NUTRIENTS & FERTILIZERS

Revised and expanded by Dr. Neil Mattson

The goal of nutrient supplementation is to provide the elements that plants require for metabolism and growth. The aim is to fertilize effectively and responsibly so that plants receive the nutrients they require for optimum performance, without applying excess. This saves fertilizer costs and prevents discharging excess nutrients to the environment.

There are many fertilizer choices available. They include prepared mixes with balanced and complete hydroponic fertilizers, liquid organic fertilizers, and slow-release solid organics. There is no one-size-fits-all fertilizer program. It will depend on the growing system, environmental conditions, and most important, the grower’s preferences.

Plant demand for essential nutrients changes throughout the life cycle. Cannabis uses more nitrogen overall during the vegetative stage than in later stages. Flowering plants require more phosphorus to promote flower formation and potassium for bud development.

Overall, to account for the plant’s changing needs for nutrients, one should adjust the nutrient supplementation program during these four life stages:

  1. propagation of seeds / rooted cuttings
  2. vegetative growth stage
  3. flowering stage
  4. ripening stage prior to harvest

Nutrient supplementation programs are adjusted for changes in environmental conditions such as light intensity, temperature, and humidity. Plant nutrient uptake is tied to transpiration, the evaporation of water from leaf stomata.

Plants transpire faster under bright light, warm temperatures, and low humidity. Because plants are using so much water, nutrients can concentrate in the canopy; therefore, nutrients (including nitrogen) should be diluted by 10-20%. When the flow rate is increased due to higher rates of transpiration and photosynthesis, nutrients build up when applied at a normal rate.

Conditions that slow down plant transpiration are low light, cool temperatures, and high humidity. Under low transpiration the nutrient solution concentration (including nitrogen) should be increased by 10-20% because plants will not absorb as much water, helping ensure sufficient nutrient supply to the plant.

N-P-K Ratios at Different Stages of Growth

Cannabis has different nutritional needs at different stages of growth.

Nitrogen (N), phosphorus (P), and potassium (K) are the nutrients needed most abundantly by plants. The amount of each nutrient varies as cannabis grows from seedling or cutting to a full-grown plant ready for harvest.

Nitrogen is necessary for creating new proteins and nucleic acids. Access to nitrogen early in the life cycle is important to develop new leaves, stems, and roots. As a result, the need to supply it to young plants is greater through the vegetative cycle, when new plant tissue growth is most substantial. The type of nitrogen (nitrate/NO3- vs. ammonium/NH4+) also plays a role. Nitrate has a near-neutral pH and keeps plants shorter, which is desirable.

Phosphorus is vital for plant growth, as it is also a component of DNA like nitrogen, but it has other roles in the plant. It is essential for photosynthesis, metabolism of sugars, cell division and elongation, and energy balance. When phosphorus is plentiful, plants grow taller and improve their flower quality. Shorter internodes typically result in higher-quality flowers, so limiting the amount of phosphorus during the vegetative and early flowering stages promotes better flower production.

Potassium has many functions in plants but is primarily responsible for regulating water movement and activating enzymes necessary throughout plant life cycles. Most nutritional guidelines suggest keeping potassium at medium to high levels from start to finish.

As plants transition from one stage in their life cycle to the next, their nutritional needs shift. They have two general stages of development, vegetative growth and flowering, but these can be broken down further:

Nutritional guides differ, but the general rules to follow are high nitrogen / low phosphorus in vegetative growth and a shift to low-to-medium nitrogen and medium-to-high phosphorus in flowering. Potassium is maintained at medium-to-high levels the entire time. Some cultivators maintain lower concentrations of potassium during vegetative and transition to high in flowering. Each nutritional guideline is dependent on the individual cultivator’s setup, preference, cultivar, and sometimes just plain trial and error for what works best for each specific system. Below are some examples of more specific N-P-K ratios that have been successful in other grow facilities.

Recommended N-P-K Ratios by Growth Stage

These are generalizations for hydroponic production. Adjust by growing conditions, media, and cultivar as needed. No matter what fertilizer instructions say, successful growers believe their eyes.

The “flush” stage is optional and is usually the last week before harvest. During a flush, some cultivators provide nothing more than clean water with no nutrients at the appropriate pH to the plants. Others will use the final ripening ratios for N-P-K, but at lower concentrations. It is all a personal choice, and there are schools of thought that support a complete flush versus a mild flush versus changing nothing during the final week of flower. (See Finishing & Flushing.)

Fertilizer Delivery Methods

The choice of fertilizers depends on the growing system and the grower’s preferences.

For example, it is common to use prepared liquid or water-soluble conventional fertilizer mixes in hydroponic systems when plants are rooted in inert, soilless substrates such as light expanded clay aggregate (LECA) pebbles, perlite, rockwool, or vermiculite. Fertigation is common in hydroponic systems using these media. The term “fertigation” is a combination of fertilizer and irrigation and refers to the fact that plants receive water-soluble fertilizer every time they are irrigated.

Planting mixes that contain peat, bark, coconut coir, or compost have greater nutrient-holding capacity, so granular-controlled or slow-release fertilizers (including organic fertilizers) can be incorporated into the potting mix. Some nutrients, such as nitrogen, phosphorus, and potassium, may be used up quickly, so they are supplied through fertigation.

Nutrient Additives/Supplements

Be wary of supplement lines. Growers have been growing great buds for decades using pure water, hydroponics, and basic nutrients. Some supplements improve yield or quality, but others just supply phosphorus and potassium at inflated prices. Try the supplement with a few plants, but leave some of the same variety untreated (controlled). That is the only way to determine the effectiveness of the supplement.

Reading a Fertilizer Label

Every fertilizer label supplies the same types of information.

The most basic information found on all fertilizer labels is the three numbers separated by dashes, such as 2-1-6, which refers to the percentage by volume of the three macronutrients nitrogen (N), phosphorus (P), and potassium (K). Note that by convention in the US the percentage of phosphorus is given in the form of phosphate (P2O5) and potassium is given in the form of potash (K2O).

Second, the label lists the guaranteed minimum analysis for macronutrients, calcium, sulfur, and magnesium (in percentage). There is no maximum analysis, so recipes may differ from the minimum guarantee.

Third, the label lists fertilizer salts the nutrients are derived from, for example, ammonium phosphate, magnesium carbonate, and so on. The derived-from information is particularly helpful if the grower wishes to get more in-depth at combining multiple fertilizer products. Some fertilizer salts are incompatible, thus referencing the derived-from information with fertilizer compatibility charts is useful. As a general rule, sticking to commercial products in the same series that are recommended for use together avoids compatibility issues.

Fourth, information on the application rate. Some labels chart application rates according to crop growth stage and how to use the fertilizer along with other products in the same series. It may list mixing instructions, and other instructions for use.

Fifth, storage instructions.

Water-Soluble Fertilizers

Water-soluble fertilizers are nutrients that are liquid or dry fertilizer that are readily dissolved in water and applied through a hydroponic or irrigation system. Water-soluble fertilizers can come from a single fertilizer salt (such as calcium nitrate or monopotassium phosphate), but more commonly they are commercially prepared by blending multiple fertilizer salts into a formulation that is semi complete or complete, and contains many or all the essential plant nutrients.

Great care must be taken when mixing fertilizer salts. Some fertilizer salts can interact with each other and cause precipitation, a chemical reaction in which the salts flocculate, or recombine into an insoluble form, usually a sludge that does not dissolve in water and can clog irrigation equipment.

A common precipitate issue occurs when combining calcium-based fertilizer salts (such as calcium nitrate) with fertilizer salts that contain phosphates or sulfates. Precipitation is mainly a problem when using concentrated fertilizer salts—this is why most hydroponic nutrients come as two- or three-part systems, since they can’t be combined at high concentrations.

In general, it is safest to combine products only from within the same product line. Because of the precipitation issue, when preparing fertilizer solutions, always fill the reservoir with water first. Then completely dissolve one fertilizer before adding the next one.

When purchasing water-soluble fertilizers, many people prefer preformulated liquid fertilizer products because they are easy to measure (by volume), dissolve quickly in the nutrient reservoir, and usually have clear instructions for mixing with other products in the same product line to provide a complete nutrient solution.

Reiziger® Grow Booster is widely used by craft cannabis cultivators because it is formulated with a high concentration of powerful botanical ingredients that provide the plant the energy it needs to fortify its natural defense process and elevate terpenes, increase resin production, and elevate aroma and color.

However, liquid fertilizers are very expensive compared with the quantity of dry fertilizer salts they contain. In addition, shipping these products is costly because of the weight of the water they contain. Dry powdered hydroponic water-soluble fertilizers contain the complete set of plant essential nutrients. They are much cheaper per unit of nutrient solution and more sustainable to ship. However, initially they require a bit more care to measure out and dissolve in the nutrient reservoir.

Home garden fertilizers can be used for cannabis, but care must be taken to make sure all plant essential nutrients are provided. For example, a common garden fertilizer is 10-10-10. This product provides N-P-K but may not provide the other essential nutrients. Other fertilizers are more complete and suitable for use without supplementation.

Advanced Nutrients Big Bud® has a proven track record of helping plants pack on bud weight throughout the bloom phase. A team of scientists engineered this bloom booster with an optimal potassium and phosphorus ratio to amplify yields and maximize bud bulk while reducing the risk of heavy metal contamination.

These two-part base nutrient systems feature precise NPK ratios for vigorous vegetation and robust flowering. Advanced Nutrients’ scientists engineered these revolutionary products with their proprietary pH Perfect Technology®, ensuring a balance-free pH that frees growers from tedious adjustments.

When larger gardens use fertigation, it is common to hold concentrated fertilizer in stock tanks. As compared with mixing nutrients at the dilute amounts the plants would receive in a larger reservoir, concentrated fertilizer stock tanks save space and the time spent frequently remixing the fertilizer.

Using a fertigation system, the concentrated fertilizer is introduced to the irrigation water using a proportioner to dilute the fertilizer to the final concentration the plants receive. This dilution can be done by a simple proportioner, which uses suction to draw up the concentrated fertilizer and mix it with the water source at the proper ratio.

A positive displacement injector is more likely to be used in commercial gardens. These injectors use a piston with a fill chamber to more accurately inject the proper ratio of nutrients into the irrigation water. These injectors typically have a higher ratio, for example, 1:64 to 1:200 1:100 is quite common and makes calculations easier (the fertilizer stock tank is prepared with 100x concentration). Remember that some fertilizers cannot be combined in a concentrated form. To account for precipitation, growers may have two to three or more fertilizer stock tanks with fertilizer injectors connected in series.

Controlled- & Slow-Release Fertilizers

Controlled- and slow-release fertilizers are dry fertilizers in a pellet or granular form. They are used not in solution-culture hydroponics but in soil or substrate grows. These fertilizers can be incorporated into the potting mix before transplanting or can be top-dressed onto the substrate surface.

Controlled-release fertilizers are typically formulated with all or most of the macro- and micronutrients required by the plant. They have a coating, often a polymer, which controls the release rate and pattern of the nutrients. Products are available with different release periods such as three to four month, five to six month, or eight to nine month.

The release rate is affected by substrate temperature; warmer temperatures lead to a quicker release rate. For example, a three-to-four-month product will release 90-95% of its nutrients during that time if substrate temperature averages 70°F (21°C), but will release more quickly with warmer conditions.

A benefit of controlled-release fertilizers is that they can provide the majority of the plant’s fertilizer needs, thus reducing the need for water-soluble fertilizers. These slow-release nutrient forms reduce leaching of nutrients to the environment, making the feeding regimen more sustainable.

However, it can be difficult for a plant to achieve its full yield potential using only controlled-release fertilizers. Cannabis requires high amounts of fertilizers, and it can be difficult to meet all the plant’s nutrient needs using controlled release alone. It is best used as the initial base fertilizer, and then liquid fertilizer can be added to supplement it.

Another issue with controlled-release is that once the fertilizer has been added, it cannot be removed, so it is important not to over apply it. During hot conditions, the fertilizer releases faster than the labeled rate, so salts can build up quickly. Monitor root zone salinity (as described below) and leach with clear water to remove excess soluble salts if there are readings in the danger zone.

Slow-release fertilizers do not have a polymer coating, and their rate of nutrient release is not as precise. They are not typically complete fertilizers but can be an inexpensive way to add specific elements. Examples of these include sulfur-coated fertilizers (such as sulfur-coated urea, a form of nitrogen), slowly soluble fertilizers such as gypsum (calcium sulfate), triple superphosphate (a source of phosphorus), and limestone. Standard limestone is a source of calcium and is also used to increase substrate pH. Dolomitic limestone is also used to raise substrate pH and is a source of both calcium and magnesium.

Soil test kits provide individual readings of nutrients. Meters provide only an overall reading. Photo: Angela Bacca

Organic Fertilizers

Organic fertilizers come from animal or plant matter and naturally mined materials. Examples include compost, fish emulsion, guanos, limestone, manure, ground minerals, plant extracts, plant meals or rock phosphate, seaweed, and worm castings. Organic fertilizers may contain some immediately available nutrients; however, much of their nutrition is slow release and requires either weathering or microbes to break down complex organic molecules and make nutrients available. Meeting the plant’s nutrient demands with organic fertilizer requires some experience, so it may be difficult to get the same yields initially with organic fertilizers as with conventional. Their use is especially nuanced when used in substrate or hydroponic systems.

Organic fertilizers often raise pH. The benefits of using organic fertilizers include reuse of waste streams (compost) as well as the potential for reduced leaching to the environment (if applied properly) because they are slow-release nutrient forms. Some connoisseurs consider organically grown buds to be the highest quality.

There are many organic fertilizer mixes on the market. Many are solid (granular, powdered, or pelleted) and are added directly to the soil or substrate prior to transplanting or to the topsoil while the crop is growing. Other organic products are readily available, soluble formulas added to the water. Many will include macronutrients as well as micronutrients; however, read the label carefully and ensure all essential nutrients are supplied either from one product or by combining materials.

Traditionally, single-source materials were used (such as plant or animal meals or manures). Most single-ingredient organic fertilizers are not complete; they are more often used as a supplement or combined with other ingredients. Many complete, balanced, organic fertilizers, containing several ingredients, are available.

Time is required for microbe-mediated nutrient availability; however, much of organic fertility is ultimately available after the first four to six weeks of application. For this reason, more available nutrients such as liquid organics may be required to spur initial growth. Then, as the microbes consume the organics in the planting mix, nutrients will gradually be released. The mix may be more “live” and fertile when it is used for a second crop. As long as the mix retains its structure and is not subject to infection or disease, it can be reused.

Neptune’s Harvest Fish & Seaweed Fertilizer (2-3-1) is an organic (OMRI listed) cold-processed fertilizer made from post-fillet ocean fish from Norwegian kelp in the North Atlantic that not only contains macro- and micronutrients, but also trace elements, vitamins, minerals, amino acids, enzymes, and omega fatty acids. Neptune’s Harvest Fish & Seaweed Fertilizer repels deer, eliminates powdery mildew as a foliar feed, and is a great rescue product for plant stress.

Neptune’s Harvest Crab & Lobster Shell (5-3-0) is a dry organic fertilizer made from organic matter that breaks down slowly while both aerating soil and helping it retain moisture. It is high in calcium (12-17%) and magnesium (1.3-1.7%) and controls root nematodes, grubs, ants, slugs and fungus.

Water-soluble organic fertilizers are delivered through the irrigation system. They work best when there is a substrate or soil where microbes are abundant, because they break down the complex nutrient compounds into soluble salts that the plants can absorb. They are not recommended for hydroponic systems because they ferment in reservoirs and lead to biofilm, which can coat roots, clog up systems, and deprive the plants of sufficient dissolved oxygen.

Compost

Compost is organic matter subjected to decomposition. It is a valuable soil or substrate amendment that increases organic matter content, nutrient-holding (or cation exchange) capacity, and is a source of beneficial microbes and slow-release nutrients.

Controlled compost piles are maintained at 130-160°F (54-71°C) for pasteurization for a minimum of three days. This kills harmful microbes and weed seeds.

Compost should represent only 10-20% of the substrate. Percentages depend on the quality of the soil that it is being added to and the level of dissolvable salts in the compost. Used as a large percentage of some mixes, it holds too much water and is too rich in nutrients. Only mature compost should be used. Immature compost can be high in ammonia and volatile organic compounds, which injure the plants.

Care should be taken to avoid transmitting human pathogens such as E. coli or Salmonella when animal manures are used. Manures should be fully composted or heat-treated before using as fertilizers, or if they are added to soil, they should be incorporated at least 90 days before harvesting a crop. Pet waste (dog and cat) can contain a number of pathogens and parasites harmful to humans and should never be used as a fertilizer or substrate amendment.

Compost teas are made by soaking organic matter or compost in water, which serves to transfer beneficial microbes, fine particulate organic matter, and some soluble nutrients to the water. The beneficial microbes can improve nutrient retention and availability to the roots as well as help the plant fight off diseases and insects. Commonly used ingredients for compost tea include vermicompost (worm-worked compost), well-composted manures, blood meal, coffee grounds, plant meals, minerals/ground rock, and oyster shells.

Similar to using manures, only materials known to not be a source of human pathogens should be used. Live (aerated) compost tea requires equipment to “brew” it with a lot of oxygen or airflow. Without proper aeration, compost tea will become infected by anaerobic bacteria, which causes plant diseases. Some hydroponic stores brew teas on-site.

Compost tea must be used within a few hours after brewing or kept aerated until use. Extracts or non aerated compost tea can also be used. With extracts, oxygen is not introduced during the steeping process, and the feedstock may be steeped in water for several days. An extract will not have the same large number of microbes as aerated compost tea; however, it is more shelf-stable and does not have to be used right away. The nutritional value of compost teas and extracts varies according to the ingredients and processor. These are rarely well described on their labels. It is important to find a processor that uses high-quality ingredients and follows a consistent process that provides reliable results.

Some compost tea recipes suggest using molasses as an ingredient of the brew. However, teas using it may become infected with pathogenic E. coli bacteria. For this reason, use honey, beet juice, malt, or sugar as a substitute.

Monitoring Plant Fertility

The pH of the hydroponic reservoir or root zone substrate has a large impact on nutrient availability. The pH is a measurement of how acidic or basic (alkaline) a solution is and varies from 0 (most acidic) to 14 (most basic). (See Water.) The pH affects nutrient availability; at a higher pH, 6.3-6.7 in hydroponics and soilless substrates, micronutrients such as iron, manganese, zinc, and copper become increasingly unavailable.

At a lower pH, calcium and magnesium become increasingly unavailable, and some micronutrients become available at toxic levels for the plant. Nutrient disorders are often due to root zone pH, rather than a lack or excess of nutrients.

A pH meter is used to measure pH. There are many models available. Handheld meters are accurate and invaluable tools. Other models are placed in a tank or reservoir and constantly monitor the water. Automated devices automatically adjust the pH.

These meters can be used to measure the pH of the source water, fertilizer solution, or hydroponic nutrient reservoir. To test root zone pH, a solution sample must be prepared (described below). There are pH meters available with soil probes.

Periodically, the meter should be calibrated using at least two standards (pH 4 and pH 7). The pH sensor reading can drift over time, so it is important to calibrate frequently according to the manufacturer’s instructions (typically every day to every week) to have accurate readings. If the meter can be calibrated with only one standard, it is only accurate close to that standard reading.

The sensor end of the pH meter, the electrode, should be kept in a pH storage solution between uses. It should not be stored in distilled water or allowed to dry out. It may need to be soaked in a storage solution for one to two hours if the sensor dries out. The pH electrode needs to be replaced when it does not calibrate or give reliable readings.

The pH of hydroponic reservoirs and in the root zone of soilless substrates can fluctuate quite rapidly depending on the alkalinity (a measure of the carbonates and bicarbonates in the water) and in response to plant uptake of nutrients. The larger the reservoir and the higher the alkalinity, the slower pH changes.

In particular, the form of nitrogen in the fertilizer has a large impact on pH. Ammonium-based fertilizer in the rhizosphere causes the pH to decrease, while nitrate-based fertilizer causes the pH to increase slightly.

In “true” hydroponics, where plants are growing directly in a nutrient solution, or in an inert substrate such as LECA or rockwool, it is recommended to test and adjust the pH daily to be around 5.5-6.0. In soilless substrates with organic components (coir, peat, or compost), pH does not fluctuate as rapidly, so it should be tested weekly and maintained at 5.8-6.2.

It’s also important that the fertility level of the hydroponic reservoir or growing medium is checked so that the right amount of fertilizer is added to the water. The concentration of nutrients in the reservoir and substrate changes over time as the plants absorb nutrients and evaporate water. An accurate measurement is the only way to be sure that the plants are getting the optimal level of ingredients for maximal growth. At a minimum, growers should have parts per million (ppm), electrical conductivity (EC), or total dissolved solids (TDS) meters, which give a measure of overall fertility. However, they do not measure the levels of individual nutrients. Chemical test kits are available that are more detailed; they provide a measurement of each nutrient.

All three meters measure nutrient levels indirectly, based on the amount of electricity the nutrient solution conducts. The meters measure how efficiently electrons travel between probes through the solution. Pure, distilled water conducts virtually no electricity. The more nutrients and minerals in the solution, the more electricity is conducted. Electrical conductivity is the opposite of electrical resistance (measured in ohms), so the unit is the mho or, in metric units, the siemens. Since the current in even highly concentrated solutions is tiny, meters typically read in either 1/1,000th of an mho (a milli-mho per centimeter, abbreviated as mMhos/cm), or 1/1,000th of a siemens (a milli-siemens, abbreviated as mS/cm). Tap water EC usually varies from 0.2 to 1.0 mMhos/cm (= mS/cm). A moderately strong fertilizer solution will have an EC of 1.5 to 2.5 mMhos/cm (= mS/cm). Thus, if tap water EC is 0.4 and fertilizer EC is 1.8, the resulting fertilizer solution would have an EC of 2.2 mMhos/cm (= mS/cm). Some EC meters are meant for more precise applications and use units of micromhos (1/1,000th of a mMhos); the units are µMhos/cm. Thus, 1,000 µMhos/cm = 1.0 mMhos/cm. Following the example above, the fertilizer solution EC of 2.2 mMhos/cm = 2,200 µMhos/cm.

Some meters provide readout in EC, and others read out as total dissolved solids or parts per million. Even those that read directly in TDS or ppm are measuring EC and then using a conversion to estimate the concentration of solids. A measurement of 1,000 ppm means that 1,000 units of nutrients are present for every million units of water.

The ppm/TDS readouts don’t provide any more information than EC meters; they simply measure EC and then use a conversion factor to give a ppm readout. The conversion calculation is based on what manufacturers consider a typical hydroponic solution. The most common are based on measurements of two types of solutions—either 4-4-2 (40% sodium sulfate, 40% sodium bicarbonate, 20% sodium chloride) or sodium chloride (NaCl)—but they produce different results. The conversion factor for a 4-4-2 solution is approximately 700 x EC in mS/cm. The NaCl conversion is roughly 500 x EC. This means that the same solution, producing the same EC of 3.0 mS/cm, converts to either 2,100 ppm or 1,500 ppm, depending on the scale the manufacturer has chosen. These conversion differences reflect the disparity in conductivity between different nutrients.

Just like pH meters, EC/TDS meters need to be maintained through periodic recalibration as well as careful cleaning and storage. If the probe is contaminated or dirty, it affects the accuracy of the meter’s reading. Rinse the probe in distilled water between readings and blot to dry with a lint-free wipe. Manufacturers include instructions for how to properly store the probe. EC electrode cleaning solution is also recommended for some models.

Nutrient types and calibration are not the only things that can affect an EC or TDS meter’s readings. The EC of the solution also varies based on its temperature, since the speed of electron travel is measured. It increases as the solution gets warmer. Some EC/TDS meters measure temperature and automatically compensate for it (automatic temperature compensation, ATC).

If a meter does not have automatic compensation, measurements should be taken at close to the same temperature each time. This isn’t an issue if the system is heated with an aquarium heater or other method of maintaining consistent solution temperature. If the gardening space (and nutrient solution) gets warmer after the lights have been running, nutrient readings should be taken at the same time of day or point in the light cycle.

Even a properly calibrated and maintained meter used under perfect conditions will not provide a precise reading of the nutrient concentration. Even expensive meters are only accurate in the range of approximately +/-5%.

EC/TDS meters are used to directly measure the overall fertilizer strength (total ions) in a hydroponic nutrient solution. EC/TDS changes over time as plants absorb water and nutrients.

Consider the situation when plants transpire 50% of the water in a reservoir. The solution’s salt concentration (EC/TDS) can become dangerously high. Similarly, because plants take the nutrients they need from the solution and leave the rest, unused salts can build up because there is an imbalance between what the fertilizer provides and what the plant uses.

As the EC (ex: > 2.5) or TDS (ex. > 1,250-1,750, depending on the conversion factor) levels climb, roots have an increasingly difficult time extracting the water they need from the root zone because the osmotic pressure is strong and they must use more energy to extract water from the salt solution.

Because plant fertility needs change over time, a lower EC is used during the propagation and early vegetative stages and a higher EC for late-vegetative stage and flowering.

The general strategy to use for EC/TDS is to monitor and adjust over time. If EC goes up over time, the plants are being overfertilized. If EC goes down over time, the plants are being underfertilized, providing less fertilizer than the plant needs in proportion to its water usage.

In hydroponic gardens, the nutrient solution’s EC should be checked every one to two days. This is especially important if the reservoir is small in relation to the garden. Add water to replenish the reservoir and decrease EC, and then add fertilizer if EC goes below target. In soilless substrates, the pH and EC should be checked and adjusted weekly. Because the probes cannot be placed directly into the substrate, a solution sample needs to be collected. This is usually done using either the PourThru method or the 1:2 dilution method.

The PourThru Method

This method does not require the collection of the medium, but it is less accurate. A small sample of distilled water is poured on top of the medium in the container and the leachate sample collected in a bowl. Measure using an EC meter. This method is most accurate when conducted on 5 to 10 pots using an average reading.

The 1:2 Dilution Method

Small samples of substrate are taken from 5 to 10 pots. This is mixed and combined with two parts of distilled water by volume. Because the two methods use different volumes of water, the recommended EC ranges are different depending on the method. Allow to soak for 60 minutes and measure using an EC meter.

Saturated Media Extraction (SME)

SME is more accurate than the PourThru method but is more time-consuming. The sample should be taken when the medium is moderately moist so that the root ball stays together. Samples are gently collected from the middle of the container of 5-10 pots and placed into another container. Distilled water is added to the point of saturation, with a little moisture visible on the surface. Allow to soak for 60 minutes and measure using an EC meter.

Source: “Nutrient Monitoring in Cannabis Cultivation: A Step-By-Step Guide”. Cannabis Business Times (Cockson and Veazie 2020)

Steps for the PourThru method:

Steps for the 1:2 dilution method:

Note that fertilizer solution EC is different from root zone EC. Usually root zone EC is a bit higher than fertilizer solution EC because of accumulated salts. If root zone EC is substantially higher than the fertilizer solution (or higher than recommended PourThru or 1:2 dilution EC), then leaching with clear water to flush out excess salts is indicated. This should be done by irrigating the substrate with clear water until about 30% of the water leaches out of the pot each time.

Overall, careful monitoring of the hydroponic reservoir and substrate pH/EC/TDS reaps dividends in plant health and yield. Because EC and TDS only give a total measure of fertilizer strength (and total salts), they do not indicate specific deficiencies.

Periodic use of chemical test kits provides more detailed information than meters because they test the levels of specific elements. Commercial laboratories can test nutrient solution, substrate, and leaf tissue to determine specific mineral element levels.

Visual Symptoms of Nutrient Deficiencies and Corrective Actions

Beyond monitoring the nutrient solution and root zone pH, EC, and TDS, one can detect nutrient deficiencies by carefully observing plant leaves. Healthy leaves have a vibrant green to purple color. Leaves turning yellow (chlorosis) or brown (necrosis), or exhibiting other growth abnormalities such as curling, rolling, or misshapening, are important clues to the problem. Beyond the color of the symptoms, it is also important to note their pattern.

Depending on the deficiency, yellow or brown patterns can appear across the entire leaf, on its edges (margins), or between the veins (interveinal). Finally, the place on the plant where nutrient deficiencies first appear tells a lot about which nutrient is deficient. The location of the leaf on the plant (top or bottom of the individual shoots or branches) correlates to the age of the leaf. Young, immature leaves show different deficiencies from older, more mature leaves, which is very informative for determining specific nutrient problems in the plant.

Leaf age is an important diagnostic criteria because some nutrient deficiencies show up first on young leaves. That indicates that the deficient nutrient(s) are immobile; the plant cannot move them from older lower leaves to new leaves growing at the top of the canopy.

Immobile nutrients where deficiencies are apparent first on young leaves include iron, manganese, copper, zinc, boron, and calcium. Boron and calcium are the least mobile, and deficiencies show up on the youngest parts of the plant.

Nutrients that are mobile can be translocated from older leaves to provide nutrients to the new leaves. Mobile nutrients, including nitrogen, phosphorus, potassium, and magnesium, show deficiency symptoms first on old leaves.

When plants show visual symptoms of nutrient deficiency, their growth and yield may already be impaired. Therefore, beyond managing pH/EC, it is important to carefully and frequently observe plants and correct nutrient deficiencies as quickly as possible.

If the correct nutrient is added to cure a deficiency, the plant usually responds in apparent ways within three to five days, depending on the deficiency. Usually the first indication is that the symptom stops spreading and plant parts that were only slightly damaged begin to repair themselves, with the exception of a calcium deficiency, which causes permanent damage during cell growth. Calcium is one of the nonmobile nutrients.

Leaves and other parts that were slightly discolored may return to normal, although plant parts that were severely damaged, or suffered necrosis, will not recover. The most dramatic changes can be monitored through new growth: an observer can easily differentiate between plant parts that grew before and after the deficiencies were corrected.

Some nutrient deficiencies can look similar. The best way to differentiate between them is to note the part of the plant in which they are occurring. If in doubt, the only way to truly determine the issue is a laboratory tissue analysis.

Macronutrients

Nitrogen (N)

Nitrogen deficiency is the most commonly occurring nutrient deficiency of cannabis. N is the first number of the three numbers found on all fertilizer packages, which list N-P-K always in that order.

Photo: Ed Rosenthal

Role in plant nutrition

N is directly responsible for the production of chlorophyll and amino acids, and it is essential to photosynthesis. It is an essential element of tissue; without it, growth quickly stops.

Symptoms

N can travel anywhere on the plant. Usually deficiency starts on the lower portion of the plant because N travels to new growth. Lower leaves first appear pale green. Then the leaves yellow and die as the N travels to support new growth. The deficiency symptoms travel up the plant until only the new growth is green, leaving the lowest leaves to yellow and wither. Lower leaves die from the leaf tips inward. Other symptoms include smaller leaves, slow growth, and a sparse profile. The stems and petioles turn a red-purple tinge.

Too much N causes a lush dark green growth that is more susceptible to insects and disease. The stalks become brittle and break from lack of flexibility.

General discussion

Without high amounts of N, especially during the vegetative growth stage, the plant’s yield is greatly reduced. Water uptake slows from vascular breakdown in the plants. N issues happen throughout the entire growth cycle. Plants should never experience an N deficiency during vegetative growth. However, overfertilizing with N causes problems too.

As the photoperiod changes, tapering off the use of N promotes flowering rather than vegetative growth. However, a small amount of N is always necessary in order for the plant to manufacture amino acids, which use N as an ingredient. This supports flower growth and utilization of P and K. Some “Bloom Boosters” have N-P-K ratios of “0-50-30.” While high numbers sound impressive, using this fertilizer too early causes the flowers to be smaller than they could have been. If there is not enough N available, the plants are not getting the most out of the fertilizer.

In the middle to the end of the flowering stage, plants frequently show an N deficiency. They’re using the nutrients that were stored in the leaves and dropping their oldest, bottom, fan leaves. To prevent the deficiency from getting extreme, switch over to bloom nutrients gradually unless the bloom fertilizer contains some N.

Problem solving

Any water-soluble N (especially nitrates, NO3) is quickly available to the roots. Insoluble N (such as urea) needs to be broken down by microbes in the soil before the roots can absorb it. After fertilization, N-deficient plants absorb N as soon as it is available and start to change from pale to a healthy-looking kelly green. Deficient plants usually recover within a week, but the most-affected leaves do not recover.

Any water-soluble fertilizer much higher in N than P and K can be used to solve N deficiencies very quickly. Most hydro “Vegetative Formulas” fall into this category.

Calcium nitrate (Ca(NO3)2) is water-soluble and fast acting. It can be used as a foliar fertilizer and in the water-nutrient solution. It is often one of the two ingredients in CalMag. Magnesium sulfate, known as epsom salts (MgSO4), is the other.

Urine, fish emulsion (5-1-1), and high-nitrogen bat or seabird guano also act quickly. In soils, high-N fertilizers such as alfalfa and cottonseed meals, manure, feather meal, and fishmeal all supply N fairly quickly.

Phosphorus (P)

Phosphorus deficiency occurs occasionally. P is the second number found on fertilizer packages. The numbers are always listed in the order N-P-K.

Photo: Kristen Angelo

Role in plant nutrition

P aids in root and stem growth, influences the vigor of the plant, and helps seedlings germinate. P is extremely important in the reproductive stages and flowering. Plants use higher amounts of P during flowering. Without adequate or even abundant supplies, it results in lower yields.

Symptoms

Plants deficient in phosphorus grow slowly and are stunted with small leaves. P is mobile, so older leaves are affected first. They turn dark green and become weak, then develop dull blue or purple hues. The edges of the leaves turn tan/brown and curl downward as the deficiency works its way inward. The lower leaves turn yellow and die.

The stems and petioles turn purple or red. Some cultivars, however, normally possess red or purple stems and petioles, so these traits are not a surefire sign of P deficiency.

General discussion

Deficiency during flowering results in lower yields, but overfertilizing can result in a chemical and/or metallic taste or burn the plant. Cold weather (below 50°F/10°C) makes P absorption very troublesome. For this reason, highly available P, such as found in water-soluble bloom formulas, can add flower yield in cool weather.

Problem solving

Water-soluble fertilizers containing high P fix the deficiency. Bloom fertilizers are high-P formulas. High-P guano also provides readily available P. Rock phosphate and greensand are also high in P and gradually release it. The affected leaves do not show recovery, but no additional growth is affected and new growth appears healthy.

Potassium (K)

Potassium is the third number found on fertilizer packages, always listed in the order N-P-K. K deficiency occurs occasionally in both planting media and outdoors in soil, but rarely in hydroponics.

Photo: Kristen Angelo

Role in plant nutrition

K is found in the whole plant. It is necessary for all activities having to do with water transportation, as well as all stages of growth; it’s especially important in the development of buds. K aids in creating sturdy and thick stems, disease resistance, water respiration, and photosynthesis.

Symptoms

Plants suffering from minor deficiencies look vigorous, even taller than the rest of the population, but the tips and edges of their bottom leaves die or turn tan/brown and develop necrotic spots. Mottled patches of red and yellow appear between the veins, which remain green, accompanied by red stems and petioles. These leaves eventually turn brown and die off. Deficiency results in slower growth, especially during the vegetative stage. Severe K shortages cause leaves to grow smaller than usual.

Excess K causes fan leaves to show a light to dark yellow or white color between the veins. K is mobile.

General discussion

Even in rich, well-fertilized soil, plants often suffer from mild K deficiencies, usually caused by improper fertilization. Many organic fertilizers such as guano, fish emulsion, alfalfa, cottonseed, blood meals, and many animal manures contain minor amounts of K relative to N and P.

Cold weather slows K absorption, as does too much calcium or ammonium (NH4+). High levels of sodium displace K.

Problem solving

Although symptoms of minor K deficiency affect the cosmetic look of the plant, it does not seem to affect plant growth or yields.

Water-soluble fertilizers containing high K fix the deficiency. Bloom fertilizer usually contains high K levels. Highly alkaline K is used in the formulas mostly to balance the pH of highly acidic P.

Liquefied kelp, bloom fertilizers, and wood ash are commonly used and work quickly to correct K deficiencies, as does potassium bicarbonate (KHCO3), potassium sulfate (K2SO4), and potassium dihydrogen phosphate (KH2PO4). Potassium silicate (K2SiO3) is used to supply silicon and has 3% K in it. Granite dust and greensand take more time to get to the plant and are not usually used to correct deficiencies, but to prevent them.

Damaged leaves never recover, but the plant shows recovery in four to five days with applications of fast-acting products.

Calcium (Ca)

Calcium is a secondary nutrient generally listed as part of a label’s guaranteed analysis. Ca deficiency is rare outdoors except in very acidic soils.

Photo: Mel Frank

Role in plant nutrition

Ca strengthens plant cell walls and therefore stems, stalks, and branches, and it aids in root growth—mostly the newer root hairs. It plays a role in plant response to stimuli by regulating the permeability of stomata. It travels slowly and concentrates on roots and older growth. It plays a role in pistil development. Ca also enhances the uptake of K.

Symptoms

Ca deficiency stunts plant growth and makes the leaves turn dark green. Large necrotic (dead) blotches of tan, dried tissue appear mostly on new growth but also on other plant parts along leaf edges. Young shoots crinkle and get a yellow or purple color. In severe cases they twist before they die. Necrosis appears along the lateral leaf margins. Problems migrate to the older growth, which browns and dies. Stems and branches are weak, lack flexibility, and crack easily.

The root system does not develop properly, leading to bacterial problems that cause root disease and die-off. The roots discolor to a sickly brown. Ca is semi mobile.

General discussion

The deficiency is occasionally found in planting mixes and is more common in hydroponics. However, Ca should be used when planting mixes are being constructed or when the pH of the mix is too low.

Distilled and reverse osmosis water lack dissolved Ca. This can lead to Ca deficiency unless the water is supplemented with it.

Rhino Skin is a next-generation potassium silicate additive that increases stalk rigidity, bolstering plants with an optimal foundation for supporting heavy bud weight. With availability in mind, Advanced Nutrients’ scientists carefully selected the most soluble forms of this naturally thick and solid element for cannabis plants.

Some hydro fertilizers contain only small amounts of Ca, as the amount of Ca dissolved in the supply water varies. If the water contains more than 125 parts per million dissolved solids, it is probably providing the plants with enough Ca. If the water contains less, Ca has to be added to the water to bring it up to 125 ppm.

Problem solving

Outdoors, add Ca to acidic soils to bring them into the pH range of 5.8-6.3. Dolomitic lime, or garden lime, can be added to planting mixes before potting. It provides Ca and also helps stabilize pH over time.

Both planting media and hydro systems can be fertilized as directed using a commercial calcium-magnesium (Ca-Mg or “CalMag”) formula; this provides instant availability to the plant. It can also be used in planting mixes. Growers often use Ca-Mg acetate.

Calcium nitrate Ca(NO3)2 is a water-soluble fertilizer that supplies both Ca and N. It provides a very soluble form of Ca to the roots and can also be used as a foliar spray. This formula gets Ca to the plant very quickly.

DIY CalMag

Dissolve 2.25 ounces of calcium nitrate Ca(NO3)2 and 6 ounces epsom salts (MgSO4) into one gallon of water. Use at same rate as commercial CalMag.

There are a number of brands of liquid Ca or liquid lime that are absorbed by the roots.

One teaspoon of hydrated lime per gallon of water provides relatively fast absorption. Dolomitic limestone, which contains Mg and Ca, takes longer to absorb. It is a good ingredient to amend into planting mixes to prevent deficiency.

Ground eggshells, fish bones, and seashells also break down over the season and add Ca to the soil. They can be softened and made more available by soaking in vinegar or lemon juice before use.

Gypsum, Ca sulfate (CaSO4), can be added to soils to increase Ca content without affecting the pH too much. It should not be added to soils with a pH below 5.5 because it interacts with aluminum (Al), making it soluble and poisonous to the plants.

Magnesium (Mg)

Role in plant nutrition

Magnesium helps support healthy veins and maintains leaf production and structure. It’s required for chlorophyll production and enzyme breakdowns.

Symptoms

Mg is mobile, so deficiency symptoms start in the lower leaves. The veins remain green while the rest of the leaf turns pale yellow, exhibiting chlorosis. The leaves eventually curl up and die. The edges of affected leaves feel dry and crispy.

As the deficiency continues, it moves from lower leaves to the middle to upper half. Eventually the growing shoots change from a pale green to white color. The deficiency is quite apparent in the middle leaves. At the same time, the stems and petioles turn purple. Some cultivars, however, normally possess red or purple stems and petioles, so these traits are not a surefire sign of deficiency.

General discussion

Mg deficiency is common in all constructed soilless media and hydro. It is not commonly deficient outdoors. It occurs more frequently if using distilled, reverse osmosis water or tap water that has low ppm count. Usually water with 125 ppm dissolved solids contains adequate amounts of Mg.

Problem solving

Mg deficiency is one of the easiest nutrient deficiencies to diagnose and cure.

Water-soluble nutrients containing Mg fix the deficiency. Such nutrients are Mg sulfate (MgSO4, epsom salts) and Ca-Mg for fast absorption, and dolomite lime or garden lime and worm castings for moderate absorption.

In hydro and planting mixes Mg deficiencies are easily fixed using 1 teaspoon (5.7 cc) of epsom salts per gallon (1.3cc per liter) of water in reservoirs. In planting mixes use 1 teaspoon per quart (5cc per liter) of soil in planting mixes. After the first treatment, use one-quarter dose with each watering or change of reservoir. Ca-Mg can also be used.

For fastest action, epsom salts can be used as a foliar treatment at the rate of 1 teaspoon per gallon (1.3cc per liter). Ca-Mg can be used foliarly as directed.

Dolomitic limestone contains large amounts of Mg. It can be used to raise the pH of soils and planting mixes and supply Mg at the same time.

Sulfur (S)

Sulphur deficiency is rare. Like iron (Fe), S moves slowly in the plant. Warmer temperatures make S harder for the plant to absorb. Unlike Fe, S is distributed evenly throughout the plant, mainly in the big fan leaves.

Role in plant nutrition

S is essential during vegetative growth and plays an important role in root growth, chlorophyll supply, and plant proteins.

Symptoms

The first signs of S deficiency are yellowing of young leaves. S deficiency starts at the back of the leaves and creeps toward the middle.

Leaf growth is slow; leaves become brittle and narrower than usual, and are small and mutated. Buds die off at the tops of flowering plants. Overall growth is stunted. Some S deficiencies show orange and red tints rather than yellowing. In severe cases the veins of the growing shoots turn yellow with dead areas at the base of the leaf where the blades join. The stems become hard and thin, and may be woody. They increase in length but not in diameter.

Too much S stunts the plant and leaf size, and the leaves look brown and dead at the tips. An excess of S looks like salt damage: restricted growth and dark color damage. This is rare.

Problem solving

Both organic soils and inorganic fertilizers contain high levels of available S, so plants are unlikely to suffer from a lack of the element. However, a deficiency is easily solved using epsom salts (MgSO4). The plant is watered with epsom salts until the condition improves. One teaspoon per gallon (1.3-2.6cc per liter). It can be applied both foliarly and to the irrigation water.

One of the reasons why S deficiency is unlikely is that many commercial fertilizers contain nutrients as sulfates, so adding nutrients containing S fixes the deficiency. Mix at recommended strength to avoid nutrient burn. Any water-soluble fertilizer that uses S in the trace minerals also works. Other sources are elemental garden S, potassium sulfate (K2SO4), which is often used in commercial fertilizers.

Gypsum, composed of Ca and S, is sometimes used to improve problematic soils, since it contains two secondary nutrients and helps make the soil more friable. Do not use gypsum on acidic soil (pH less than 5.5); it affects the availability and absorption of soil aluminum, which is toxic to plant roots at relatively low levels.

Micronutrients

Boron (B)

Boron deficiency is not common. B is not mobile.

Role in plant nutrition

B is important in the processes of maturation, pollen germination, and seed production. It also aids in cell division, protein formation, healthy leaf color, and plant structure formation. Proper amounts keep stems, stalks, and branches strong and help plant cells maintain rigidity. It helps calcium maintain solubility.

Symptoms

The first sign of a B deficiency is the browning or graying of the growing tips followed by their death. Soon after, the lateral shoots start to grow, but then die. Shoots appear sunburned, twisted, and bright green. The leaves develop small brown necrotic dead spots that look like strawberry seeds and are surrounded by an area of dying tissue between leaf veins. B deficiency resembles a calcium deficiency, but can be differentiated by the small size of the necrotic areas.

Stems and petioles (leaf stems) are brittle and show signs of hollowness. B deficiency only affects newer growth.

Roots become stunted, and the smaller secondary roots become short and swollen as the root tips die. The roots are vulnerable to fungal and bacterial attacks that rot the root hairs and cause discoloration.

Excess B is more common in outdoor soil gardens. It is sometimes delivered in irrigation water and then becomes part of the soil matrix. Indoors and in containers it is rare and usually caused by overuse of fertilizers. B deficiency causes the yellowing of the leaf tips, which progresses inward. The leaves drop and the plant dies.

Problem solving

Even using reverse osmosis, B is difficult to remove. Treat a B deficiency foliarly or through the irrigation water, using one teaspoon (4.9cc) of boric acid (available in drug stores) per gallon of water (1.3cc per liter). Fast-acting solutions also include borax, compost, and compost teas.

Copper (Cu)

Copper deficiencies are rare. Cu has a low mobility.

Role in plant nutrition

Cu is essential to healthy plant production and reproduction and maturity, and assists in carbohydrate metabolism and oxygen reduction.

Symptoms

Cu deficiency first appears in young leaves, which exhibit necrosis and coppery, bluish or gray with metallic sheen at the tips and margins. The young leaves turn yellow between the veins.

Other symptoms include limp leaves that turn under at the edges and eventually die, and wilting the whole plant. New growth has difficulty opening. Flowers do not mature or open in males, and in females the stigmas don’t grow properly.

Cu toxicity is rare but fatal. As the plant approaches death, its leaves turn yellow from its inability to use iron (Fe). The roots are abnormally sized and then start to decay.

General discussion

Cu deficiencies are often confused with overfertilization.

Problem solving

Foliar feeding with copper fungicides such as copper sulfate (CuSO4) and chelated copper adjusts a deficiency. Any hydroponic micronutrient formula containing Cu helps as well. Compost, greensand, and kelp concentrates are good natural sources.

Soaking dimes or quarters in water and then using the water to irrigate the plants also supplies Cu, because these coins are 92% Cu and 8% zinc. (Pennies contain mostly zinc.) An acid solution such as pH Down, K fertilizer, lemon juice, or vinegar dissolves the Cu faster.

Iron (Fe)

Iron deficiency occasionally occurs outdoors and in planting media.

Role in plant nutrition

Fe is necessary for enzymes to function and acts as a catalyst for the synthesis of chlorophyll. Young actively growing tissues need Fe to thrive.

Symptoms

Fe deficiency starts in the new leaves, which lack chlorophyll but have no necrotic spots. This causes interveinal chlorosis, or the yellowing of the leaves except for the veins, which remain green. New leaves start to experience chlorotic molting, first near the base of the leaflets so that the middle of the leaf appears to have a brown mark. The veins remain dark green. Note that a Fe deficiency looks similar to a Mg deficiency except for its location. Fe deficiency affects the new growth but not the lower leaves, while Mg deficiency affects the middle and lower leaves first. Fe moves slowly in the plant.

Problem solving

An Fe deficiency may indicate a pH imbalance. Fe precipitates with even a moderately high pH. Fe may be present but not available to the plants. If it is present, it will dissolve if the pH is lowered to the preferred range (5.8-6.2).

Foliar feed with Fe chelated fertilizer containing Fe, Zn, and Mn, since these deficiencies are often found in combination. Other Fe-bearing supplements include compost, Fe chelates (often found in hydroponic micronutrient supplements), iron oxides (Fe2O3, FeO), and iron sulfate (FeSO4) for fast absorption. Supplements should be added both foliarly and to the planting medium. Adding rusty water also works.

Manganese (Mn)

Manganese deficiency is rare and almost always associated with Fe-Zn deficiencies.

Role in plant nutrition

Mn helps enzymes break down for chlorophyll and photosynthesis production, and it aids in making nitrates available for protein production.

Symptoms

Mn deficiency is generally found in the young leaves. The leaf tissues turn yellow, and small areas of tan or brown dead tissue (necrotic areas) appear in the middle of the leaf. The leaf veins usually stay green. The leaf becomes outlined in a ring of dark green along its margins. Too much Mn in the soil causes an iron deficiency. In addition, the plant shows a lack of vigor. Mn is not mobile.

Problem solving

For fast relief, foliar feed with a water-soluble fertilizer high in Mn such as Fe-Zn-Mn fertilizer, hydro micros, or Mn chelate. Then add the fertilizers to the water-nutrient solution. Compost and greensand also contain Mn but are absorbed more slowly than the water-solubles.

Molybdenum (Mo)

Molybdenum deficiency is very rare but is more likely to occur in color-changing cultivars in cold temperature conditions. Mo is mobile.

Role in plant nutrition

Mo is contained in enzymes that help plants convert nitrates to ammonia, which is required for protein production.

Symptoms

The middle leaves turn yellow. As the deficiency progresses toward the shoots, the new leaves become distorted or twisted. A Mo deficiency causes leaves to have a pale, fringed, and scorched look, along with undersized or strange-looking leaf growth. Older chlorotic leaves experience rolled margins, stunted growth, and red tips that move inward toward the middle of the leaves.

Sometimes Mo deficiency is misdiagnosed as an N deficiency. However, N affects the bottom leaves first. Mo affects leaves in the middle of the plant first and then moves up to the newer growth.

Excessive Mo in cannabis looks like Fe or Cu deficiency.

General discussion

Generally a Mo deficiency most often occurs when S and P are deficient. Mo toxicity does not tend to wreak havoc on plants, but excess Mo causes severe problems in humans, so extra precautions should be taken when using it. Follow directions carefully.

Problem solving

Foliar spraying with water-soluble fertilizers aids in overcoming the deficiency. Because plants need Mo in such small amounts, a hydroponic micronutrient mix is often the most efficient way to supply it. These fertilizers can be used as foliar sprays or applied to the soil, as well as their customary use in hydroponic nutrient solutions.

Silicon (Si)

Silicon deficiency is very rare. Si is not mobile.

Role in plant nutrition

Si has not been proved necessary for plant growth. However, the presence of Si promotes the development of strong leaves, stems, and roots. It also increases resistance to fungal and bacterial diseases and insect infestation. When available to the plant, it is added to the structure of cell walls, strengthening them and making them more resistant to a number of environmental stresses, including drought, pests, and disease. It strengthens stem and branch structure, and as a result it promotes growth and development. The plant also exhibits an increase in photosynthetic activity and overall yield increases.

General discussion

Si is abundant in nature, but it is not included in hydroponic fertilizers, so it should be used as a supplement.

Problem solving

Diatomaceous earth can be added to the soil or planting mix. The Si is dissolved by acids in the medium into a soluble form that the roots absorb.

Liquid Si is found in Si supplements. It is immediately available to the plants.

Zinc (Zn)

Zinc deficiency occurs occasionally. With low levels of Zn in the plants, the yields are dramatically reduced. Zn is not mobile, so symptoms occur mainly in the newer growth.

Photo: Mel Frank

Role in Plant Nutrition

Zn aids in plant size and maturity, as well as in the production of leaves, stalks, stems, and branches. Zn is an essential component in many enzymes and in the growth hormone auxin. Low auxin levels cause stunted leaves and shoots. Zn is also important in the formation and activity of chlorophyll. Plants with high levels of Zn can tolerate longer droughts.

Symptoms

New growth has radically twisted leaf blades. Zn deficiencies are identifiable by spotting, chlorosis, and yellowing between the veins of older leaves. Interveinal yellowing is often accompanied by overall paleness. During the flowering stage, buds may contort, twist, and turn hard. When the deficiency first appears, the spotting can resemble that of an Fe or Mn deficiency, but it affects the new growth. Zn excess is very rare, but produces wilting and even death in extreme cases.

Problem solving

Use an Fe-Zn-Mn micro mix to solve the deficiency. Zinc sulfate (ZnSO4), chelated zinc, or zinc oxide (ZnO) also adjust the deficiency.