Your plants need water, sunlight, CO2
, and nutrients to grow, stay healthy, and develop. If your plants are getting plenty of water, light, and airflow but their growth or development is lacking, you most likely have a nutrient issue. Nutrients are critical factors in virtually all of the biological processes of the marijuana plant, from photosynthesis to respiration. Macronutrients are needed in large quantities throughout the plant’s life, but micronutrients are needed in only minute amounts. Most soils and many added-nutrient blends contain more than enough micronutrients to bolster your plants.
But just providing nutrients isn’t enough because a plant actually has to take them up for their use to be effective. There are two primary factors that determine uptake of available nutrients: soil pH and the presence of other nutrients. Most nutrients are easily taken up by plants in soil with a pH level between 6 and 7, so aim for a soil pH of 6.5. If you suspect soil pH is your issue, check the pH of your watering solution and the pH of the soil via runoff water with test strips or a pH meter (see
Step 8: The Nutrients
). If these readings are outside the appropriate range, you may need to adjust the pH of your soil or watering solution appropriately.
The other common issue is blocking of nutrients by other nutrients. For example, iron uptake is blocked if there is an abundance of potassium in soil. In these situations, deficiencies are more often the problem, but toxicities—where the plant has too much of a certain nutrient—can also occur.
Soil pH
Note: Green bars cover the ranges of soil pH within which the indicated nutrients can be taken in by marijuana plants.
Nitrogen Deficiency
Nitrogen is key to the creation of chlorophyll, enzymes, hormones, and amino acids—the building blocks of plant proteins. Nitrogen is consumed more than any other nutrient during the early growth and vegetative stages and dictates the general size and health of your plants.
Signs of nitrogen deficiency include:
•
Yellowing of leaves (often during the vegetative stage), starting with older leaves toward the bottom of the plant and migrating up if left unchecked
To fix nitrogen deficiencies:
•
Check that your soil pH is in the appropriate range and adjust as necessary
•
Reassess nutrient feeding products, dilutions, and schedules to determine the root cause of the deficiency.
•
Add nitrogen via nitrogen-rich soil amendments or foliar feeding in the form of blood meal, worm castings, fish meal, etc.
Signs of nitrogen toxicity include:
•
Leaves turning deep green
•
Leaves appearing leathery in texture
•
Blades of leaves curling inward
•
Brittle, dry leaves
To fix nitrogen or other nutrient toxicities:
•
Discontinue feeding the nutrient to the plant
•
Flush soil thoroughly and modify soil pH, nutrient, dilution, and application schedules as necessary
Phosphorous Deficiency
Phosphorus is a key player in root growth, bud development, stem structure, and disease resistance. It is especially needed in the flowering stage, during which it promotes strong, dense buds.
Signs of phosphorus deficiency include:
•
Slow growth and loss of vigor
•
Brown spots on the largest leaves
•
Darkening of stems that spreads if unchecked
•
Leaf edges turning brown and curling inward
To fix phosphorus deficiencies:
•
Check that your soil pH is in the appropriate range and adjust as necessary
•
Reassess nutrient feeding products, dilutions, and schedules to determine the root cause of the deficiency
•
Add phosphorus via phosphorus-rich soil amendments or foliar feeding in the form of bone meal or flowering-stage nutrient blend
Potassium Deficiency
Potassium facilitates water circulation, respiration, disease resistance, photosynthesis, root growth, and CO2
regulation. Only small quantities of potassium are needed by plants.
Signs of potassium deficiency include:
•
Leaves turning brown before dying
•
Leaf tips and edges appearing burnt
•
Abnormal growth
To fix potassium deficiencies:
•
Check that your soil pH is in the appropriate range and adjust as necessary
•
Reassess nutrient feeding products, dilutions, and schedules to determine the root cause of the deficiency
•
Add potassium via potassium-rich soil amendments or foliar feeding in the form of wood ash or fish meal
Calcium Deficiency
Calcium plays a role in healthy root systems, strong stems and branches, and heat resistance. If you water with RO, DI, or zero ppm TDS water, you may need to find a way to include calcium in your grow. Most soil mixes contain ample amounts of calcium from lime.
Signs of calcium deficiency include:
•
Slow development of buds
•
Dying leaves
•
Dead patches on leaves
•
Unnaturally dark-colored leaves, primarily on older growth
•
Tips of leaves curling inward
To fix calcium deficiencies:
•
Flush growing media with water to remove any salts or nutrients potentially blocking uptake
•
Check that your soil pH is in the appropriate range and adjust as necessary
•
Add calcium via dolomitic lime or similar
•
Add calcium nitrate to watering solution
Magnesium Deficiency
Magnesium is a key component of the chlorophyll molecule and is essential to the plant’s ability to absorb light. It assists enzymes’ production of sugars, which are essential to big, vibrant buds.
Signs of magnesium deficiency include:
•
Leaves yellowing between veins
•
Older leaves yellowing and falling off
•
Leaves curling inward
•
Growing shoots that look white instead of green
•
Stems and petioles turning red
To fix magnesium deficiencies:
•
Flush growing media with water to remove any salts or nutrients potentially blocking uptake
•
Check that your soil pH is in the appropriate range and adjust as necessary
•
Add magnesium via Epsom salt, dolomitic lime, worm castings, or a calcium-magnesium supplemental product
Sulfur Deficiency
Sulfur plays a role in root-system development and the creation of chlorophyll, vitamins, and proteins.
Signs of sulfur deficiency include:
•
Slowing growth
•
Yellowing of new foliage
•
New leaves that are thinner and smaller
•
Buds dying prematurely
•
Yellowing that starts at the base of the leaf and moves out to the blade tips
To fix sulfur deficiencies:
•
Flush growing media with water to remove any salts or nutrients potentially blocking uptake
•
Check that your soil pH is in the appropriate range and adjust as necessary
•
Add sulfur via Epsom salt or potassium sulfate
Copper Deficiency
Copper plays a role in photosynthesis, flower development, and the synthesis of proteins and carbohydrates.
Signs of copper deficiency include:
•
Twisted or curled-under leaves
•
Decay in young leaves
•
Dark, purple, or blue leaves with yellow or pale tips and edges
•
A metallic hue to leaves
•
Abnormal flower growth
•
Leaves directly under light being most affected
To fix copper deficiencies:
•
Flush growing media with water to remove any salts or nutrients potentially blocking uptake
•
Check that your soil pH is in the appropriate range and adjust as necessary
•
Add copper via compost, kelp, copper biocides, or watering with tap water
•
Evaluate lighting intensity and adjust if necessary
Iron Deficiency
Iron is one of the more common deficiencies for indoor plants. Iron is involved in chlorophyll production and supports general plant health and vigor.
Signs of iron deficiency include:
•
Pale yellow leaves with green veins (like with magnesium deficiency), particularly on new growth
•
Stunted plant growth and development
To fix iron deficiencies:
•
Flush growing media with water to remove any salts or nutrients potentially blocking uptake
•
Check that your soil pH is in the appropriate range and adjust as necessary
•
Add iron via iron chelate as a foliar spray or compost as a soil amendment
Manganese Deficiency
Manganese plays a key role in chlorophyll production and the management of enzymes and nitrates. An abundance of manganese can cause decreased iron uptake and vice versa.
Signs of manganese deficiency include:
•
Leaves yellowing between veins
•
Brown, necrotic spots on center of leaves
•
Brittle, dry leaves
•
New growth being most affected
To fix manganese deficiencies
•
Flush growing media with water to remove any salts or iron potentially blocking uptake
•
Check that your soil pH is in the appropriate range and adjust as necessary
•
Add manganese via a water-soluble manganese spray supplement or compost as a soil amendment
Molybdenum Deficiency
Molybdenum is involved in the conversion of nitrates and ammonia and the synthesis of proteins. This micronutrient is needed in only trace amounts, so deficiencies are quite rare. Molybdenum deficiency is sometimes confused with nitrogen deficiency. The latter starts at the oldest, lowest leaves, while the former starts in the middle of the plant.
Signs of molybdenum deficiency include:
•
Yellowing of leaves starting in the middle of the plant
•
Abnormal growth of new plant tissues
•
Leaf tips and edges turning red, orange, or pink
To fix molybdenum deficiencies:
•
Flush growing media with water to remove any salts or nutrients potentially blocking uptake
•
Check that your soil pH is in the appropriate range and adjust as necessary
•
Add molybdenum via a foliar spray containing molybdenum
Zinc Deficiency
Zinc regulates enzymes, plant proteins, and growth factors. Zinc deficiencies typically arise when plants can’t uptake the nutrient due to a high or low soil pH level. Zinc deficiencies are often accompanied by deficiencies in manganese and iron, so check for these micronutrients as well.
Signs of zinc deficiency include:
•
New growth turning yellow
•
Older leaves’ veins turning yellow
•
Abnormal new growth
•
The distance between nodes shortening
To fix zinc deficiencies:
•
Remove damaged leaves
•
Flush growing media with water to remove any salts or nutrients potentially blocking uptake
•
Check that your soil pH is in the appropriate range and adjust as necessary
