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Diagnosing & Treating Cannabis Plant Deficiencies
There are 4 major steps to growing healthy, high-yield marijuana plants:
- Adequate light
- Appropriate amount of water
- Suitable climate
- Proper nutrients
Get these steps right and you will grow healthy plants that can defend themselves against most diseases and pests.
If you do not provide your plants the correct level of nutrients, they can become stressed, resulting in:
- stunted growth (reduces yields)
- susceptibility to mold and bugs
- vulnerability to disease
To be a successful cannabis grower, you need to know the signs of a deficiency of any of the essential nutrients for healthy plants.
STEP ONE: Growing Medium pH
Before planting, test the pH of your growing medium (soil, hydroponic solution, etc.). If the growing medium is too alkaline or acidic, your plants will be unable to absorb certain nutrients through their roots.
After planting, periodically re-test the pH to make sure that all nutrients necessary are available for your plants.
Soil and soil-like growing medium:
- Acceptable: 5.8 to 6.3
- Optimal: 6.3
- Acceptable: 5.5 to 6.5
- Optimal: 6.0
- Test the pH of your water; it can vary widely from source to source.
- Avoid over-fertilizing as it can rapidly lead to pH problems.
STEP TWO: Identifying cannabis nutrient deficiency
By learning the signs of cannabis deficiencies, you will be able to identify potential problems before they cause irreparable damage.
A common marijuana nutrient deficiency is lack of nitrogen which is essential throughout the lifecycle of the plant. The symptoms — which can be mistaken for overwatering — include:
- Leaves become lighter and the mature leaves near the plant’s base turn yellow
- If not addressed, the yellowing will continue up the plant and, eventually, you will see brown spots at leaf margins and leaves will curl and drop
- You will see fewer bud sites and earlier flowering
Calcium deficiency in cannabis is often found in hydroponic growing operations or outdoor planting in climates that are wet and cool with acidic soil. Symptoms include:
- Leaves near the plant’s base curl and develop brownish yellow spots and brown borders.
- Stunted plant growth caused by root tip deterioration and death.
Plants cannot grow without potassium. Symptoms of a potassium deficiency in marijuana plants — which can look like heat stress or nutrient burn — include:
- Leaves are overly green, but dull before showing brown leaf tips and yellowing.
- Eventually leaves will show more brown and display signs of dehydration. Younger growth will curl.
Zinc deficiency usually develops in a growing medium with a high pH level. Zinc deficiency is often found in dry climates with alkaline soils. Symptoms include:
- Leaves turn 90 degrees sideways.
- Young leaves wrinkle and distort.
- Leaf tips and margins discolor.
Iron deficiency is usually the result of either incorrect pH levels or excess levels of zinc, manganese or copper.
- Yellowing between the veins at the base of younger leaves (the veins stay green).
- Eventually both the young and old growth leaves will yellow between the veins while the veins stay green.
Sulfur is an important nutrient for the production of oils and terpenes. Although a sulfur deficiency in cannabis is not common, it is usually the result is caused by a high pH level in the growing medium.
- Young leaves turn lime green then yellow.
- Eventually leaf veins turn yellow and leaves become dry and brittle.
High amounts of magnesium are used by plants for photosynthesis.
Symptoms of a magnesium deficiency in marijuana plants include:
- At first, in older leaves, the area between the veins will yellow while the veins stay green. Then brownish red spots will begin to form.
- The rest of the plan will eventually start showing the same symptoms with more (and larger) spots forming.
- Finally the whole plant will look sick with some leaves curling and dropping off.
Phosphorous deficiency is uncommon, usually developing if your growing medium has a pH being above 7.0).
- The leaf stems of older leaves turn purple and eventually the leaves turn dark blue/green.
- Outward growth drastically slows.
- Dark spots appear on leaves.
- Sometimes leaves change color to a dark bronze or metallic purple.
- Leaves curl and drop.
STEP THREE: Treating Cannabis Deficiencies
Start with pH
If you have identified a potential nutrient deficiency, start by testing your growing medium. If the pH is not where it should be, adjust it to the proper pH level.
Consider Adding Targeted Nutrients
If the pH level is correct, add appropriate amounts of the deficient nutrient.
- Nitrogen – foliar feed with a soluble fertilizer with high available nitrogen
- Calcium – calcium supplement in fertilizer, foliar feed 1-2 teaspoons of dolomite lime per quart of water
- Potassium – supplement a soluble fertilizer with sufficient potassium
- Zinc – foliar feed with soluble fertilizer containing zinc.
- Iron – add fertilizer with a high percentage of iron
- Sulphur – supplement fertilizer with potassium sulfate or magnesium sulphate
- Magnesium – foliar feed or soil drench 2-3 teaspoons of Epsom salt per gallon of water
- Phosphorus – add fertilizer with a high ratio of phosphorus (blooming fertilizer)
Adjust for Overfeeding
If you have over-fertilized your plants, typically the pH at the root zone will be too acidic. Feeding your plants more will make the situation worse. Consider flushing your plants by rinsing out excessive salts with pH-balanced water:
- Drench the growing medium a few times.
- Test the soil.
- To avoid additional stress to the plant, begin fertilizing with ½ strength doses, slowly working up to full strength.
Visit Diagnosing Cannabis Nutrient Deficiencies in Sick Plants at Leafly.com for detailed growing information.