I love to grow and want all cultivators to succeed. However, farming is unpredictable — all the songs about farming are woeful. This updated excerpt from my book Marijuana Garden Saver: A Field Guide to Diagnosing and Correcting Cannabis Problems focuses on nutrient issues. Whatever the problem, be it pests, diseases, or nutrients, the first line of defense is to use your eyes and act quickly. — Ed Rosenthal
Nutrients
When plants cannot get the nutrients they need they do not function properly, adversely affecting growth and yield. This can occur in any growing medium, while using any planting mix or technique—coir, rock wool, soil, soil-less, hydroponic or aeroponic. Plant disorders are characterized by their symptoms which appear more quickly in hydroponic gardens than in planting mixes or soil.
An overabundance of nutrients can result in nutrient burn or toxicity and can also lock out other ingredients. Unless the damage is slight, individual leaves do not recover from nutrient deficiencies. Some nutrients are mobile and are translocated from older to new growth so the damage is seen in older leaves, not in new growth. Other nutrients are not mobile. Their deficiencies are apparent in the new growth.
All fertilizer packages list three numbers that identify the N-P-K percentages. They appear as three numbers with dashes between them such as 25-10-10. The first number represents Nitrogen (N), which is responsible for foliage or leaf development. Fertilizers that promote heavy leaf growth have a higher first number (N) than the other two. The second number represents Phosphorus (P), which is important for strong stems and flowering. The third number is Potassium (K), which promotes healthy metabolic function. Sometimes micronutrients are listed after the macronutrients: these are Calcium (Ca), Copper (Cu), Iron (Fe), Magnesium (Mg), Manganese (Mn), and Zinc (Zn).
All nutrients are required to be present for proper metabolic function. Most growers who use premixed nutrient systems and faithfully follow the manufacturer’s feed schedules never see deficiencies before they flush their plants. Two deficiencies that may appear when using commercial fertilizers are Calcium (Ca) and Magnesium (Mg). On the other hand, organic, living soil and outdoor plants that do not receive supplemental nutrition are more often subject to deficiencies, but only because living soil systems have more variables involved in delivering nutrients compared to concentrated nutrient products.
pH
pH is a logarithmic measure of the acid-alkaline balance in soil or water. A pH of 1 is the most acidic solution, 7 is neutral and 14 is the most alkaline. When the pH is slightly acid, the nutrients dissolve well and are available to the plants. As the pH rises above or falls below those numbers some nutrients precipitate out of solution. Plants cannot absorb nutrients when they are precipitated. Plants can only “drink” them when they are in solution so even if nutrients are present, they are available to the plants only when they are dissolved. As a result, even though sufficient nutrients may be present, if any aren’t in the water solution roots won’t have access to them. When the pH is out of range, whether in soil or water, plants grow very slowly because some nutrients fall out of the solution as they precipitate and are unavailable.
Different species of plants have adapted to living under different pH levels. Marijuana has been grown in hydroponic solutions with a pH as low as 5.5, but it does best when grown in soil or water within a pH range of 5.6-6.2, slightly acidic. This is the pH of good garden soils. All plant nutrients are water soluble in this range so they are readily available to the plants. Outside this range some become less available.
pH can be viewed as a see-saw. As fertilizers are added it can drop or rise rapidly. It’s up to the grower to keep the pH stable. It is important to adjust pH when adding nutrients. When pH levels are out of the “safe” range nutrients fall out of solution and are unavailable to the plants. pH is important for both soil and hydroponic gardening. Failing to monitor it can lead to disastrous results. The pH level directly affects plants’ ability to absorb nutrients. When the pH sinks below 5.6 or rises above 6.2 some micronutrients precipitate out of solution and are less available.
Below 5.5 Boron (B), Copper (Cu), Manganese (Mn), and Phosphorous (P) become too available. This can result in toxicity.
The only accurate way to adjust the pH is by using a pH meter or pH test papers. Guesswork won’t do.
Adjusting Your pH Outdoors & In
Outdoors, if you are adjusting the soil pH before planting, use powdered sulfur if the soil is too alkaline or lime if it is too acidic. Check with knowledgeable local nursery staff or agricultural extension agents familiar with local soils. They can give you advice on correct proportions since soils vary in their reaction to adjustments. It takes several months after the addition of these minerals for the soil to adjust.
If the plants are already in the ground and the soil is out of the preferred range, adjust the irrigation water using pH up to raise alkalinity or pH down to increase acidity. Monitor the runoff. For instance, a composed media had a pH of over 8, which is very alkaline. It was irrigated with water adjusted to a pH of 5.1, very acidic. At first the runoff was over 7. Eventually the runoff tested at 6 and the pH level of the irrigation water was adjusted higher to maintain that pH in the runoff.
Water should be pH adjusted only after soluble fertilizers are added to it because their ingredients also affect water pH. Most commercial potting soils and topsoil are already pH balanced. If the plants are to be grown in soil or planting mix, check the pH using a pH meter or test strips before you plant.
Most indoor planting media are not soils at all: they are made using bark, peat moss or coir as the main ingredient. Other materials are added to adjust porosity and water retention. These mixes can be considered disease-and pest-free.
Planting mixes can be adjusted using commercially available pH-up and pH- down mixtures. Home remedies are available but can cause problems. Commercial products tend to be more stable and are concentrated and inexpensive.
Immobile Vs. Mobile Nutrients
One factor used to diagnose nutrient problems is by location. Some nutrients are immobile. Once they are set in place in the plant, they cannot move from their location. Other nutrients are mobile. When there is a limited supply, they go where the action is—usually the top of the canopy.
Immobile Minerals
Boron (B), Calcium (Ca), Copper (Cu), Iron (Fe), Manganese (Mn), Molybdenum (Mb), Sulfur (S) and Zinc (Zn) are utilized by the plant in ways that prevent them from being moved or movable on a limited scale. These are called immobile or intermediately movable nutrients. Calcium, for example, is permanently laid down in cell walls and cannot be moved. When these nutrients are deficient, the plant cannot transport them from older leaves so new growth occurs where immobile nutrient deficiency symptoms show up as deformed leaves. With extreme deficiency they may die back. This is most likely to happen with Boron and Calcium deficiencies.
Mobile Nutrients
Nutrients that the plant can move around are called mobile nutrients. Nitrogen (N), Magnesium (Mg), Phosphorous (P), Potassium (K) and Nickel (Ni) are examples. These nutrients can be cannibalized and moved to support new growth elsewhere in the plant.
When there is a deficiency, plants move the nutrients from old growth to the top of the canopy, where they will be utilized most effectively. Deficiency symptoms appear on older leaves from which the nutrients are being removed.
Nutrient deficiencies can, at times, serve a purpose—for both the plant and the grower. If you have grown cannabis before and flushed your plants at the end of flowering, you are already familiar with nutrient deficiencies. Flushing removes nutrients from the root zone, cutting the plant off from getting the materials it needs to grow. Without an incoming supply of nutrients, plants (including cannabis) can adapt to periods of low nutrients and move some nutrients around via the xylem and phloem of the vascular system to support new growth.
Flushing starves the plant of all nutrients, creating multiple simultaneous deficiencies. Both fertilized and non-supplemented grows sometimes experience a single deficiency. In these cases, the appearance of symptoms among older generations of leaves while new growth remains healthy matches expectations for one or more deficiencies among the mobile nutrients. Deficiency symptoms of immobile nutrients show up in new growth while older leaves remain healthy looking.
Invest in two relatively inexpensive meters: a pH meter and a TDS meter. The pH meter tells whether the soil chemistry is right for good uptake, and the TDS meter quickly tells growers whether there are too little, enough or too many nutrients in the root zone. Total dissolved solids (TDS) are measured in parts per million (ppm).
A low TDS suggests a general lack of nutrients. Check the actual readings against the projected numbers printed on the instruction label. Then adjust the strength of the nutrient solution or frequency of delivery.
The TDS meter won’t highlight which nutrients are lacking, just the total amount in solution. The only way to measure the individual nutrient levels on-site is by using chemical test kits. Micronutrients are present in such small amounts compared to the major nutrients that all of the minors could be left out of a nutrient batch and the TDS value would still be 98-99% of the target value. The only way to accurately determine deficiencies is by recognizing them or by submitting soil or nutrient solution samples to a lab.
pH and TDS meters are available for solutions as well. To measure pH and TDS in the root zone the grower has to add enough distilled water to the container to get a small amount of runoff liquid out of the bottom of the container. These meters can then measure pH and TDS of that sample.
This is a painful, time-consuming and highly variable process. To measure levels in mediums, use a meter whose probes are inserted into media for quick and direct measurements.
Don’t forget the roots when you are checking for symptoms. The roots should be white and firm. Brown, blackened, mushy or stringy roots indicate symptoms of problems.
In hydroponic systems, monitor the nutrient/water solution a minimum of once a week. If the numbers haven’t changed much and the plants are growing rapidly, there is usually no reason to change nutrients. If the nutrients need to be changed, rinse the roots at the same time. This helps prevent bacterial or fungal growth that attacks cannabis roots.
A really low EC (electrical conductivity) suggests a general lack of nutrients. Supplemental growers can then adjust the strength of their nutrient solutions or frequency of nutrient delivery. Unfortunately, the EC won’t tell you if minor nutrients are deficient because, unlike major nutrients, they are present in such small amounts that they could be left out of a nutrient batch and the EC would still be 98-99% of the target value. Aside from visually recognizing deficiency and toxicity symptoms, the only way to absolutely identify a deficiency is by submitting soil and nutrient solution samples to a lab.
AI Assistance
There are a number of apps available that attempt to identify problems based on a simple questionnaire and some photos. At this point they are sometimes helpful, but check back in a few months or so and am sure they will be more refined. Two of the apps I sampled are Agrio and Plant Parent.
For all deficiency symptoms:
- Review nutrient recipe and mixing protocols to ensure that all the nutrients required are being delivered. For users of commercial concentrated nutrients, this means making sure the manufacturer’s recommendations are followed. Even so, there may be Ca and Mg deficiencies if runoff, rainwater or R/O water is used. For DIY nutrient mixers be sure the recipe is complete (all nutrients) and delivering the proper levels.
- Review the pH of nutrient solutions and media if possible to make sure the pH does not drop below 5.6.
- Plants can be and are being grown successfully outdoors in soils with a pH as low as 5 or as high as 8 and still allow for adequate uptake of all nutrients even if it may not be optimal. Growth would be enhanced by lowering pH so more nutrients dissolved. Plants can be grown outdoors in soil with a pH as low as 5 or as high as 8. Yields will suffer, however.
Diagnosing Nutrient Deficiencies
Nutrient deficiencies appear without uniformity across a plant’s leaves early in a deficiency scenario. The symptoms are apparent on either newer growth or older growth so if there is uniformity in symptoms throughout the plant, the problem is not likely to be a deficiency. If the symptoms are visible only on newer or only on older growth, a nutrient deficiency is highly probable.
Once a nutrient problem is confirmed, you need to know how to solve it. Be aware, many of the visual symptoms presented as nutrient deficiencies and toxicities can also be created by disease and environmental factors, which are discussed in subsequent chapters in this book.
Deficiency Samples
Look at the growing points of stems and branches to detect potential nutrient issues. If older generations of leaves display mottling, yellowing between veins, or drying or dying of mature leaves, the symptoms match those of deficiencies of the mobile nutrients N, P, K or Mg (Nitrogen, Phosphorus, Potassium or Magnesium). Plants scavenge these nutrients from older leaves. New growth appears healthy while the symptoms appear in the cannibalized leaves.
If new growth appears stunted, deformed, brown or dying, think deficiency of an immobile nutrient. B, Ca, Cu, Fe, Mn, Mb, S and Zn (Boron, Calcium, Copper, Iron, Manganese, Molybdenum, Sulfur and Zinc are immobile so the plant cannot move them and symptoms appear on new growth). Yellowing of new-growth leaves matches the description of iron deficiency. Sulfur deficiency, while much rarer, also results in these symptoms. Purpling of stems may be characteristic of certain strains, a sign of a phosphorus deficiency or normal plant response to cool temperatures. Patterns of chlorosis (yellowing) in leaves suggests deficiencies of both major and minor elements.
For all deficiency symptoms: Review nutrient recipe and mixing protocols to ensure that all the nutrients required are being delivered. For users of commercial concentrated nutrients, this means making sure the manufacturer’s recommendations are followed. Even so, there may be Ca and Mg deficiencies if runoff, rainwater or R/O water is used. For DIY nutrient mixers be sure the recipe is complete (all nutrients) and delivering the proper levels. Review the pH of nutrient solutions and media if possible to make sure the pH does not drop below 5.6.
Plants can be and are being grown successfully outdoors in soils with a pH as low as 5 or as high as 8 and still allow for adequate uptake of all nutrients even if it may not be optimal. Growth would be enhanced by lowering pH so more nutrients are dissolved. Plants can be grown outdoors in soil with a pH as low as 5 or as high as 8. Yields will suffer, however.
Diagnosing Nutrient Toxicities
Excessive nutrient levels often result in nutrients bonding with other nutrients to create goopy solids that fall out of solution making them unavailable for plants to uptake. This can produce any number of other deficiencies with hard to predict visual symptoms.
If leaves appear to be significantly darker green and are smaller than usual, think high nitrogen, possibly due to a general over fertilization. Dark green is not normal for cannabis. If leaves are dark and hooked downward resembling a claw, think excess nitrogen. Pests can cause hooking too no matter what the color so be sure to check for thrips in particular. If you don’t see pests but the leaves are dark, it is likely excess nitrogen.
If plants are growing poorly, check the roots. If they are blackened and stringy this is a symptom of highly excessive nutrient delivery. If you find burnt roots, remove and destroy all affected plants.
Toxicity Symptoms
For all toxicity symptoms: Flush with water for two days. Follow this procedure for container and “in the ground” growing. Review/correct nutrient recipes to ensure proper levels of nutrients and no mixing problems. Reintroduce fertilization after two days and monitor plant response.
For all toxicity symptoms:
- Flush with water for two days. Follow this procedure for container and “in the ground” growing.
- Review/correct nutrient recipes to ensure proper levels of nutrients and no mixing problems.
- Once new growth on the plant has recovered reintroduce fertilization and monitor plant response.
Identifying Specific Nutrient Deficiencies And Toxicities
This section includes illustrations that represent the symptoms plants get when they have deficiencies. Use these pictures and descriptions to help you think through a diagnosis and use meters or labs to provide data that can help confirm visual symptoms.
To make the section easy to navigate, we begin each description with a quick reality check about the problem: How common is it? This is followed by a description of the affected plant’s appearance. Next is information on the role that the nutrient plays in the plant’s nutrition and its mobility. By mobility we mean that once a nutrient is transported to a site and is used to build tissue, can it be moved or is it fixed?
Mobile nutrients move to new growth so deficiencies appear on older vegetation. Nonmobile nutrients stay put so that deficiencies appear on new growth. Each nutrient section concludes with a guide to fixing the deficiency and getting the plant back to full health.
This article is excerpted from Ed Rosenthal’s book Marijuana Garden Saver: A Field Guide to Diagnosing and Correcting Cannabis Problems, which is organized as a field guide to different issues: pests, disease, and nutrient deficiencies. Each problem and how to resolve it is described in a succinct format that begins with a quick reality check about the problem: How common is it? This is followed by a description of the affected plant’s appearance. Stay in touch with Ed at @edrosenthal420 and purchase this and other helpful books at edrosenthal.com.