When cultivating cannabis hydroponically, growers can choose between drain-to-waste and recirculating systems. In drain-to-waste systems, the nutrient solution is discarded after use; in recirculating systems, the nutrient solution is reused.
When using a recirculating system, it is important to collect and test the runoff—the excess nutrient solution that passes through the root zone—because it gives an idea of whether it is suitable for reuse or adjustments need to be made. Neglecting to monitor runoff can lead to salt buildup, resulting in nutrient toxicity. Repeatedly reusing a nutrient solution without testing and adjusting the runoff can also cause nutrient deficiencies.
This blog post addresses when to collect runoff samples, what to test for and how to adjust your nutrient solution based on the analysis.
Collecting runoff
Our grow consultants recommend collecting samples of runoff water from individual plants from the front and back of each irrigation zone within a system. Collect the runoff about a minute after it flows to yield a sample representative of the entire root zone, including any accumulated salts or nutrient deposits.
Measurements to monitor
Once a sample has been collected, growers should measure the following parameters and apply any necessary corrective measures:
pH
Maintaining a stable pH is critical for optimal nutrient absorption. If runoff pH is higher than the nutrient solution, it may indicate that the growing medium is releasing basic cations, leading to alkalinity. This may indicate that the plants are underfed or inefficiently absorbing nutrients. Conversely, if runoff pH is lower, plants may be overfed. A low pH can signal acidification, possibly from nitrate uptake or acidic residues in the root zone. A high EC usually accompanies this drop, indicating nutrient buildup in the growing medium.[1]
Root pathogens like Pythium and Phytophthora can also affect runoff pH by disrupting nutrient and water uptake. Damaged roots may struggle to absorb nutrients, leading to pH fluctuations, while decomposing roots can release acidic compounds.[2]
The ideal pH is 5.8–6.3. If the pH is too low, then flush the growing medium with a nutrient solution that has a balanced pH until the runoff returns to acceptable levels. If the pH is too high, then increase the nutrient feed concentration or adjust the irrigation frequency.
Buffer products like Emerald Harvest’s pH Up and Down also help bring pH into balance.
Electroconductivity (EC)
EC indicates nutrient accumulation or depletion in the growing medium. The EC should be tested at multiple points throughout the hydroponic system to see if plants are absorbing the right balance of nutrients. Consider measuring it at the dripper to check that the nutrient solution is still delivering the optimal concentration. Testing the root zone is also important for determining how plant absorption is affecting EC.
When checking the EC of runoff, it’s normal for it to be slightly higher than the EC of the nutrient solution, as this shows that plants are absorbing nutrients as expected. However, if the runoff EC is significantly higher—more than 20%—it may indicate salt accumulation, overfertilization, insufficient irrigation or poor drainage.[3]
If runoff EC is too high:
- Increase irrigation frequency to reduce the time between irrigations, limiting evaporation that causes salts to concentrate in the growing medium.
- Increase the nutrient solution volume by 25–30% to saturate the growing medium and flush out excess salts. Once the EC normalizes, reduce the volume of the nutrient solution, as needed.
- Reduce the nutrient concentration For example, if the EC of the nutrient solution is 2.0 and the EC of the runoff is 2.5, lower the concentration to 1.5–1.9 until the runoff stabilizes at 2.0.[4]
While it is rare, the EC of the runoff can be lower than the EC of the nutrient solution. This may happen if plants are absorbing nutrients too quickly or if the growing medium is retaining nutrients. To address this, reduce irrigation frequency and slightly increase the nutrient solution’s concentration.[5]
Water quality
Testing irrigation water quality before using it in hydroponics is essential, but monitoring water quality should also continue throughout the growing cycle to prevent the accumulation of harmful pathogens or materials.
Runoff provides a valuable opportunity to test for the following:
- Total Dissolved Solids (TDS): Runoff with high EC should also be tested for TDS, which measures the total dissolved materials in water, such as sulfates, bicarbonates, sodium, carbonates, calcium and magnesium. High TDS can introduce unwanted salts and minerals, disrupting nutrient absorption and potentially causing nutrient burn.
- Pathogens: Pathogens like Pythium and Phytophthora—which thrive in moist, nutrient-rich environments—can alter the runoff pH. Test the runoff for these pathogens to ensure they are not developing, particularly if the runoff pH is out of balance.
- Biofilms: Testing for organic matter or microorganisms can indicate if biofilms are forming in the system. In addition to acting as reservoirs for the above-mentioned pathogens, biofilms can deoxygenize water and clog irrigation components.
If runoff tests high for any of these issues, the irrigation system may require cleaning or water treatment, in addition to adjustments to irrigation practices or the nutrient solution.
Test runoff regularly
Regularly analyzing runoff is critical for high-value crops like cannabis, where precise nutrient management ensures maximal yield and optimal quality. By consistently collecting and testing the runoff pH and EC, and the water quality, growers can ensure plants receive the right nutrient balance they need and prevent issues that could disrupt development.
Emerald Harvest Team
