Iron (Fe) is an essential plant nutrient that is required for protein synthesis, chloroplast development, and the photosynthetic process of energy storage in cannabis.
Iron deficiency is commonly observed as interveinal chlorosis of the upper foliage (Fig. 1). Iron is considered an immobile nutrient and signs of a deficiency will first appear in the newly developing portions of the plant, as it cannot be translocated from other plant parts.
There are a wide variety of potential causes for Fe deficiency that should be evaluated, including high pH, overwatering, nutrient antagonism, and lack of supply.
Figure 1: Iron deficiency observed as interveinal chlorosis of the upper foliage as a result of high substrate pH.Courtesy of Brian WhipkerHigh pH Can Cause Iron Deficiency in Cannabis Crops
One of the most common problems associated with Fe deficiency is high substrate pH, also referred to as high alkalinity. Cannabis has a recommended substrate pH of 5.8–6.2. Substrate pH levels of more than 6.5 make Fe change to less bioavailable forms, leading to a deficiency.
High substrate pH is commonly observed in areas with water alkalinity levels above 2 milliequivalents (i.e., containing more than 120 parts per million (ppm) bicarbonate). Alkalinity refers to the concentration of bicarbonates and carbonates in irrigation water, which act as a buffering system that resists pH change.
If high alkalinity water is not counteracted with acidic fertilizer or water treatment, then the substrate pH will increase over time. It is therefore important to test your water supply to account for its mineral content and how it might impact plant growth and substrate pH. Monitoring substrate pH can be done non-destructively using the Pour Thru method.
Overwatering Limits Oxygen Availability
Iron deficiency can also result from overwatering. Overly frequent irrigation or failure to reduce watering during lower demand periods can lead to oversaturation of the root zone, limiting oxygen availability and reducing the plant’s ability to take up Fe.
This issue often arises when plant water needs decrease, such as during seasonal transitions in greenhouses from summer to fall, but irrigation practices are not adjusted accordingly.
Beyond nutrient uptake limitations, oversaturation also predisposes roots to rot caused by Pythium and other pathogens. Allowing the root zone to dry down between irrigation events is important to promoting root growth and preventing root rots or Fe deficiency.
Ensure Your Iron Is in Available Form in Hydroponic Systems
In hydroponic systems, Fe must be supplied in a soluble and plant-available form as the absence of a soil or organic fraction eliminates natural buffering and mineralization processes.
The most effective sources are chelated forms of Fe, such as Fe-EDDHA, Fe-DTPA, or Fe-EDDHA, which prevent iron from precipitating (separating from the liquid) and maintain solubility across a range of pH levels. The choice of chelate is critical, since some forms are stable only under certain pH ranges.
Failure to supply a plant-available Fe source in hydroponics can quickly lead to deficiency symptoms, as Fe becomes unavailable even when present in solution.
Iron deficiency remains one of the most frequent nutritional challenges in greenhouse and hydroponic crop production. Growers who routinely monitor substrate or solution pH using tools such as the Pour Thru method are better able to detect early shifts that may limit nutrient availability. By combining chemical, cultural, and monitoring strategies, the risk of Fe deficiency can be minimized, ensuring healthy crop growth and marketable quality.
For more resources on iron fertility and plant nutrition, read:
Figure 1: Iron deficiency observed as interveinal chlorosis of the upper foliage as a result of high substrate pH.Courtesy of Brian Whipker
