Nutrient Deficiencies
How to spot, diagnose, and treat problems of
nutrient deficiency in your indoor garden, in two parts.
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 CuSO4.
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