chlorosis (deficiency) and stunted growth. Fewer branches grow, and the roots become dark, thick, and slow growing. Toxic conditions accelerate quickly in acidic soils. Hydroponic gardeners must carefully monitor their solution to avoid copper excess.
    Treat toxicity: Flush the soil or the growing medium to help expel the excess copper. Do not use copper-based fungicides.
Molybdenum (Mb) - immobile
    Practical Information: Molybdenum is seldom deficient.
    Technical Information: Molybdenum is a part of two major enzyme systems that convert nitrate to ammonium. This essential element is used by cannabis in very small quantities. It is most active in roots and seeds.
    Deficiency: This micronutrient is almost never found deficient in cannabis. Deficiency promotes nitrogen shortage. First, the older and middle-aged leaves yellow, and some develop interveinal chlorosis; then the leaves continue to yellow and develop cupped or rolled-up margins as the deficiency progresses. Acute symptoms cause the leaves to become severely twisted, die, and drop. Overall growth is