A plant may experience severe nutrient deficiencies. Possible causes of leaf yellowing and death include:
- Unsuitable pH levels
- Light burn and leaf discoloration from grow lights
- Underwatering or overwatering
- Over-fertilization or nutrient deficiency
- Unsuitable temperature/humidity levels
Nitrogen (N) Deficiency
Symptoms of nitrogen deficiency begin in the lower leaves and gradually progress upwards. Severe nitrogen deficiency causes lower leaves to drop, hinders the formation of strong branches, and significantly slows plant growth.
Phosphorus (P) Deficiency
Phosphorus deficiency causes yellowish discoloration and brown spots on leaves (sometimes leading to dead patches). This deficiency results in an underdeveloped root system, limiting the plant's ability to absorb nutrients effectively.
Potassium (K) Deficiency
Potassium deficiency starts with yellowing at the edges of leaves and the appearance of brown speckled spots, eventually leading to leaf death. It affects flower formation and reduces overall yield.
Calcium (Ca) and Magnesium (Mg) Deficiency
Calcium and magnesium deficiencies weaken plant stems, impairing nutrient absorption. Affected leaves become yellow, lose their healthy color, and experience reduced photosynthesis.
Zinc (Zn) Deficiency
Zinc deficiency causes newly emerging leaves to yellow from the tip and eventually dry out. This results in delayed flowering and the formation of deformed buds.
Iron (Fe) Deficiency
Iron deficiency leads to yellowing or whitening between leaf veins, stunting growth, and reducing yield.
Copper (Cu) Deficiency
Copper deficiency causes dark blue or purple discoloration on leaf veins and stems. In severe cases, brown spots form, leading to leaf death.
Boron (B) Deficiency
Boron deficiency causes leaf yellowing, deformation of leaves and buds, and brittle branches. Buds become fragile and prone to damage.
Manganese (Mn) Deficiency
Manganese deficiency initially causes slight yellowing between veins on young leaves. Over time, the leaves curl and wither.
Sulfur (S) Deficiency
Sulfur deficiency causes new leaves to yellow while older leaves remain green. Prolonged deficiency turns the upper leaves bright yellow.
Preventing Nutrient Deficiencies
To prevent nutrient deficiencies, maintain proper fertilization, regulate pH levels, ensure adequate nutrient availability, and control temperature and humidity.
For cannabis plants, pH levels should be maintained between 5.5 and 6.5 for optimal nutrient uptake.
Once the pH is adjusted and nutrients are properly dissolved, essential minerals become available and active for the plant.
Temperature and Humidity Issues
Improper temperature and humidity levels disrupt cannabis seed development, hinder photosynthesis, reduce aroma, and limit mineral absorption. They also increase the risk of bacterial, parasitic, fungal, and other harmful diseases.
Maintaining ideal temperature and humidity prevents plant drying, rotting, and disease development. Cannabis leaves are approximately 3°C cooler than the surrounding room temperature.
Proper Watering
Underwatering dries the soil, halting photosynthesis and preventing cell regeneration. Overwatering causes root rot due to oxygen deprivation, stopping photosynthesis and nutrient absorption.
Watering frequency depends on soil type, pot size, and plant growth stage:
- Vegetative Stage: Water 10-20% of the pot's volume every 2-3 days when the top 2-3 cm of soil is dry.
- Flowering Stage: Water 20-30% of the pot's volume every 2-3 days.
- Ripening Stage: Flush the plant with 50-100% of the pot's volume. If flushing with 50%, repeat 3 times; with 100%, repeat 2 times.
Coco coir dries faster than soil, requiring watering every 1-2 days, ideally mixed with perlite to retain moisture.
Foliar Feeding
Foliar feeding is recommended when plants suffer from nutrient deficiencies due to pH imbalance, inadequate fertilization, or unsuitable temperatures that hinder nutrient absorption through roots.
Spraying leaves directly provides a fast and effective way to deliver nutrients and prevent disease. Preventive sprays should be applied in advance to protect against pests.
- Best Time for Foliar Feeding: Early morning or late evening outdoors; during dark hours indoors. Avoid spraying under direct sunlight to prevent leaf burn.
- Optimal Temperature for Foliar Feeding: Avoid spraying above 27°C, as stomata close and hinder nutrient absorption. Below 20°C increases the risk of mold growth.
Pest Control
Pests typically nest on the underside of leaves, so thorough spraying on both sides of leaves is crucial for effective pest control.
Importance of CO2
Plants require carbon dioxide (CO2) for photosynthesis and cell regeneration. Insufficient CO2 slows plant growth and eventually causes gradual death.
If the room temperature exceeds 27°C and light intensity surpasses 800 watts, plants struggle to absorb CO2, making a CO2 generator necessary during light hours.
- CO2 Levels Based on Growth Stage:
- Vegetative Stage: 800-1200 ppm
- Flowering Stage: 1200-1500 ppm