Cannabis loves nitrogen and requires high levels of it during vegetative growth and then tapers off during flowering. Nitrogen is easily washed away and must be replaced regularly, especially during vegetative growth. Nitrogen is the most common nutrient deficiency. When deficient, growth slows. Lower leaves cannot produce chlorophyll and become yellow between the veins, leaving the veins green. Yellowing progresses throughout the leaf eventually causing it to die and drop off.

Nitrogen regulates the plants ability to make proteins essential for new protoplasm in the cells. Electrically charged nitrogen allows the plant to tie proteins, hormones, chlorophyll, vitamins, and enzymes together. Nitrogen is essential for the production of amino acids, enzymes, nucleic acids, and chlorophyll and alkaloids. This important nutrient is mainly responsible for leaf and stem growth, as well as overall size and vigor. Nitrogen is most active in the young buds, shoots, and leaves. Too much nitrogen can burn foliage.

Treat deficiencies by fertilizing with a complete NPK fertilizer. You should see results within 4 or 5 days. Fast acting organic sources of nitrogen include seabird guano, fish emulsion, and blood meal.

In the images at the right, the first is classical nitrogen deficiency. The second image is a slight overdose of nitrogen, notice how the leaves take on a claw like appearance with the leaf tips curling downward and are dark green in color.

Cannabis uses the highest levels of phosphorus during germination, seedling, cloning, and flowering. Phosphorus is necessary for photosynthesis and provides a mechanism for the energy to transfer within the plant. Phosphorus is associated with overall plant vigor, resin, and seed production. The highest concentrations of phosphorus are found in root-growing tips, growing shoots, and vascular tissue.

A lack of phosphorus causes stunted growth and smaller leaves. Leaves turn bluish-green and dark blotches often appear. In some cases, the stems, leaf stems, and main veins turn reddish purple on the undersides of the leaves. The leaf tips of older leaves turn dark and curl downward. Severely affected leaves develop large blackish necrotic lesions throughout the leaves surface. These leaves will later dry, shrivel up and contort before dropping off. Flowering is often delayed and overall yield is diminished, seed yield is poor, and plants become vulnerable to fungal and insect attack. Soggy and acidic growing mediums aggravate phosphorus deficiencies.

Deficiencies are somewhat common and are often misdiagnosed. Deficiencies are most common when the growing medium pH is above 7 and phosphorus is unable to be absorbed properly. When the soil is too acidic and has a pH below 5.8, the nutrients become chemically bound and unavailable for absorption. High levels of iron or zinc can also retard the absorption of phosphorus in the growing medium.

Potassium is used during all stages of growth. Soils with a high level of potassium increase the plant’s resistance to bacteria and mold. Potassium is essential in the accumulation and translocation of carbohydrates. It is necessary to make the proteins that augment the oil content and improve the flavor in cannabis plants. It also encourages strong root growth and is associated with disease resistance and water intake. Mixing Langbeinite into the soil prior to planting can really give plants a boost throughout the growing season by providing both potassium and magnesium.

Potassium starved plants initially appear healthy. Deficient plants are susceptible to disease. Symptoms include: older leaves develop necrotic spots and leaf edges can appear burnt. Severely affected leaves will turn dark yellow and die. High salinity levels in the soil can lock out potassium, requiring a complete flushing of the growing medium with clean water.

Cannabis uses high levels of magnesium and as a result, deficiencies are common, especially in acidic soils where the pH is below 7. Adding dolomite lime to soil before planting will help to stabilize the pH, plus is will add both magnesium and calcium. Garden variety epsom salts are the quickest way to address a magnesium deficiency.

Magnesium is found as the central atom in every chlorophyll molecule, and it is essential for 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.

Magnesium deficiency is common indoors. The lower leaves, and later the middle leaves, develop yellow patches between dark green veins. Rusty brown necrotic spots appear on the leaf margins, tips, and between the veins as the deficiency progresses. The brownish leaf tips curl upward before dying. The entire plant can discolor in a few weeks, and if severe, turn a pale yellow before browning and dying. A minor deficiency will cause little or no problem with growth. However, minor deficiencies escalate and cause 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 too acidic and cold. Magnesium can become bound in the soil where there is an excess of potassium, nitrogen, and calcium. A high EC of the nutrient solution will slow water transpiration and diminish magnesium availability.

No deficiency symptoms are visible during the first 3 to 4 weeks of growth. In the 4th to 6th week of growth, the first signs of deficiency appear. Interveinal yellowing and irregular rust brown spots appear on the older and middle aged leaves. Younger leaves will remain healthy. The leaf tips will brown and curl upward. As the deficiency advances, rust brown necrotic spots multiply and interveinal yellowing increases and works its way from the bottom of the plant towards the top affecting younger growth. Magnesium deficient leaves can fold upward taking on the appearance of a taco shell.

Cannabis requires nearly as much calcium as the other macronutrients. Avert deficiencies in the soil and in most soilless mediums by adding dolomite lime or using soluble hydroponic fertilizers that contain adequate levels of calcium.

Calcium is fundamental to cell manufacturing and growth. Calcium is necessary to preserve membrane permeability and cell integrity, which ensures proper flow of nitrogen and sugars. It stimulates enzymes that help build strong cell and root walls. Cannabis must have some calcium at the growing tip of each root.

Calcium deficiency is somewhat uncommon indoors. Frequently, plants can process more calcium than what is available. Deficiency signs can be difficult to detect. Deficiencies start with weak stems, very dark green foliage, and very slow growth. Young leaves are affected first and show the first signs. When deficient, new growing shoots develop yellowish to purple hues and disfigure before shriveling up and dying. Flower development is inhibited, the plants become stunted, and harvest is diminished.

Deficiencies can be treated by dissolving 1/2 teaspoon of hydrated lime per gallon of water. Water deficient plants with the calcium and water solution for as long as the symptoms persist. Or use a complete hydroponic fertilizer that contains adequate amounts of calcium. Keeping the growing medium stable will help mitigate calcium deficiencies.

Many fertilizers contain some form of sulfur and as a result deficiencies are seldom. Gardening variety epsom salts often contain both magnesium and sulfur. When using epsom salts to cure either a sulfur or magnesium deficiency, make sure that the crystals completely dissolve prior to treating deficient plants.

Sulfur is an essential building block of many hormones and vitamins. Sulfur is an indispensable element in many plant cells and seeds. Sulfate is involved in protein synthesis and is part of the amino acids, cystine, and thiamine, which are the building blocks of proteins. Sulfur is essential in the formation of oils and flavors, as well as for respiration and the breakdown of fatty acids.

Deficiency symptoms typically start with younger leaves that turn a lime green to yellowish color. As deficiencies progress, leaves yellow interveinally and lack succulence. Veins typically remain green, and leaf stems and petioles turn purple. Leaf tips can burn, darken, and hook downward. Deficiency symptoms are most obvious in the older leaves. Sulfur deficiency resembles a nitrogen deficiency. Acute sulfur deficiency causes elongated stems that become woody at the base. Sulfur deficiency occurs indoors when the pH is too high or when there is excessive calcium present.

Zinc is the most common micronutrient found deficient in arid climates and alkaline soils. Zinc works with manganese and magnesium to promote the same enzyme functions. Zinc cooperates with other elements to help chlorophyll as well as prevent its demise. It is an essential catalyst for most plants enzymes and auxins, and it is crucial for stem growth. Zinc plays a vital part in sugar and protein production. It is fairly common to find zinc deficiency in cannabis plants and are more frequent in soils with a pH of 7 or more.

First younger leaves exhibit interveinal chlorosis, and new leaves and growing tips develop small, thin blades that contort and wrinkle. The leaf tips, and later the margins, discolor and burn. Burned spots on the leaves could grow progressively larger. These symptoms are often confused with a lack of manganese or iron, but when zinc deficiency is severe, new leaf blades contort and dry out. Flower buds also contort into odd shapes, turn hard, and dry. A lack of zinc stunts all new growth including flower buds.

Treat zinc deficiencies by lowering the pH to 5.8-6.5 and adding a complete micronutrient fertilizer.

Manganese deficiencies are relatively common indoors. Manganese is engaged in the oxidation and reduction process associated with the photosynthetic electron transport. This element activates many enzymes and plays a fundamental part in the chloroplast membrane system. Manganese assists nitrogen utilization along with iron in chlorophyll production.

Young leaves show symptoms of deficiency first. They become yellow between the veins (interveinal chlorosis), and the veins remain green. Symptoms spread from younger to older leaves as the deficiency progresses. Necrotic spots develop on severely affected leaves which become pale and falloff; overall plant growth is stunted, and maturation may be prolonged. Severe deficiency looks like a severe lack of magnesium.

Treat deficiencies by lowering the pH and adding a complete micronutrient fertilizer. Sometimes, flushing the growing medium to remove toxic salt buildup will also be required.

Iron is available in a soluble chelated form that is immediately available for absorption by the roots. Deficiency indoors is common in alkaline soils. Iron is fundamental to the enzyme systems and to transport electrons during photosynthesis, respiration, and chlorophyll production. Iron permits plants to use energy provided by sugar. A catalyst for chlorophyll production, iron is necessary for nitrate and sulfate reduction and assimilation. Acidic soils typically contain adequate iron for vegetative growth.

Iron deficiencies are common when the pH is above 6.5 and uncommon when the pH is below 6.5. Symptoms may occur during rapid growth or stressful times and disappear by themselves. Young leaves are unable to draw immobile iron from older leaves, even though it is present in the soil. The first symptoms appear on the smaller leaves as veins remain green and areas between the veins turn yellow. Interveinal chlorosis starts at the opposite end of the leaf tip, the apex of the leaves attached by the petiole. Leaf edges can turn upward as the deficiency progresses. Leaves fall off in severe cases.

Treat iron deficiencies by lowering the pH to 6.5 or less. Avoid fertilizers that contain excessive amounts of manganese, zinc, and copper, which inhibit iron uptake. High levels of phosphorus compete with the uptake of iron. Improve medium drainage; excessively wet soil holds little oxygen to spur iron uptake. Damages or rotten roots also lower iron uptake. Increase root zone temperature. Apply chelated iron in liquid form to the root zone. Complete, balanced hydroponic nutrients contain iron, and deficiencies are seldom a problem.

Be careful not to kill your plants with kindness. Over fertilizing is a common mistake made by beginning gardeners, and then when signs of overdose show up, the symptoms are often misdiagnosed as a deficiency leading to more fertilizers being added to the growing medium. Many store bought potting soils are fortified with nutrients that will sustain cannabis needs for the first month of growth.

When beginning a feeding regimen, avoid consecutive days of fertilizing. Instead, establish a routine of feeding with nutrients on one watering followed by regular water at the next watering, and then a mild organic tea at the next watering. Repeat this cycle throughout vegetative growth. Some varieties of cannabis are more sensitive to fertilizer than others, so you may need to adjust this schedule up or down depending on the strain that you are growing.