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Ed Rosenthal’s Guide To Understanding Nutrients

Updated: Mar 16, 2022

Feeding pot plants properly is essential for growing clean and flavorful cannabis. In this excerpt from Marijuana Garden Saver, the “Ganja Guru” showcases deficiencies and how to treat them.

Healthy cannabis bud/ Dabsel Adams

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, rockwool, soil, soilless, 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 new growth.

All fertilizer packages list three numbers that identify the N-P-K ratio. They usually 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), manganese (Mn) and zinc (Zn).

All nutrients are required to be present for proper metabolic function. Most growers use premixed nutrient systems and faithfully follow the manufacturer’s feed schedules, and they never see deficiencies before they flush their plants. Two deficiencies that may appear when using commercial fertilizers are calcium and magnesium. 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 with concentrated nutrient products.


The pH level is a logarithmic measure of the acid-alkaline balance in soil or water. A pH level of 1 is the most acidic solution. 7 is neutral and 14 is the most alkaline. When the pH level is within the 5.8-6.3 range, slightly acid, the nutrients dissolve well and are available to the plants. As the pH level 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 only available to the plants when they are dissolved. As a result, even though sufficient nutrients may be present, if plant roots do not have access to them the plants will indicate deficiencies. Plants that are growing in water or soil outside the proper pH range grow very slowly.

Different species of plants have adapted to living with different pH levels. Marijuana has been grown in hydroponic solutions with a pH level as low as 5.5, but it does best when grown in soil or water within a pH range of 5.6-63, slightly acidic. This is the pH level 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.

The pH level can be viewed as a seesaw. As fertilizers are added, the pH level can drop or rise rapidly. It’s up to the grower to keep it stable. It is important to measure pH after adding nutrients. When pH levels are out of the “safe” range, nutrients fall out of solution and are unavailable to the plants. The pH level is important for both soil and hydroponic gardening. Failing to monitor it can lead to disastrous results. The pH level directly affects a plant’s ability to absorb nutrients. When the pH level rises above 6.2, some micronutrients precipitate out of solution and are less available. Below 5.5, boron (B), copper, manganese and phosphorus become too available. This can result in toxicity.

The only accurate way to adjust the pH level is by using a pH meter or pH test papers. Guesswork won’t do.

Adjusting Your pH Outdoors & Indoors

Outdoors, if you are adjusting the soil’s pH level before planting, use powdered sulfur if the soil is too alkaline or lime if it is too acidic. Check with a 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 a pH Up mixture to raise alkalinity or a pH Down mixture to increase acidity. Monitor the runoff. For instance, a composed medium had a pH level of over 8, which is very alkaline. It was irrigated with water adjusted to a pH level 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 level in the runoff.

Water should be pH adjusted only after soluble fertilizers are added to it because their ingredients also affect water’s pH level.

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 level using a pH meter or test strips before you plant.

Most indoor planting mediums 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 way to diagnose nutrient problems is by their location. Some nutrients are immobile. Once they are set in place in the plant, they cannot move from that location. Other nutrients are mobile. When there is a limited supply, they go where the action is—usually to the top of the canopy.

Immobile Nutrients

Boron, calcium, copper, iron (Fe), manganese, molybdenum (Mb), sulfur (S) and zinc are utilized by the plant in ways that prevent them from being moved, or are only 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 immobile nutrient deficiency symptoms show up as deformed leaves where new growth occurs. With extreme deficiency, growth 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, magnesium, phosphorous, potassium 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 typically move the nutrients from old growth to the top of the canopy, where they will be utilized most effectively. Deficiency symptoms then show up on the 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 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 a Good Set of pH and EC/TDS Meters

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, just enough or too many nutrients in the root zone. Total dissolved solids (TDS) are measured in parts per million (ppm).

A low TDS level 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 with the major nutrients that all of the minors could be left out of a nutrient batch and the TDS value would still be 98 to 99 percent 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 the pH and TDS levels 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 the pH and TDS levels of that sample.

This is a painful, time-consuming and highly variable process. To measure levels in the medium, use a meter whose probes are inserted into the medium for quick and direct measurements.

Don’t forget the roots when you are checking for symptoms. They should be white and firm. Brown, blackened, mushy or stringy roots are 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) level 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 level 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 level would still be 98 to 99 percent 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.

Diagnosing Nutrient Deficiencies

Nutrient deficiencies appear without uniformity across a plant’s leaves early. 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, though, that many of the visual symptoms presented as nutrient deficiencies and toxicities can also be created by disease and environmental factors, which are discussed in depth in my book Marijuana Garden Saver.

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.

A yellowing of new-growth leaves matches the description of iron deficiency. A sulfur deficiency, while much rarer, also results in these symptoms.

A purpling of stems may be characteristic of certain strains, a sign of a phosphorus deficiency or just a normal plant response to cool temperatures.

Patterns of chlorosis (yellowing) in leaves suggest 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 calcium and magnesium deficiencies if runoff, rainwater or reverse-osmosis (RO) water is used. For DIY nutrient mixers, be sure the recipe is complete (all nutrients) and delivering the proper levels.

Review the pH levels of nutrient solutions and the medium if possible to make sure the they do not drop below 5.6.

Plants can be and are being grown successfully outdoors in soils with a pH level as low as 5 or as high as 8, still allowing for adequate uptake of all nutrients even if the level may not be optimal. Growth would be enhanced by lowering the pH level so that more nutrients dissolve. Plants can be grown outdoors in soil with a pH level 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 absorb. 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 excess nitrogen, possibly due to a general overfertilization. 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 burned roots, remove and destroy all of the 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.

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