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How many times have we heard that a balanced diet is essential to our health? Plant nutrition isn’t much different. Cannabis plants can thrive or struggle depending on how you design your fertilization programs. Plants require 17 essential elements for growth. They obtain hydrogen (H), oxygen (O), and carbon (C) from irrigation water or as a gas via the atmosphere. Growers must provide the remaining 14 elements to help cannabis plants thrive.

These include the macroelements:

  • nitrogen (N)
  • phosphorus (P)
  • potassium (K)
  • calcium (Ca)
  • magnesium (Mg) and
  • sulfur (S).

And they also include the microelements:

  • boron (B)
  • chlorine (Cl)
  • copper (Cu)
  • iron (Fe)
  • manganese (Mn)
  • molybdenum (Mo)
  • nickel (Ni) and
  • zinc (Zn).

The quantity of macroelements and microelements required varies by type of plant, but they are all required by the plant for growth.

Cobalt (Co), selenium (Se), silicon (Si) and sodium (Na) are four beneficial elements that promote plant growth but are not considered necessary to complete the plant life cycle. With cannabis production, many growers are providing Si to promote overall plant health.

Providing a complete fertilizer package that contains all the essential elements is important to maximize plant growth. When designing a fertility program, it is also important to keep in mind some key principles that ensure there is a nutritional balance. Just like with human health, where we must balance our intake of sugar, protein, grains, fruits and vegetables, too much of one item can lead to imbalances in the plant due to antagonistic effects. For example, if too much P is provided, it can hinder the uptake of other elements and lead to Fe, Mn and Zn deficiencies.

Key Considerations

Nitrogen Form. Nitrogen in fertilizers primarily comes from three sources. Nitrate-nitrogen is the main form the commercial floriculture greenhouse industry uses for its plants. While the cost is higher, it’s the preferred form of N because it promotes compact plant growth. In the cannabis industry, the market currently lacks plant growth regulators registered for use on cannabis, so you can opt for nitrate as a source for N to help avoid excessive overgrowth, also known as plant stretch.

The other two forms of N contained in most fertilizers are ammoniacal nitrogen and urea. Many organic fertilizers have N in these forms. In commercial floriculture greenhouse production, these two forms of nitrogen are strategically deployed but are only used to a limited extent. Many growers will use ammoniacal nitrogen and urea-based fertilizers during the first two weeks after plants have been transplanted into the final container to help establish the plants and encourage a flush of new growth. After that point, most growers rely upon a nitrate-nitrogen based fertilizer.

The key takeaway here is to supply most of your N from nitrate-nitrogen. Do an evaluation of your fertilizer type by reading the fertilizer label to ensure the N form you are providing is on target.

Moderating P Applications. Phosphorous fertilization strategies have been studied extensively over the past five years. One of our graduate students, Josh Henry, worked on optimizing P fertilization rates for his master’s degree thesis. In essence, plants require a baseline level of P to grow adequately (Fig. 1). For a continuous fertilization program for plants grown in a soilless substrate, the target concentration is between 8 ppm and 15 ppm of P. Providing levels below that will result in less plant growth, while concentrations above that level provide little benefit while costing more money. Phosphorus is also the primary contributor to plant stretch. Too much P will lead to excessive internode elongation and tall plants. That’s why it’s important to limit excessive P applications.

Figure 1: Understanding P Rates. The response of Alternanthera (pictured) to increasing concentrations of phosphorus (P) from 0, 5, 10, 20 or 40 ppm (left to right). Phosphorus concentration can be used to control excessive plant growth, but adequate levels must be provided to avoid deficiency situations.

Plant nutrition experts are currently debating whether the P rate needs to be amplified just before flowering in cannabis to improve quality. Research led by Manitoba crop nutrition specialist John Heard on a dual-purpose hemp crop grown for seed and fiber suggests that extra P is not required by cannabis (Fig. 2).

Figure 2: The uptake of phosphorus (P) by a dual-purpose hemp crop grown for seed and fiber. The illustration is modified from work conducted by Heard et al. in Manitoba, Canada. The gray box area represents the start of long nights and corresponding reproductive growth, which is assumed to occur with >9.5-hour night lengths in Winnipeg, Manitoba, starting around Aug. 15. Accumulation of P into the seed and during seed maturity would not apply to greenhouse-grown cannabis plants; therefore, higher levels of P should not be required. This brings into question the need to increase P applications during late flowering of greenhouse-grown cannabis crops. *Phosphorus pentoxide “Nutrient uptake and partitioning in industrial hemp.” J. Heard et al., Manitoba, Canada

The researchers found that hemp plants front-end load P during the first half of the growing season as seen with the plant’s upward accumulation of P. At midseason, the total accumulation in the plant plateaus. This indicates that the plant uptakes limited additional P. Hemp relies upon those internal P reserves and translocates (moves) P if it is required in other parts of the plant. This suggests that adequate P levels should be provided to cannabis during the first half of the production cycle for the plant to accumulate an adequate reserve that can be translocated if needed later. Providing a P boost late in the growing season appears to not be needed. There is a need to conduct a scientifically based trial to clear up this uncertainty with greenhouse-grown cannabis.

While the target level of P required by greenhouse-grown cannabis is not currently known, we would speculate based on the scientific data from other species that levels of 15 ppm to 20 ppm P supplied on a constant basis should be all that is required. At North Carolina State University we have begun an experiment looking into optimal P rates supplied at a constant level throughout the cannabis crop cycle and will be able to further refine those recommendations in the near future.

Ratio of K to Ca to Mg. Providing the proper balance of K to Ca to Mg is important for greenhouse production of cannabis. Too much of one element does not in itself result in toxicity symptoms. Instead, excessive levels of one element has an antagonism against the others. For instance, excessive K will result in either a Ca deficiency or an Mg deficiency being observed in a plant. Many instances of Mg deficiency observed in cannabis may be due to excessive K being supplied and not due to the lack of available Mg to the plant. Figure 3 illustrates the trend that is observed in the leaf tissue concentration of a plant when K is excessive.

Figure 3: The relationship of how increasing the concentration of K can have an antagonistic effect on both Ca and Mg uptake by a plant. A balanced fertilization approach of providing K to Ca to Mg in a 4:2:1 ratio to optimize uptake of all three nutrients is required. Illustration adapted from research conducted on poinsettias.

For cannabis, K, Ca and Mg all appear to be needed in larger quantities compared to other greenhouse floriculture species. In general, the rule is provide K, Ca and Mg in a 4:2:1 ratio to avoid antagonisms. For commercial poinsettia production, we recommend a similar ratio around 200 ppm K to 100 ppm Ca to 50 ppm Mg; this would be a good starting point for cannabis (which, like poinsettias, is a short-day plant) until scientifically based research can determine optimal rates. Also, keep in mind that excessive sodium (Na), which can come from your fertilizer source or irrigation water, can also interfere in K, Ca and Mg uptake.

Dialing in the Micros. Managing microelements can be a challenge in most fertilizer programs. The concentration difference from deficient to adequate to excessive rates is very narrow. Until you become comfortable mixing your own micronutrient fertilizer salts, it’s safer to rely upon premixed micronutrient packages or the micros provided in commercial fertilizer blends.

All in Balance. Providing all the essential elements is the key to optimizing plant growth. Equally important is balancing the proportion of elements provided to cannabis so plants remain healthy.

Brian Whipker, Paul Cockson, James Turner Smith & Hunter Landis are from the Department of Horticultural Science, North Carolina State University, Raleigh, N.C.