Part II - Treating Nutrient Deficiencies Nitrogen. N is the most common deficiency of cannabis indoors or out. The first sign is a gradual, uniform yellowing of the large, lower leaves. Once the leaf yellows, necrotic tips and areas form as the leaves dry to a gold or rust color. Symptoms that accompany N deficiency include red stems and petioles, smaller leaves, slow growth, and a smaller, sparse profile.
Remedy by fertilizing with any soluble N fertilizer or with a complete fertilizer that is high in N. If your diagnosis is correct, some recovery should be visible in three or four days. New growth will be much more vigorous and new stems and petioles will have normal green color. Indoors, you should expect plants to need N fertilization a few times during growth. Once a plant shows a N deficiency, you should fertilize regularly to maintain healthy and vigorous growth.
Phosphorus. P deficiency is characterized by slow and sometimes stunted growth. Leaves overall are smaller and dark green; red color appears in petioles and stems. The leaves may also develop red or purple color starting on the veins of the underside of the leaf. Generally the tips of most of the leaf blades on the lower portion of the plant die before the leaves lose color. Lower leaves slowly turn yellow before they die. Remedy with any soluble P-containing fertilizer. Affected leaves do not show much recovery, but the plant should perk up, and the symptoms do not progress.
Potassium. Often, K-deficient plants are the tallest and appear to be the most vigorous. Starting on the large lower leaves, the tips of the blades brown and die. Necrotic areas or spots form on the blades, particularly along the margins. Sometimes the leaves are spattered with chlorotic tissue before necrosis develops, and the leaves look pale or yellow. Red stems and petioles accompany potassium deficiencies.
K deficiencies can be treated with any fertilizer that contains potassium. Wood ashes dissolved in water are a handy source. Recovery is slow. New growth will not have the red color, and leaves will stop spotting after a couple of weeks.
Calcium. Ca deficiencies are rare and do not occur if you have added any lime compound or wood ash. But Ca is added primarily to regulate soil chemistry and pH. Make sure that you add lime to soil mixtures when adding manures, cottonseed meal, or other acidic bulk fertilizers. An excess of acidic soil additives may create Mg or Fe deficiencies, or very slow, stunted growth. Remedy by adding one teaspoon of dolomitic lime per quart of water until the plants show marked improvement. Foliar feeding is most beneficial until the soil's chemistry reaches a new balance.
Sulfur. S deficiencies sometimes can be confused with N deficiencies and may also occur because of an excess of other nutrients in the soil solution. S deficiency symptoms usually start at the top of the plant. There is a general yellowing of the new leaves. In pots, the whole plant may lose some green color. Both S and Mg deficiencies can be treated with the same compound, epsom salts, or bathing salts, which are inexpensive and available at drug stores.
Magnesium. Mg deficiencies frequently occur in soilless mixtures, since many otherwise all-purpose fertilizers do not contain Mg. Mg deficiencies also occur in mixtures that contain very large amounts of Ca or Cl. Symptoms of Mg deficiency occur first on the lower leaves. There is chlorosis of tissue between the veins, which remain green, and starting from the tips the blades die and usually curl upward. Purple color builds up on stems and petioles.
You may first notice Mg symptoms at the top of the plant. The leaves in the growing shoot are lime-colored. In extreme cases, all the leaves turn practically white, with green veins. Treat Mg symptoms with one-half teaspoon of epsom salts to each quart of water, and water as usual. The top leaves recover their green color within four days, and all but the most damaged should recover gradually. Continue to fertilize with epsom salts as needed until the plants are flowering well.
Iron. Fe is likely to be deficient when the soil is very acid or alkaline. Under these conditions, the iron becomes insoluble. Remedies include adjusting the pH before planting; addition of rusty water; or driving a nail into the stem. Commercial Fe preparations are also available. If the soil is acidic, use chelated iron, which is available to the plants under acidic conditions. Symptoms appear first on the new growing shoots. The leaves are chlorotic between the veins, which remain dark green. Iron symptoms are usually most prominent on the growing shoots.
Manganese. Mn deficiency appears as chlorotic and then necrotic spots of leaf tissue between veins. They generally appear on the younger leaves, although spots may appear over the whole plant. Mn is present in many all-purpose fertilizers.
Boron. Symptoms of B deficiency first appear at the growing shoots, which die and turn brown or gray. The shoots may appear "burned." A sure sign of boron deficiency is that, once the growing tip dies, the lateral buds will start to grow, but will also die. B deficiency can be corrected by application of boric acid, which is sold as an eyewash in any drugstore. Use one-fourth teaspoon per quart of water. Recovery occurs in a few days with a healthy growth of new shoots.
Molybdenum. Mb is readily available at neutral or alkaline pH. Mb is essential for nitrogen metabolism in the plant, and symptoms can be masked for a while when N fertilizers are being used. There is a yellowing of the leaves at the middle of the plant. Mb is included in many all-purpose fertilizers.
Zinc. Zn-deficiency symptoms include chlorosis of leaf tissue between the veins. Chlorosis or white areas start at the leaf margins and tips. More definite symptoms are very small, new leaves which may also be twisted or curled radially. Galvanized nails can be buried or pushed into the stem. Commercial preparations of zinc are also available.
Copper. Cu deficiencies are rare; be careful not to confuse their symptoms with those of overfertilization. Starting with the younger leaves, which become necrotic at the tips and margins, leaves will appear somewhat limp, and in extreme cases the whole plant will wilt. Treat by foliar-spraying with a commercial fungicide such as CuSO[SIZE=-1]4[/SIZE].
