Understanding the symptoms of nutrient deficiencies and toxicities is vital for serious growers
By Erica Hansen
Plant nutrition is a complicated topic for many growers. Over the years, a great deal of research has been conducted on mineral nutrition and how it impacts plant health.
Should growers halt nitrogen during flowering?
Many cannabis growers believe
By Erica Hansen
The concept: Adequate nitrogen availability in vegetative crops leads to leafy, green, sometimes sprawling growth. Excess nitrogen can lead to much darker green plants that may be overly leafy, which could lead growers to believe that the plant is prioritizing vegetative growth over flower production. Growers could hypothesize that reducing nitrogen once flowering has begun will reduce the amount of energy a plant is putting toward vegetative production, enabling it to focus on flowering.
The theory: Many cannabis growers believe nitrogen concentrations should be drastically reduced or cut out completely during flowering. They believe this will result in larger flowers and less vegetative growth, and that too much nitrogen will lead to undesirable growth patterns that affect the aroma and taste of the flowers.
The science: Nitrogen is the primary nutrient needed for health plant growth. Every plant, regardless of species, needs nitrogen. Healthy plants use nitrogen throughout their growth cycle for numerous tasks. Nitrogen is included in proteins, membranes, amino acids, nucleic acids, and enzymes. Throughout the growth cycle, biological processes are continually producing, processing, breaking down and reallocating any number of these types of compounds. All living organisms produce and use proteins, which are essential to life. Amino acids are the building blocks of these proteins, and nitrogen is a key component in their production. Problems in any one of these processes can lead to acute health issues.
Nutritional requirements change depending on growth stage and plants will only take up nutrients as they are available and as they are needed. Tomato producers have long switched to a lower concentration of nitrogen once flowering begins, but nitrogen is never entirely eliminated. This recipe change allows growers to save money by reducing the amount of nitrogen that must be added to their fertilizer solution. Nutrient absorption into roots is also an active process for some compounds, meaning that plants expend energy to move certain nutrients from the soil or media into root cells. As such, the absorption rates of most nutrients are self-regulated by the plant. Reducing the concentration of nitrogen and boosting the concentration of other nutrients (phosphorus and potassium) will not force the plant to absorb more than it can use. Many of the effects of nitrogen toxicity seem to arise from physical damage associated with exposure to high concentrations of nutrient.
Unfortunately, very little university research on the topic of cannabis cultivation is available. The idea that excess nitrogen availability affects the aroma and taste of buds at harvest deserves an in-depth, scientific investigation. Researchers have shown with other crops that varying salt concentrations can affect the flavor of food crops at harvest; a similar mechanism may be in play with cannabis. However, without controlled environments and a devotion to rigorous research methods, no concrete conclusion may be drawn about nitrogen levels, except that it is important to every plant.
“Factors affecting mineral nutrient acquisition by plants,” by David T. Clarkson, published in the Annual Review of Plant Physiology (1985).
“Raven Biology of Plants,” by R.F. Every and S.E. Eichorn (2013).
“Symptoms of Deficiencies and Toxicities by Element,” published by Greentrees Hydroponics (www.hydroponics.net).
Plant nutrient classifications can be divided several ways. One way is to compare relative nutrient amounts needed by the plant, classifying them as macronutrients and micronutrients. Macros are needed in significantly larger quantities than micros. Macronutrients include nitrogen, phosphorus, potassium, calcium, magnesium and sulfur. Micronutrients include iron, manganese, zinc, copper, boron, molybdenum and chlorine. Some crops are also able to utilize silicon, nickel and sodium, but not all texts refer to them as essential elements.
Alternatively, nutrients can be classified by their functions within the plant, leading to four separate groups.
Group one nutrients (sulfur and nitrogen) are included in carbon compounds such as sugars or structural compounds.
Group two nutrients (phosphorous, silicon and boron) are important for energy storage and strengthening plant structures. They are found throughout plant tissues.
Group three nutrients (potassium, calcium, magnesium, chlorine, manganese and sodium), which perform their functions as ions within the plant cell, are often related to some type of signaling or actively catalyzing biological reactions. These activities commonly help plants respond to shifting environmental conditions, such as changing availability of water or nutrients in the root zone. While other nutrients are often tightly bound within compounds, these ions move freely in water.
Finally, group four nutrients (iron, zinc, copper, nickel and molybdenum) are involved in oxidizing or reducing reactions. Oxidation and reduction reactions, also known as “redox” reactions, are important for all living organisms. By understanding what role each nutrient plays, growers can tweak fertilizer ratios for better growth and diagnose health issues with the plant.
Nutrient deficiencies and toxicities
Recognizing symptoms of nutrient deficiency and toxicity is an important skill for every grower. However, determining the nutrient responsible for problems can sometimes be a tricky task. High levels of one nutrient can even lead to a decreased ability for crops to absorb other nutrients.
Here’s a look at symptoms of deficiencies and toxicities for eight nutrients that are important for plants:
– Nitrogen: Appropriate levels of nitrogen promote healthy, green foliage growth. While plants are growing, nitrogen tends to be directed mostly to new developing leaves. This means that when available nitrogen levels fall, older leaves will show symptoms first. Nitrogen deficiency often appear as an overall yellowing of the plant called chlorosis, with the lower leaves being more heavily affected. General growth is also affected, leading to smaller, spindly plants. When nitrogen deficiency occurs over a longer period of time, foliage can develop a purple tint and stems can become woody. In contrast, an excess of nitrogen takes the form of overly leafy, deep green foliage with a tough, waxy skin.
– Sulfur: The symptoms of sulfur and nitrogen deficiencies often look strikingly similar. A key difference between the two is the developmental pattern of chlorosis. Nitrogen is able to be transported throughout the plant, meaning it moves from older leaves to younger growth as needed. The older leaves then become chlorotic before any other tissues.
– Phosphorus: Some nutrients, such as phosphorus, do not exhibit toxicity symptoms. Phosphorus deficiencies manifest as slow overall growth, with dark green lower foliage. Phosphorus deficiencies may also lead to purple coloration forming in plant tissues like flower buds. However, phosphorus deficiencies are not accompanied by chlorosis, which is a key trait to differentiate between plants lacking in phosphorus compared to nitrogen.
– Potassium: Potassium plays important roles in both regulating the amount of water that plants can absorb and the process of photosynthesis. It is easily transported throughout the plant, meaning any symptoms of a problem will appear on the mature lower leaves first. Potassium-deficient plants will show signs of a mottled pattern of chlorosis, leading to necrosis of the leaf tips, margins and interveinal areas. Weak stems are also a sign of potassium deficiency, leading to plants that can be easily bent over.
– Calcium: Calcium is an important component of cell walls, making it a key element in the structural support system of plants. Calcium deficiency or transportation problems are often the culprit behind tip burn issues. Calcium can be transported throughout the plant, but it is often not moved fast enough to quickly growing areas such as shoot tips. Tip burn often begins in these areas. Problems at the meristem, or growing point, of a plant can lead to general malformation in younger leaves as well.
– Magnesium: Magnesium ions are included in the structure of chlorophyll, playing an important role in photosynthesis and many other biological processes. Deficiencies in magnesium lead to interveinal chlorosis that appears as a yellowing of leaves in between the veins. With magnesium, chlorosis will characteristically show up in mature leaves first due to magnesium’s mobility. Magnesium toxicity does not tend to exhibit any negative symptoms.
– Iron: In contrast to magnesium, iron is not freely mobile, so symptoms of deficiency arise in developing leaves first. Interveinal chlorosis is the end result of iron deficiency, much like magnesium. Prolonged deficiency may lead to such severe chlorosis that leaves may turn almost entirely white. Iron is important in quite a few biological processes such as protein synthesis and chlorophyll production process.
– Copper: Copper deficiencies begin to manifest as dark green, malformed young leaves which may eventually die.
Some micronutrients, while essential to plant growth, quickly cause problems when present in excess. Copper is one such micronutrient. Toxicities show themselves as overall stunted growth or even as iron deficiencies. Root zones are affected as well, appearing dark, stunted, and with reduced branching. With the existence of copper-based fungicide and pesticides, growers must be conscious of the effects of overdosing their crops.