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Cannabis Nutrient Testing: How to Grow Cannabis Like a Pro, Even When You're Not

Updated: 2 days ago


Table of contents


  1. Cannabis Nutrients Testing: On-Site Monitoring of Key Parameters for Real-Time Understanding of Plant Requirements

  2. pH, EC, TDS and their connection to nutrients availability

  3. Individual Nutrient Monitoring: A Key to Accurate Nutrition

  4. Nutrient Testing: Lab Foliar Analysis vs. On-Site Field Sap Analysis

  5. Fertilization strategy approach for optimal plant growth

  6. How to test for nutrients in hydroponic solution and soilless substrates

  7. How to sample cannabis before fresh tissue analysis / plant sap analysis

  8. Results and Interpretation for Soilless Substrates, Nutritive & Hydroponic Solution Analysis

  9. Nitrogen recommendation for cannabis plants

  10. Potassium recommendation for cannabis plants

  11. Phosphorus recommendation for cannabis plants

  12. Calcium recommendation for cannabis plants

  13. Reference table for nutrients level (Cannabis grown in Hydroponic or Nutritive Solution)

  14. Results and Interpretation for cannabis plant sap (plant fresh tissue)

  15. Tools for monitoring nutrient levels and self-testing of cannabis crops



1. Cannabis nutrients testing: on-site monitoring of key parameters for real-time understanding of plant requirements


Cannabis and plant nutrition monitoring encompass the analysis of the growing medium (whether soil or soilless substrates) and the hydroponic solutions.


This analysis involves assessing key chemical characteristics such as pH, Electrical Conductivity (EC) levels, and nutrient concentrations (NPK).


The primary purpose of these tests is to evaluate the availability of essential elements, ensuring they align with the theoretical plant requirements for optimal growth.


Beyond soil and growing medium analysis, comprehensive plant nutrition monitoring extends to the testing of nutrient levels in the plant fresh tissue (sap). This reflects the amount of nutrients that have been absorbed by the plant.


As the requirements for each nutrient vary throughout the growing cycle, monitoring their fluctuations becomes essential. This approach helps to understand the balance between key nutrients and the actual plant nutritional status at different growing stages to ensure optimal nutrition.


It is easy to implement these tests yourself by using various field nutrient testers, so in this article we will delve into the critical aspects of plant nutrition management for cannabis growers.


2. pH, EC, TDS and their connection to nutrients availability

 

The pH and conductivity of soil, soilless substrates or hydroponic solutions are the most frequently tested parameters by commercial and recreational growers.


 pH stands out as the most critical parameter, influencing nutrient availability, solubility, and the activity of microorganisms responsible for breaking down organic matter.


The Electrical Conductivity (EC) or Total Dissolved Solids (TDS) concentration is often associated with nutrient levels.



However, it is essential to note that conductivity is affected by ANY type of salt in the sample, making it unable to differentiate between beneficial salts (nutrients) and undesirable salts (e.g., NaCl), negatively impacting soil fertility.


Additionally, the TDS and Conductivity readings won't inform about the concentration of each key nutrient, which is essential for advanced plant nutrition.

 

3. Individual Nutrient Monitoring: A Key to Accurate Nutrition


Cannabis plants require three macronutrients in large quantities - Nitrogen (N), Phosphorus (P) and Potassium (K).


These three elements are the main components of commercial fertilizers with varying NPK ratios, depending on the growing stage.


The vegetative growth stage has a higher demand for Nitrogen (N), while the Phosphorous (P) and Potassium (K) play a critical role during the flowering stage.


These keys nutrients can be measured easily and regularly in the hydroponic solution as well as in the plant sap to ensure the plant is accumulating these minerals properly. This helps to understand the ratio between elements and allows for diagnosing possible nutrient-specific deficiencies.


4. Nutrient Testing: Lab Foliar Analysis vs. On-Site Field Sap Analysis


Some growers may be familiar with foliar (dry leaf tissue) analysis to assess nutrients concentration in the plants. This laboratory-based method requires sending fresh whole leave samples to a lab, where they are dried out, grinded in powder and then analyzed. This lab-based analysis provides a comprehensive report, including micronutrients levels.


However, it falls short in delivering real-time insights since optimal crop nutrition can be time sensitive.


Nutrient deficiencies, imbalances, and excesses may take weeks to manifest visible symptoms on the plant.

By the time these issues become apparent, it's often too late to send leaf samples for analysis, since growers may need to take critical actions for immediate fertilization adjustments.


Laboratories can also take days to deliver their results, and by then, the crop conditions may have significantly changed.


As a solution, growers can quickly and easily perform real-time tests on their own to assess the nutritional status of their plants.

This involves exctracting plant sap by squeezing the leaves and then analyzing it using field meters.


The fresh plant tissue (sap) is a good matrix to measure mobile ions absorbed by the plant, which represents the amount of nutrients available for the plant growth.


Squeeze the sap and the meters will give you result in ppm

5. Fertilization Strategy Approach For Optimal Plant Growth

A proactive approach involving continuous monitoring of nutrient uptake is essential for high-value crops like cannabis.

Combining nutritive solution and plant sap analysis provides growers with valuable real-time information about plant nutritional status that will help them to adjust their feeding programs and ensure optimal plant growth.

The analysis of the nutritive / hydroponic solution informs the grower about the quantity of nutrients being fed to the plant, while sap analysis reflects how each nutrient has been absorbed.


Additionally leachate tests reveal the amount of nutrients that remain unconsumed.



6. How to test for nutrients in hydroponic solution and soilless substrates

 

It is relatively straight forward to test a hydroponic solution sample. For most parameters, HORIBA LAQUAtwin meters can be immersed in the solution and deliver instant results in seconds.


Ensure to calibrate each meter correctly before any measurement (LAQUAtwin Calibration video).


LAQUAtwin meters use flat glass and Ion Selective Electrode method: these electrodes can measure pH, Electrical Conductivity (EC), Nitrogen/Nitrate (N/NO3), Potassium (K), Calcium (Ca) and Sodium (Na).


To measure Phosphorous (P) on-site, we suggest using a field meter from Hanna Instruments Phosphorus Checker HI-706.

This is a photometer (measures color change of the sample) that uses reagents and needs sample dilution for each measurement.


For cannabis grown in soil, read this blog post that covers recommended soil solution preparation method of 'soil slurry'.


7. How to sample cannabis before fresh tissue analysis / plant sap analysis


Tip 1: Sampling consistency is key for testing quality

 

Samples must be collected at the same time each day, ideally when the plant is active in the morning. The optimal window is before 11 AM, when ambient temperature ranges between 58-82°F (14°C & 28°C) and the relative humidity is 60-85%.

 

Tip 2: Leaf quality and representation matters

 

  • Collect 50 grams of leaf material from various representative plants picked randomly across the lot to analyze.


  • Only collect the Most Recent Mature Leaves (MRML) making sure to keep the leaflets and petiole intact.


  • Do not collect leaves from plants showing signs of nutrients deficiencies, disease or presenting effect of insect infection/attack.

  • If leaf samples are not analyzed immediately on the field, keep the leaves in a zip bag and leave them in a fresh environment (cooler box or fridge).

 

Tip 3: Why petioles aren't the perfect fit


While for majority of greenhouse crops it is commonly accepted to do the measurement in petioles, in case of cannabis the petiole is often too thin.

Growers would have to collect too many leaves to be able to analyze the samples.

Since this could be an issue for smaller producers, the best option is to use the complete leaf material to perform this analysis in cannabis plants.

 

Plant sap collection procedure


  1. Cut the leaves in small pieces and squeeze them using a garlic press: the liquid extract is the fresh leaf sap which will be used for analysis.

  2. Aim to gather 2 ml of plant sap, if you want to test for all 6 parameters

  3. If you struggle to obtain this quantity, the sap can either be diluted or transferred from one LAQUAtwin sensor to another after you finish measuring each parameter.

 

Note: in case of Phosphorous tester, the instrument measurement range (0 to 15 ppm) is not wide enough to measure Phosphorus concentration in fresh sap, therefore we recommend diluting the sample 50 times to bring down the concentration within the instruments working range.


To do so, use 0.2 ml of plant sap sample and 9.8 ml of deionized water. Once the meter delivers the reading, this value would need to be multiplied by the dilution factor (50 times – 1/50) to represent the real concentration of Phosphorous in the sap before the dilution.





Dilute your sap to measure phosphorus

(do not forget to correct your result according to the dilution factor)


NB: the Phosphorus checker has a measurement range (0 to 15 ppm) which is not wide enough to measure phosphorus concentration in fresh sap. Therefore, we recommend diluting the sample 50 times to bring down the concentration within the instrument's working range.


To achieve this, we suggest using 0.2 ml of plant sap sample and 9.8 ml of deionized water. Once the meter delivers the reading, this value needs to be multiplied by the dilution factor (x 50) to determine the actual concentration of P in the sap before dilution.





8. Results and Interpretation for Soilless Substrates, Nutritive & Hydroponic Solution Analysis


pH


Cannabis plants thrive best in a slightly acidic growing medium.


The optimum pH for Cannabis in soil is around 6.3 – 6.8, while in soilless or hydroponic systems, the optimum pH is around 5.5 – 6.1.


Electrical Conductivity (EC)


Here are the recommended EC levels for cannabis split between different growing mediums (hydroponic or soil/substrates).



The optimal range of conductivity increases together with the plant’s growth.


EC Values for Cultivating in Hydroponic


Vegetative growth:


  • During the first weeks, use water with an EC of 0.5 to 0.8 mS/cm then increase weekly up to 1.1 to 1.3 mS/cm depending on the strain.


Blooming:


  • During the first few flowering weeks: 1.3 to 1.4 mS/cm

  • During the fattening stage, increase to 2.0 - 2.2 mS/cm

  • Over the last few weeks, lower level from 0.0 to 0.4 mS/cm.


For sativa strains you may want to reduce the maximum amounts by 0.2 - 0.3 mS/cm.



EC values for soil and soilless substrate


Vegetative growth:

  • First weeks 0.8 to 0.9 mS/cm, then increase up to 1.1 to 1.2 mS/cm

Blooming:

  • Start with a range from 1.2 to 1.3 mS/cm then increasing weekly until 1.9 - 2.0 mS/cm

  • Over the last few weeks, lower level from 0.0 to 0.4 mS/cm.




9. Nitrogen recommendation for cannabis plants


Nitrogen is mobile in the plant in the form of nitrate (NO3-). Cannabis plants require nitrogen more than any other nutrient. It is the main element in fertilization and is vital during the vegetative state to ensure plant growth and foliage production.


Nitrate (NO3) also plays a fundamental role in energy metabolism, protein synthesis, photosynthesis activity, and chlorophyll formation in plants.


The ideal range for nitrogen in a hydroponic solution is around 160 mg/l (ppm) of nitrogen (N), which is equivalent to 700 mg/l of Nitrate (NO3-).


  • Deficiency of nitrogen results in reduced growth with apparent leaf chlorosis and lower yield.

  • Excess nitrogen leads to smaller plants and smaller organs (roots and leaves).


Source: Frontiers Plant Science, 2020


10. Potassium recommendation for cannabis plants


Potassium is a key element for plants as it plays a major role in multiple physiological and metabolic processes.

It represents one of the most important nutritional requirements for the cannabis industry, especially during blooming and fruit development phases.

Potassium is also referred to as "the quality element" because it affects yield quality, such as color, taste, and aroma.


Source: Agronomy Journal, 2022, A. Saloner & N. Bernstein


Studies have shown that for cannabis grown hydroponically, the optimal potassium concentration during the vegetative stage is between 175 and 240 mg/l of K+, depending on the cultivar.

As plant progresses to the flowering stage, the potassium concentration ranges between 100 and 400 mg/l of K+, with no visible effect of deficiencies.


11. Phosphorus recommendation for cannabis plants


Phosphorus (P) is an indispensable element for energy storage, playing a vital role in the vegetative state by contributing to root growth, enhancing stalk and stem strength, promoting flower formation, and facilitating metabolic activities in cannabis.


Within plants, phosphorus is a mobile element engaged in numerous metabolic key functions. This mobility allows phosphorus to be translocated from older leaves to satisfy the demands of newer leaves growth.


  • Phosphorus deficiency symptoms appear first in older leaves and will lead to decrease of the reproductive development, delays flowering initiation and impacts the yield

  • Excess phosphorus can reduce the uptake of magnesium and micronutrients.


Research has shown that symptoms of phosphorus deficiency will develop quickly in cannabis plants, especially during the mid-vegetative portion of the growth cycle and during the flowering stage.


For phosphorous in hydroponically grown cannabis the recommended range is from 15 to 30 ppm.

For optimization of biomass production, a minimum concentration of 10 ppm should be used for plants grown for flower buds.

The results also showed that a fertilization rate of 30 ppm of phosphorus resulted in a significantly larger bud fresh weight compared to a lower concentration.


Supply of higher phosphorus concentrations (60 and 90 ppm) did not show any additional improvement.


Source: Cannabis Business Time / Sivan Shiponi, Nirit Bernstein 2021


12. Calcium recommendation for cannabis plants


Calcium is a secondary nutrient that acts as a fundamental structural element for cell wall development, helps reduce soil salinity, and improves water penetration when used as a soil amendment.

The cannabis plant needs to develop a robust structure to support its mass during the vegetative and flowering stages. Calcium is required throughout most of the growing cycle.


Calcium deficiency is a common issue for cannabis growers, and the most common symptom observed is the appearance of brown or bronze spots and yellowing on the leaves.

In most cases, the deficiency is a resulf of an unsuitable pH value at the root level.


The upper tolerance of calcium for plants is not known, and most fertigation programs recommend a concentration between 150 and 300 ppm, extractable from the growth media or in solution.


It's important to note that calcium will interact with phosphate in solution, so it should not be applied in feeds containing phosphorus.


13. Reference table for nutrients level (Cannabis grown in Hydroponic or Nutritive Solution)


14. Results and Interpretation for cannabis plant sap (plant fresh tissue)


pH


The pH of the plant sap is paramount, with the optimal range typically around 6.4 for most crops. A pH value below 6.4 may expose the plant to fungal diseases due to deficiencies in calcium, potassium, and magnesium.


Conversely, if the plant sap pH is higher than 6.4, there may be a reduction in the availability of essential mineral nutrients like nitrate, phosphorus and sulphur. Higher pH levels can make the plant more susceptible to pest attacks.


Conductivity values for plant sap


When measuring the conductivity in plant sap, values usually fall within the range of 10 - 15 mS/cm.


Nutrient reference ranges in cannabis plant sap



15. Tools for monitoring nutrient levels and self-testing of cannabis crops


Nutrient management and monitoring in plant sap play a crucial role in cannabis cultivation.


Cannabis plants have specific nutrient requirements throughout different growth stages, and maintaining proper nutrient levels is essential for optimal plant health and productivity. While traditional methods such as soil or hydroponic solution analysis provide valuable information, they may not reflect the real-time nutrient status of the plants.


If you would like to measure pH, EC, Nitrogen Nitrate, Phosphorus, Potassium, Calcium and Sodium of your plants in real-time, you can look at the option of getting your own version of Agronomist kit.



Each kit comes with generous amount of calibration solutions, reagents for intensive testing and a comprehensive grower's manual with recommendation on sampling techniques, field testing and nutrient reference ranges for cannabis and other crops.





Conclusion


By analyzing plant sap, growers can obtain immediate insights into nutrient uptake, deficiencies, imbalances, and excesses. This enables them to make timely adjustments to the feeding program, ensuring that the plants receive the right nutrients at the right time. Continuous monitoring of plant sap allows growers to proactively address nutritional issues and maximize crop yields. It serves as a valuable tool for precision nutrient management, contributing to the overall success of cannabis cultivation in terms of both quality and quantity.

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