Try this simple rule of thumb for fixing nutrient problems in plants:

Look at the whole plant. If it has spotted leaves (and there are no bugs or molds), it is probably over-fed or the pH is too low. If it has chlorotic leaves at the top of the plant, add micro nutrients. If it has chlorotic leaves at the bottom of the plant and the chlorosis is progressing upwards, add macro nutrients (N especially).

I also caution that if you attempt to identify single-nutrient deficiency or toxicity you will soon find yourself immersed in a byzantine quagmire of second guesses and impossible-to-confirm (unless you can do leaf-tissue analysis) diagnoses. It is a sure route to headaches and compounded problems. Further underscoring the futility of such an approach is the simple fact that you will probably not be able to buy single-nutrient supplements!

Here is my flow chart for solving problems. It is broken into mobile and immobile nutrient deficiencies (not toxicities). Begin at the top of the mobile chart:

84-DraggedImage

85-DraggedImage

The chart below addresses each nutrient individually, the ionic forms used by the plant (note that these are inorganic; plants cannot use organic nutrients), its function in the plant, symptoms of its deficiency, toxicity, and its mobility in the plant and soil.

Nutrient

Ionic Forms Used

Function

Deficiency

Toxicity

Mobile in Plant?

Mobile in Soil?

Nitrogen (N)

NO3-, NH4+

Used in amino acids, needed for photosynthesis, facilitates carbohydrate utilization, stimulates root growth, makes other nutrients useable.

Stunted, slow growth, uniform yellowing of lower leaves progressing upward. Common.

“Eagle-clawing” of new leaves, stunted growth, dark green color. Extremely common owing to new growers’ tendency to fertilize their plants into a toxic state.

Yes

Yes as NO3-

No as NH4+

Phosphorus (P)

H₂PO4-, HPO42-

Component of ATP,

component of DNA and RNA, used in proteins, metabolic processes, photosynthesis, and respiration; affects root development, maturation (ripening), flowering and fruiting.

Dark green leaves progressing to purple coloration in petioles, stems, veins on underside of leaves, starting at bottom of plant, progressing upwards. Stunted growth, poor root growth. Unlikely.

Interferes with micronutrient and N absorption, so can appear as micronutrient deficiency. Unlikely.

Yes

No

Potassium (K)

K

Associated with overall vigor of the plant, regulates many functions including opening and closing of stomata, water uptake, essential for photosynthesis. Remains in ionic form.

Chlorosis of young leaf edges, moving inward leaving green veins with inter-veinal browning and spots of necrosis. K deficient plants are susceptible to disease, correct immediately. NB: Lesions will not disappear after correction, look for cessation of new chlorosis. Can be alleviated with NA. Common.

Competes with N uptake, causing a N deficiency. Unlikely.

Yes

Yes

Calcium (Ca)

Ca2+

Needed for healthy shoot and root tips, used in fruiting, cell wall component. Remains in ionic form.

Intimately connected with watering–too much or too little water will cause problems with Ca. Famous for blossom-end-rot on tomatoes, for pot it affects budding, making buds small and causing them to abort entirely in extreme cases; new shoots “clump”, leaves cup. Common.

Creates a high-pH crisis, which locks out many micronutrients. Common because growers spook after learning about Ca deficiency, then add too much.

See Mg, below.

No

Yes

Sulfur (S)

SO4-

Used in amino acid production, contributes to aroma, flavor (not always pleasantly).

Not likely unless you are growing in very sandy soil. Entire plant yellows rapidly from bottom up. Looks like N deficiency, but with pinkish veins. Highly unlikely.

Almost as unlikely as deficiency.

No/Poorly

Yes

Magnesium (Mg)

Mg2+

Needed in chlorophyl, enzymatic, fat, an sugar making processes. Needed for seed germination. Remains in ionic form.

Characterized by inter-veinal chlorosis accompanied by red/purple petioles, this is a fairly common deficiency; marijuana has a good appetite for magnesium. Epsom salts at 1 tsp/gal of water will bring it around.

Blocks Ca uptake.

Mg/Ca balance determines how “tight” soil is. The two cations make up the bulk of bound nutrient in the colloidal component of your soil. Ca flocculates (opens) soil, Mg coagulates (closes or tightens) soil.

Yes

no

Silicon (Si)

SiO44-

Structural component in cell walls, improves draught resistance.

Increased lodging (tipping over) tendency.

Not happening.

No

Yes

Chlorine (Cl)

Cl

Used in shoot and root growth. Remains in ionic form.

Practically impossible. Merely touching plant provides adequate supply.

Scorching, increase in succulence.

Yes

Yes

Iron (Fe)

Fe2+, Fe3+

Catalyzes chlorophyl synthesis.

Dramatic, high-contrast (almost unmistakable) inter-veinal chlorosis quickly spreading down from the top to whole plant.

Rare. Bronzing of leaves as plant attempts to neutralize its oxidizing effect.

No

No

Boron (B)

B3BO3, B(OH)3

Used in metabolism of N, pollen germination, cell division, fruiting, hormone movement.

Death of terminal buds accompanied by blackening of tissue. Young leaves curl and die from proximal end out. Common.

Marginal leaf scorch, resembling total ppm burn. Fairly common.

No

Yes

Manganese (Mn)

Mn2+

Used in synthesis of chlorophyl, co-enzyme. Remains in ionic form.

Necrotic spots become white/translucent and crack open or even fall out of the leaf.

Occurs in highly acid situations, appears as combination spotting and inter-veinal chlorosis.

No

Yes

Sodium (Na)

Na

Used in pyruvic acid cycle, synthesis of chlorophyl, can substitute for K in many plants. Remains in ionic form.

Exceedingly unlikely.

Competes with K for uptake. Excess NaReverses water potential, resulting in wilted, scorched plants. Fairly common outdoors.

Yes

Yes

Zinc (Zn)

Z

Used in enzyme activity, formation of chloroplasts, auxin, and starch.

Inter-veinal chlorosis, puckering, followed quickly by pitting. Veins remain green. New leaves are small, internal distance shortens dramatically, guttation prevalent.

Occurs when plants are unwittingly potted in galvanized metal pails. Severe stunting, reddening. Not common.

No

No

Copper (Cu)

Cu, Cu

Enzymatic activity, chlorophyl synthesis, catalyst in respiration.

Curling, wilting, new growth chlorotic, stunted, eventual death in terminal leaf buds.

In low pH situations, appears as Fe deficiency. Unlikely.

No

No

Nickel (Ni)

Ni

Necessary for seed development, key to N metabolism.

Distal tip of the leaves will suffer necrosis, progressing inward.

Unlikely.

yes

yes

Molybdenum (Mo)

MoO4

Used in conversion of nitrate to useable form.

Because it is used mainly–if not only–as a component in nitrate reductase, it looks like (and in fact, is) N deficiency but without redness under leaves. Leaves curl up, twist, deficiency progresses to small necrotic spots in inter-veinal tissue.

Brilliant yellow, orange or purple color. Rare.

Poorly

Yes

* In an earlier version of my book, Small Spaces, Big Yields, nickel (Ni) was incorrectly listed among the immobile nutrients. For the record, nickel is mobile and I regret the error.

If you are finding this information useful, why not pick up a copy of Marijuana Cultivation Reconsidered? It contains this information, plus so much more. Marijuana Cultivation Reconsidered is over 300 pages, with over 90 images and illustrations. Danny Danko describes it as “meticulously researched and enormously useful. . . a must-read for any grower striving to learn more than the basics and think outside the parameters of ‘conventional wisdom’ and horticulture folklore.”