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Nutrient Matters is a recurring series focusing on plant nutrition by university researchers. Marijuana pH and water For marijuana growers, whether using soil, soilless or hydro methods, pH levels can make a huge difference to your success and yields. In this article we will take a closer Best Practices for Monitoring pH for Cannabis 05 Mar Best Practices for Monitoring pH for Cannabis Best Practices for Monitoring pH for Cannabis Editor’s note: Balancing pH is a

New Research Results: Optimal pH for Cannabis

Nutrient Matters is a recurring series focusing on plant nutrition by university researchers.

The recommended substrate pH for cannabis (Cannabis sativa) varies widely. A recommended substrate pH of 6.0 to 6.5 often is cited as the standard. However, we have observed pH ranges between 4.0 and 5.0 or greater than 7.0 during our visits to commercial greenhouses growing cannabis.

Our observations from those greenhouses indicated that plants grown at acidic (lower) pH appear to have normal growth and lack the symptomology of lower leaf blackening or bronzing typically seen with other greenhouse species at lower pH levels. In crops grown at higher pH, only in rare instances did we observe interveinal chlorosis of the upper (youngest) leaves (a common symptom of greenhouse crops grown at high pH). These observations suggest that cannabis is not prone to iron deficiencies when the substrate pH nears 7.0.

Returning to our labs at North Carolina State University (NCSU), we began our work to understand the underlying factors affecting pH in substrates and how those fluctuations impact the Cannabis sativa plant in order to determine the optimal pH range for cannabis.

This is what we discovered.

pH on the Range

Substrate pH is important to plant nutrition because it directly impacts plant nutrient availability.

Substrate pH levels below 5.0 result in increased micronutrient availability that can lead to iron (Fe) toxicity or manganese (Mn) toxicity, or both. Symptoms of Fe toxicity and Mn toxicity may appear as black speckling on older leaves (observed with zinnias or gerbera) or as bronzing (observed with marigolds or seed geraniums). While some crops display low pH symptoms on lower leaves, others may only display low pH through stunted growth, as is the case with poinsettias.

In our initial experiments at NCSU, we did not observe symptomology of lower leaf blackening or bronzing on Cannabis sativa plants, only stunted growth. These preliminary results suggested that cannabis can regulate micronutrient uptake under low substrate pH conditions similar to a poinsettia.

When the substrate pH becomes too high, many species develop interveinal chlorosis (yellowing) on the youngest leaves. This is a common situation that occurs with many greenhouse-grown species (such as petunias) when the elevated pH makes micronutrients, such as Fe, unavailable to the plant, even if adequate levels are provided in the fertilizer.

We did not observe symptomology of interveinal chlorosis in cannabis plants grown at pH 7.0 in our initial experiments at NCSU, either. Iron uptake can also be hindered by over-irrigation, cold substrate temperatures or root disease such as Pythium. In other experiments at NCSU, we did observe the development of interveinal chlorosis on plants with a pH of 7.8. This observation provides more refinement of the upper pH limit and would suggest that the pH should be lower than 7.5.

The preliminary results of NCSU’s research imply that vegetative stock plants of cannabis have a wide substrate pH range in which the plants will optimally grow. That range appears to be as wide as pH 5.0 to 7.0. Based on experience with other species, a narrower range of 5.5 to 6.5 may be more appropriate to target, as this will allow growers to more easily adjust as pH approaches the ends of the targeted range.

For growers, utilizing these pH values in a monitoring system such as the PourThru method implies that the safe pH zone to target would be a narrower 5.8 to 6.2. If pH drops below 5.8 or above 6.2, corrective procedures should begin to adjust the pH back into the 5.8 to 6.2 range.

(Note: These values are for plants grown in greenhouse conditions with a soilless substrate (growing medium). Optimal ranges for field-based growing conditions typically would move up the lower and the upper range by 0.5 pH units to 6.2 to 6.7—as field soils latch on to more nutrients than a soilless substrate such as peat or coir, and risk of nutrient lock-up is lessened.)

How to Correct pH

Again, when the pH drifts into unwanted territory, adjustments must be made. Below are standard corrective procedures used to modify substrate pH for plants grown in soilless substrates in greenhouses and adapted for cannabis.

1. Low Substrate pH Correction When Fe toxicity and Mn toxicity become a problem, raise substrate pH to the recommended pH range. Corrective procedures to raise low pH levels are listed on p. 52 (Table 1). Switching to an alkaline fertilizer when substrate pH is nearing the lower limit will help stabilize the pH.

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Flowable Lime

To make small adjustments of roughly 0.5 pH units, mix 1 to 2 quarts of flowable lime per 100 gallons of water. If using an injector, avoid using higher concentrations of flowable lime as it will damage it. If higher rates are needed, then split your applications to avoid damaging your injector. To avoid leaf burn, it is best to rinse the foliage after treatment if any flowable lime comes in contact with the leaves.

Hydrated Lime

For more stubborn low-pH problems, use a hydrated lime mixture. Mix 1 pound of hydrated lime in 3 to 5 gallons of warm water. Mix it twice and let it settle after each mixing. Once the sediment collects in the bottom of the container, pour the liquid through your injector system set at a 1:15 ratio. This product is corrosive, so rinse the foliage as soon as possible to avoid leaf burn, and avoid skin contact.

Potassium Bicarbonate (KHCO3)

Potassium bicarbonate should be handled with care. It can throw off your substrate chemistry as it provides 993 ppm of potassium (K) in mixtures of 2 pounds per 100 gallons of water. Rinse the foliage immediately after application via injectors and leach heavily the following day with a complete fertilizer to reduce substrate electrical conductivity (EC) and restore nutrient balance. The 2-pounds rate will increase the substrate pH by roughly 0.8 pH units. Rates greater than 2 pounds per 100 gallons of water can cause phytotoxicity.

As always, remember to recheck your substrate pH to determine if reapplications are needed.

2. High Substrate pH Correction The target pH for cannabis is between 5.8 and 6.2. Higher pH values may result in Fe deficiency and create interveinal chlorosis on upper leaves. Check the substrate pH to determine if it is too high. Be careful when lowering the substrate pH, as low pH can be more problematic and difficult to manage.

Cannabis plant growth is less vigorous at low substrate pH levels.

Acid-Based Fertilizer

If the substrate pH is beginning to increase, consider switching to an acidic-based fertilizer. These ammoniacal nitrogen (N)-based fertilizers are naturally acidic, and plant nitrogen uptake will help moderate the substrate pH over a week or two.

Acid Water Drench

Some growers use this intermediate correction when pH levels are not excessively high and a quick lowering of substrate pH is desired. Sulfuric acid is recommended to acidify irrigation water to a pH 4.0 to 4.5. Apply this acid water as a substrate drench, providing 5 percent to 10 percent excessive leaching of the substrate. Rinse the foliage to avoid phytotoxicity. Results should be visible within five days. Retest the substrate pH and repeat if needed.

Iron Drench

If the levels are excessively high, then a Fe chelate application can be made to the substrate.

  • Iron-EDDHA can be mixed at a ratio of 5 ounces per 100 gallons of water.
  • Iron-DTPA can be mixed at a ratio of 5 ounces per 100 gallons of water.
  • Iron sulfate can be mixed at a ratio of 4 ounces to 8 ounces per 100 gallons of water.

Apply the iron as a substrate drench with enough volume to leach the pot, and rinse the foliage immediately after application.

Conclusions

Based on grower observations and initial experiments at NCSU, it is possible to grow cannabis with a wider soilless substrate pH range than most other species. Cannabis plants do not appear prone to develop leaf symptomology when substrate pH is too low or too high compared to the current general greenhouse standards—only plant stunting occurs at sub-optimal conditions.

Therefore, based on research and experience with other species, a wider range of 5.5 to 6.5 may be used. When adapting these values to a monitoring system, the recommended pH zone to target would be 5.8 to 6.2. By monitoring the substrate pH over time, one can assure that plants are within the optimal range.

Brian Whipker, Turner Smith and Paul Cockson are from Department of Horticultural Science, North Carolina State University, Raleigh, N.C. Hunter Landis is from North Carolina Dept. of Agriculture and Consumer Services, Raleigh, N.C.

Marijuana pH and water

For marijuana growers, whether using soil, soilless or hydro methods, pH levels can make a huge difference to your success and yields. In this article we will take a closer look at pH values and discuss why you need to know about them and how that knowledge can help you become a better grower.

What is pH?

So, let’s start at the beginning. pH is the measure of the acidity or alkalinity of a given substance. Technically it is about the concentration of hydrogen ions. A pH of 7.0 is totally neutral, 1.0 to 6.9 is acidic and 7.1 to 14.0 is alkaline. When using a pH scale it is important to know that the decimal points really count. The pH scale is a logarithmic scale which means that for every one point of pH, the concentration changes by a factor of ten. For example, an increase in pH from 7.0 to 8.0 is actually a tenfold increase, so be aware!

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Why is pH Important in Growing marijuana?

  • The optimum pH for marijuana in soil is around 6.3 – 6.8
  • The optimum pH for marijuana in soilless or hydro is around 5.5 – 6.1

pH in Soil

When growing marijuana in soil you are less likely to run into pH problems, especially if you are using especially mixed soils that feed the plant throughout its life, without having to add any liquid nutrients. It is said that the soil acts as a buffer, this means that it helps slow the change of pH values as opposed to hydro systems where changes in pH take effect much more quickly.

  • The same thing happens when you grow marijuana in soil in containers. When you add water the pH of the soil changes and the whole range of nutrients become available to your plant as the soil slowly dries out again;
  • This means that some nutrients that are otherwise dormant in the soil become available when it is wet.
  • If you are using tapwater, make sure you let it sit in a bucket or reservoir for a few days to allow it to dechlorinate. Then check the pH to make sure it is within your required range.

pH in Hydro

If you grow marijuana in a hydroponics system then pH management is a much more important issue. With no soil to act as a buffer changes in your pH values take effect much more quickly. With hydro grows allowing your pH to fluctuate within a prescribed range is important as it allows all of the nutrients in the solution to become available to the plant in turn. Luckily, this happens naturally as the pH of the solution in your hydroponic reservoir will drift over time. When making up the nutrient solution for your hydro set up, always mix the separate components in the water. Never mix them together directly. This can cause them to chemically react with each other and may change their desired properties. Mix your nutrients gently. Overly vigorous mixing adds oxygen to the solution and this will temporarily raise pH levels. Some growers like to shake their solution vigorously to add oxygen which is good for the roots. If you like to do this, do it after you have checked and adjusted the pH. Make your mix in a clean reservoir and check the pH. Let the mix stand for an hour and check the pH again. The pH of nutrient solutions often changes quite quicklywithin the first hour so you should recheck and adjust as necessary.

Making adjustments to pH

If you are topping up your solution, or adjusting the pH in your reservoir, try not to subject the plants to drastic changes in pH. Large swings in pH will stress them. Make changes slowly. Do not mix nutrients or pH regulators directly into the plants’ reservoir. Make a mix in a separate container first then add that to your reservoir so that changes take place slowly. Remember that pH drift is not only normal, it’s desirable. Allow the pH in your reservoir to change gradually, but make sure you keep within the range of 5.5 to 6.5.

Checking pH

Although many soil growers don’t bother, a pH tester is a crucial piece of kit for the serious marijuana grower. There are a couple of options that are available to you. Digital Meter – This is by far the easiest, most popular and most accurate method for checking your pH levels. Digital pH meters are easy to work, just insert the probe and read the pH levels off of the digital read out. pH Strips – Cheaper to buy initially, but more expensive in the long run, and more hassle. pH strips turn a specific color depending on the pH. You then compare the color to an index and that gives you the pH value. If you are trying to measure the pH of your soil you will need to make up a soil solution in water. You should measure pH periodically as part of your plant maintenance program. With experience you will need to measure less often as you get your set up dialled in. Special care should be taken to measure pH when you seriously change the nutrient regime you are following, when flipping to 12/12 for example.

Adjusting pH

So, having checked the pH levels of your nutrient solution you find that it is out. How do you adjust it? The best answer is to buy proprietary pH Up and pH Down solutions. There are lots of forum posts by people who add vinegar or baking powder to adjust pH. Whilst there is some convincing evidence of this working, we recommend using proprietary solutions for reliable results.

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pH Up is a strong alkali formula for raising pH. The one from General Hydroponics is made from a base of Potassium Hydroxide and Potassium Carbonate.

pH Down is an acid based formula for lowering pH. General Hydroponics up/down is made from a base of Phosphoric Acid.

As explained above, adjust the pH of your solution a little at a time. Try to use only either Up or Down. If you overshoot with one and then have to readjust with the other you can end up unnecessarily stressing your plants. Mix up a little of the required pH adjuster in a separate jug. Then add them a little at a time to your reservoir. Allow time for the whole reservoir to even out and settle. Better to get it right with 3 slight adjustments than have it wildly swinging up and down.

Best Practices for Monitoring pH for Cannabis

05 Mar Best Practices for Monitoring pH for Cannabis

Best Practices for Monitoring pH for Cannabis

Editor’s note: Balancing pH is a critical component to ensuring nutrient solubility and uptake for cannabis. As part of the upcoming release of the Fluence Cannabis Cultivation guide (available later this month), we are releasing tips on how to measure and monitor pH to ensure your fertigation strategy is not a limiting factor to your use of high-intensity LED lighting.

The pH scale — which ranges from zero to 14.0 — provides insight into how chemical compounds will interact with one another based on their ionic state. It is good to remember, pH reflects the concentration of hydrogen ions in a solution. More intuitively however, pH from a practical sense can be understood in terms of acids (vinegar, ammoniacal nitrogen) and alkaline bases (baking soda, potassium bicarbonate).

Caption: Nutrient availability increases or decreases in response to pH. A pH level of 5.8 – 6.2 is appropriate for cannabis.

On the pH scale, values less than 7.0 indicate acidity; values greater than 7.0 indicate alkalinity. Deionized water has a neutral pH of 7.0. The pH scale is a logarithmic function, so even small changes in pH values are significant. For example, a pH level of 4.0 is ten times as acidic as a pH level of 5.0. For plants, pH is important because it affects the form of the nutrients in the substrate.

For example, when pH is low, the solubility of some micronutrients like iron and manganese increases, making them more available to plants. This can cause toxicity. However, when pH increases, micronutrients, along with phosphorus, become less soluble and less available to plants.

Regardless of the concentrations of your nutrient solution, unbalanced pH levels can create an antagonistic environment for nutrients and will make them unavailable to your plants. The figure above shows how nutrient availability relates to pH.

Cannabis, like many plants, prefers slightly acidic soil conditions. It tolerates a wide pH range (5.0-7.0) without symptoms of bronzing or interveinal chlorosis (yellowing of top leaves), but pH levels outside of the optimal range of 5.8 – 6.2 will limit growth. To maintain optimal pH levels, cultivators should test pH levels every two days, and adjust the pH as needed.

Testing pH with a Digital Meter

Testing pH is easy. Combination pH/EC meters are relatively inexpensive and require little training to use. They offer a permanent solution to disposable pH test strips and dye kits, which are cumbersome and must be subjectively interpreted by the color of the reactive test material.

  1. Calibrate the meter: Before testing, meters require calibration against a known standard. In this case, calibrating against purified water has a neutral pH value of 7.0. Make sure the water you are using is deionized. Submerge the probe of the meter into the water and adjust the display to read pH 7.0.
  2. Test the solution: Submerge the probe into a container of the fertilizer solution or directly into the tank. Read the digital display.
  3. Test the growing media: The electrochemical environment of the root zone can be different than that of the fertilizer solution. Salt buildup can cause nutrient concentrations at the roots, which causes pH levels to be different from that of the fertilizer. To test the pH at the roots, stay tuned to the Fluence blog, as we will be releasing a best practices guide to testing for electrical conductivity (EC) in the next few weeks.

We hope these tips have been a helpful reminder of how to leverage pH tests in your grow.

For more tips and best practices, keep an eye on the Fluence blog, as well as stay up to date via Instagram, LinkedIn, Facebook and Twitter to gain access to the Fluence Cannabis Cultivation Guide, which will be available later this month.

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