Understanding Nutrient Mobility in Cannabis Plants

Correctly identifying the deficient nutrient ensures appropriate management of nutrient mobility. It prevents unnecessary or ineffective treatments. Nutrient deficiencies in plants often present symptoms that closely resemble one another. Many also share visual similarities, making accurate diagnosis challenging.

It’s crucial to observe where on the plant symptoms first appear—on older, middle, or younger leaves. This location provides vital clues, as different nutrients are mobile to varying degrees within the plant.

How Nutrients Move

People generally divide nutrients into two categories: mobile and immobile.

Mobile nutrients such as nitrogen (N), phosphorus (P), potassium (K), and magnesium (Mg) can be translocated. They move from the lower foliage to the new developing growth when nutrient demands exceed what the plant can uptake.

Immobile nutrients include iron (Fe), manganese (Mn), calcium (Ca), boron (B), zinc (Zn), sulfur (S), copper (Cu), and molybdenum (Mo). Immobile elements cannot be translocated throughout the plant. They cannot satisfy the demands of new growth. When assessing visual nutrient deficiencies, utilizing the location of the visual symptoms is important in identifying the problem.

For example, when the leaf tissue between the veins turns yellow while the veins remain green, it creates a distinct pattern known as “interveinal chlorosis.” This symptom typically indicates nutrient deficiencies in elements like magnesium (Mg), iron (Fe), or manganese (Mn).

Because magnesium (Mg) is a mobile element, plants first show interveinal chlorosis on the lower, older foliage, and the symptom then progresses to the middle portions of the plant. In contrast, plants show iron (Fe) deficiency symptoms first in the new growth and developing foliage.

Diagnosing Nutrient Deficiencies

Visual signs and symptoms of mobile nutrient deficiencies

Plants communicate their nutrient needs through visual cues. When mobile nutrients are lacking, specific signs and symptoms appear, primarily in the older leaves.

For example:

• Yellowing or discoloration starting from the bottom leaves and progressing upwards.
• Stunted growth and reduced leaf size.
• Leaves turning pale or showing interveinal chlorosis (yellowing between leaf veins).
• Premature leaf drop or leaf death.

Considerations to Aid in Visual Diagnostics

There are a variety of considerations that can also help confirm visual diagnostics. Fe deficiency can result from various factors, such as oversaturated media, high pH (>6.5), or nutrient antagonism. Fe is included in most fertilizers that include a micronutrient package and, as a result, is generally supplied in sufficient amounts.

Mg can be supplied to the plant in a variety of ways, including dolomitic limestone as a preplant charge, through the irrigation water, or through the fertilizer. Testing the irrigation water is important to determine what elements are included naturally. It also identifies what elements must be supplemented.

In areas with low alkalinity in the water during high-growth demand periods, such as floral development, Mg deficiency can be observed. Additionally, not all fertilizers include Mg. For growers with insufficient Mg in their water, utilizing a calcium-magnesium (Cal-Mag) supplement or supplying Mg from Epsom salts or an Mg nitrate fertilizer is important. This avoids late-season nutrient deficiencies once the Mg from dolomitic lime is no longer available.

Additionally, taking leaf tissue samples can be a very useful tool. From previous literature, adequate Fe should be between 59–169 parts per million (ppm) of the leaf. Mg should be within 0.25%–0.81% for vegetative plants. While these values will vary slightly based on cultivars and growing conditions, they can be a good range to start with.

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Conclusion 

Accurate identification of nutrient mobility and deficiencies is critical for effective plant management. Understanding the mobility of nutrients like Mg and Fe is key to diagnosing issues based on symptom location. While both elements are essential for healthy growth, their differing mobility within the plant results in distinct visual patterns. These patterns can guide growers toward more precise solutions.

By combining visual observations with environmental assessments, fertilizer practices, fertigation systems, and tissue testing, growers can make informed decisions to correct deficiencies early, prevent future issues, and support optimal plant performance throughout the growing cycle.