What is pH, and Why Does it Matter to Plants?
The pH of a substance measures its relative acidity or alkalinity. Soil pH is one of the chief concerns of gardeners and growers when it comes to the health of their plants.
Acidic substances are naturally called acids, but alkaline substances are called bases. Gardeners sometimes refer to acidic soils as “sour” and basic soils as “sweet.”
The scale for pH measurements ranges from 0 to 14, with lower values corresponding to acidity and higher values to alkalinity. A pH of 7 is considered a neutral measurement, so it is neither a base nor an acid. Pure water should typically be neutral. Vinegar is an example of a typical household acid. Conversely, baking soda is an alkaline powder familiar to most people.
You may be wondering, “But what does it mean to be alkaline or acidic? What does pH actually measure?” The amount of hydrogen ions in matter is what pH indicates directly. An excess of hydrogen ions drives pH lower, making substances more acidic. Meanwhile, the relative absence of these ions is what defines bases.
Why pH Matters to Plants
This measurement is gravely important in the realm of raising plants, mainly because the pH of the soil affects how well plants can pass nutrients between the substrate and their root systems. If the pH is too extreme or outside of a plant’s ideal range, it will struggle to move nutrients across its roots and can thus become nutrient deficient.
Besides its direct effects on the root function of plants, pH indirectly affects a plant’s health by influencing other elements of the soil that contribute to its survival. In return, changes in those individual soil factors can alter pH. For example, pH values can change with:
- Nutrient availability/ leaching
- Soil bacteria
- Presence of toxins
- Soil structure
Imbalances in soil pH and microorganism diversity affect beneficial macro species as well. Worms, for instance, are capable of providing numerous services to plants. They aerate the soil, improve water filtration, and concentrate nutrients in their castings, sometimes making chemical compounds more available to root systems. If you grow your plants in soil where worms can reach, extreme pH levels may drive these helpful creatures and others like them away.
You will want to monitor and manage pH around the roots of whatever you happen to be growing, whether it is a cannabis plant or something in your garden. This way, you will be better able to ensure that your plants can tolerate their environment, grow optimally, and deliver their best yield.
What Are the Symptoms of High pH in Vegetables/Fruits?
In an alkaline environment (in the 7-14 pH range), some nutrients can become less soluble and thus less available to plants. This situation can result in too high a concentration of certain nutrients (nutrient toxicity) and too low a concentration of others (nutrient deficiency). Whether it is nutrient overabundance or scarcity ailing a crop, physical signs can appear that advertise the plant’s distress.
Here are some recognizable high pH symptoms and the conditions associated with them.
Stunted Growth:
If your plant’s growth is mysteriously stunted, a phosphorous deficiency could be the culprit.
Corrupted New Growth Points/ Leaves:
Calcium deficiencies tend to affect young leaves and new growth most conspicuously. This growth can appear twisted, wilted, or stunted. An iron deficiency similarly impacts young shoots, causing them to die off, starting from the tip. Copper deficiencies also heavily impact new leaves near the tops of plants and can cause stunting or wilting.
Dark Green Leaves with Bronze, Purple, or Red Tinges:
This specific symptom is often a telltale sign of a phosphorous deficiency.
Leaf Brown Spots and Necrosis:
Copper deficiencies may be responsible for spots of necrosis forming on leaves. Brown spots and necrosis are symptoms of a phosphorous deficiency as well.
Mature Leaf Die Off:
The death and dropping off of mature leaves may be caused either by a phosphorous deficiency or copper deficiency.
Interveinal Chlorosis:
Interveinal chlorosis is when the spaces between veins on leaves turn yellow, while the veins themselves remain green or dark. Plants exhibiting interveinal chlorosis could be suffering from an iron deficiency or copper deficiency.
Yellowing or Browning of Leaves:
At a high pH, the nutrient molybdenum can accumulate to the point of becoming poisonous. A general yellowing or browning of leaves can signal molybdenum toxicity.
Dropping Flowers & Diseased Fruit:
Calcium deficiencies may cause fruits to look underdeveloped and diseased. The issue may reveal itself through “blossom-end rot” in plants like tomatoes and peppers. Blossom-end rot is when dark spots and rot develop at the bottom of fruits. If the plant is still flowering, this deficiency will cause blossoms to drop off prematurely. Flower impairment can occur in cases of copper deficiencies.
What Are the Symptoms of High pH in Cannabis?
Symptoms of high pH in cannabis can vary greatly because they will span a variety of nutrient deficiency symptoms due to nutrient lockout. The healthy, vibrant, green color will often be replaced by yellowing leaves. Vigorous plant growth stops, and your plant’s health will struggle until the correct pH for cannabis is achieved. Things to watch out for:
Iron Deficiency:
Yellowing leaves and yellow between leaf veins.
Nitrogen Deficiency:
Wilting leaves and yellowing leaves.
Phosphorus Deficiency:
Darkening of leaves. Pink or purple on leaves.
Copper Deficiency:
Yellow leaf tips and brown patches on leaves.
Magnesium Deficiency:
Yellow leaves. Yellow between leaf veins.
Calcium Deficiency:
Bronze or brown patches. Spots.
What Can These Symptoms Be Confused With?
High pH in cannabis and other plants can generate signs of ailment that may appear to be caused by unrelated conditions. Take a look at some of the other possible causes of the above symptoms, which may be confused with excess alkalinity.
Overwatering Plants
The fallout from overwatering plants can mimic high pH issues. Either may result in stunted growth or brown and wilting leaves.
Aphids
The presence of aphids can hinder growth in many plant species. Their eggs appear like brown or yellow spots on leaves, which the untrained eye might mistake as evidence of malnutrition.
Leafhoppers
Leafhoppers will create spots or markings on plant parts that may resemble spots from conditions caused by alkalinity.
Mites
The damage mites inflict on plants may cause them to grow aberrantly. Russet mites can stunt plant growth, while broad mites induce twisted leaves. Those who do not know any better could erroneously blame either issue on a high pH.
Tobacco Mosaic Virus
The Tobacco Mosaic virus generates unnatural patterns of green and yellow in leaves. Gardeners and growers could misidentify the problem to be interveinal chlorosis arising from pH problems.
How long does it take for cannabis plants to recover from high pH?
How long it takes for cannabis plants to recover from high pH is a difficult question to answer with precision. High pH cannabis symptoms will depend on the severity of the pH issue is and how long it has been present. Your plant could recover from symptoms within a few days if you properly flush the soil with plenty of water. Or, in the worst circumstances, your plant could be permanently stunted or die if left to sit in a very high pH environment for too long.
What Are the Symptoms of Low pH in Vegetables/Fruits?
A marginally low pH is best for many fruits and vegetables to grow in because it allows the most nutrients to become available for root uptake. Very acidic substrates, however, can quickly cause harm. Excessively low pH causes some nutrients to build up to toxic levels while making others less accessible (just like when the pH is too high).
In other words, low pH symptoms in garden vegetables and fruits arise from nutrient deficiencies and toxicities. There are some conditions and symptoms that low and high pH both cause, but there are also several differences. Below is a list describing possible symptoms of excessive acidity and which nutrient problems might result.
Stunted Growth
Both magnesium deficiencies and manganese toxicity caused by low pH can stunt a plant if left unaddressed for too long.
Interveinal Chlorosis
An overly acidic substrate can lead to a magnesium deficiency, manganese toxicity, or iron toxicity. Interveinal chlorosis is a possible symptom of all three nutrient imbalances.
Necrosis
If a plant has a magnesium deficiency, parts of the plant may eventually suffer from necrosis.
Yellow or Brown Spots
In cases of manganese toxicity, yellowish-brown spots can appear in the interveinal area and spread. Lower leaf blades and sheaths may also exhibit brown spots on the veins. If the problem is iron toxicity, tiny brown spots will start at the tip of the leaf and spread.
Dried Leaf Tips
Leaf tips drying up can be a symptom of manganese toxicity or iron toxicity. If it is the latter, the leaves may turn orange-brown or orange-yellow as they become dry. In severe cases of iron toxicity, they can even take on a purple-brown color.
Reduced Tillering
Manganese toxicity and iron toxicity caused by a highly acidic pH can each be responsible for reduced tillering in plants.
Damaged or Dead Roots
One additional symptom that might point to iron toxicity is dying, dead, or damaged roots.
What Are the Symptoms of Low pH in Cannabis?
If pH is too low for your cannabis plants and they are left in an acidic environment, you will find many of the symptoms above. When pH is too low, certain micronutrients become more mobile and are absorbed in excess of what the plant needs, resulting in a potential for several toxicities. It is important to monitor your pH and, upon first symptoms, diagnose the issue immediately.
What Can Low pH Symptoms Be Confused With?
Fusarium Wilt
Fusarium wilt could be confused with nutrient imbalances caused by low pH. It similarly causes wilting, yellow leaves, and growth stunting.
Septoria Leaf Spot
Septoria leaf spot inflicts circular brown spots on the affected plant, which someone could mistake for those caused by manganese or iron toxicity.
Dehydrated Plants
Watering plants too little can lead to a range of poor health signals, such as wilting stems and leaves. Planters or gardeners could misattribute the overall degeneration of the plant to a nutrient problem related to acidity.
Cannabis Root Rot
Cannabis root rot can create the appearance of dying roots, as they become slimy and emit a foul odor. Since iron toxicity is also associated with unhealthy root systems, it is easy to confuse the two conditions.
Cannabis Bud Rot
Both iron toxicity and cannabis bud rot may cause leaves to turn purple and die. However, mold on the buds of cannabis plants is a symptom of bud rot only.
What is the Best pH for Cannabis?
Similar to garden vegetables, cannabis plants also thrive in slightly acidic soils. That said, cannabis is somewhat more tolerant (can do well in a broader pH range) than most fruit and vegetable plants. Researchers have recorded pH values as low as 4 or 5 and above 7 in commercial greenhouses.
Soil
In soil, the ideal range to aim for is between 6.0 and 7.0 pH. In the wild, cannabis grows most often in soils between 6.2 and 6.9. Because soil is not a purely liquid environment, the pH should be a little more resistant to change, so you should not have to micromanage it as much as you would with other substrates. In all cases, your goal should be to keep within a range, not constantly adjust for a single value.
Hydro/ Soilless/ Coco
For cannabis grown using either hydroponics or soilless mediums like coco coir, the best pH range is between 5.5 and 6.5. You will notice this is a little more acidic than that recommended for growing in soil. You can consult pH charts online for each kind of substrate to visualize which nutrients are best absorbed in what ranges and where they overlap.
When all the nutrients come from liquids, as with hydroponics and soilless growing, you will need to keep a closer eye on pH and manage it a bit more carefully. That way, you can be sure to avert any issues arising from going outside the target range. However, it is still important to allow pH to fluctuate moderately within those parameters. That gives your plants the chance to absorb as many vital nutrients as possible. Recall that various nutrients are most available for uptake at slightly differing ranges.
What is the Best pH for Greenhouse and Garden Soils?
Soil pH in the United States naturally ranges between 5.5 and 8.5, with 5.5 being very acidic and 8.5 being very basic. Most fruits and vegetables prefer a narrower range, hovering close to neutral or slightly acidic. Most gardeners recommend keeping the soil in your pots or beds within a range of 6.0 to 7.0, and many consider 6.5 to be ideal.
There are a few notable outliers – for example, potatoes and berries fair best in more acidic soils. On the other hand, cabbages and asparagus are happiest at approximately a neutral pH. If you are not sure what your specific plants need in terms of pH, you can find charts online which list popular fruits, vegetables, and garden species alongside their optimal pH ranges.
How to Test pH
There are a couple of different tools you can use to test current pH levels in your growing medium. For soil, you will probably want to employ a digital pH meter. For hydroponics and situations where you will only test the water, you may use a digital meter, a pH pen, or even low-tech drops and strips if desired. Our personal favorite is the Blue Lab Combo Meter.
Soil
The first method of assessing soil pH involves merely testing the runoff water after watering the soil. To test the soil directly and not just the runoff water, you may need to follow additional steps to prepare it for the digital meter. Collect a sample of your garden soil that is about 4 or 5 inches deep. Then, change it to the consistency of mud by adding water and mixing. Make sure the water does not come straight from the tap. Use rainwater, or let tap water sit for 24 hours before mixing.
Once the soil is like mud, stick in a meter to attain a reading. Get three pH readings total, then take an average of all of them. When testing multiple soil samples, clean the meter between readings, turn it off, and turn it back on before repeating the steps.
Hydro/ Soilless/ Coco
Fortunately, the process for testing pH in hydroponic and soilless systems is relatively straightforward. You only need to test the water in the reservoir for hydroponic set-ups. For soilless mediums, test any water you add to the substrate. A pH pen works well to check liquids, but you can use a digital meter instead if you choose.
How to Correct Bad pH
In general, the steps to correcting an improper pH include assessing the current pH, making adjustments, and continuing with monitoring and maintenance going forward. If you plan to add any nutrients or supplements to your water, do so before you begin testing or correcting. Ensure that the solution is well-mixed by lightly shaking or stirring the water. Test this water using a meter, pen, strips, or drops.
Hydroponics
To get the root pH for a hydroponics system, test the water in the reservoir. Like soilless set-ups, the range you want to aim for is 5.5-6.5. If the pH is too high, you can add a minuscule amount of a product called “pH down” to the water. Retest the water after a few minutes. If the original pH is too low, you can add a little “pH Up” solution to the water. Again, check the pH a second time once a few minutes have passed.
Repeat these steps if necessary. However, a small amount of either solution should be potent enough to effect significant change on the first try. You may also use pH Up and pH Down to adjust the water added for the other two substrate types, soilless and soil.
Soilless
For soilless mediums, test the runoff at your next watering. If the pH is between 5.5 and 6.5, you do not need to make any alterations. When the runoff comes out to be less than 5.5, the next watering should be at pH 6.5. If it tests at more than 6.5, the water you add next should have a pH of 5.5. Repeat until the pH comes back into range.
Soil
To correct an inappropriate pH in soil, begin by assessing your current root pH. To do so, test the water intended for your plants before their next watering. Then, check the runoff draining past the bottom of the roots. The pH of the water you are adding should always be in the 6 to 7 range. When runoff water also proves to be in the correct range, you do not yet need to change anything.
If you find that the runoff tests at less than 6, then for your next watering, add water with a pH of 7. Unsurprisingly, you do the reverse in the opposite situation. When the runoff pH is more than 7, add water that has a pH of 6 at your next watering. Repeat until you have gradually brought the pH back inside the recommended parameters.
Alternative Substances Used to Adjust pH in Cannabis
While pH Up and pH Down are often sufficient for making pH adjustments in small amounts of water, other standard methods exist. Growers and gardeners might rely on acidic or alkaline fertilizers to do the work instead, especially in the context of greenhouses where large reservoirs of pH-adjusted water are applied to many plants simultaneously. The following are documented strategies for adjusting pH that commercial growers have applied in cannabis greenhouses using soilless substrates.
Cannabis High pH Correction: Acid-Based Fertilizer
- Transitioning to an acid-based fertilizer can be advantageous if cannabis pH is too high
- Ammoniacal nitrogen-based fertilizers are a naturally acidic option
- Nitrogen uptake of plants will help moderate pH in the following two weeks after making the change
- Take care when lowering pH, as acidity problems can be more challenging to cope with than if the cannabis runoff pH is too high
Cannabis High pH Correction: Acid Water Drench
- Medium-level correction for situations where cannabis pH is too high
- Drops substrate pH rapidly
- To bring irrigation water down to 4.0 or 4.5 pH, sulfuric acid is an often-recommended choice
- Apply sulfuric acid as a substrate drench
- Provide 5 to 10 percent leaching of the substrate
- Rinse the foliage (prevent phytotoxicity)
- You should see results in five days or fewer
- Retest and, if cannabis runoff is too high, repeat steps
Cannabis High pH Correction: Iron Drench
- Iron chelate application
- For addressing excessively high pH
- Apply as a substrate drench
- Apply enough to induce leaching
- Rinse foliage immediately after
- Iron sulfate mixture option: 4 to 8 ounces per 100 gallons of water
- Iron-DTPA mixture option: 5 ounces per 100 gallons of water
- Iron-EDDHA mixture option: 5 ounces per 100 gallons of water
Cannabis Low pH Correction: Flowable Lime
- For minor adjustments of approximately 0.5 pH units when runoff pH is low in cannabis
- Use 2 quarts mixed per 100 gallons of water
- Split and space out applications if more is needed (avoid injector damage)
- Rinse foliage afterwards
Cannabis Low pH Correction: Hydrated Lime
- For more severe pH changes when runoff pH is low in cannabis
- Mixture: 1 pound in 3 to 5 gallons of warm water
- Mix twice
- Let settle after each mixing
- Once settled, pour solution through injector (at 1:15 ratio)
- Rinse foliage immediately
- Avoid contact with skin (product is caustic)
Cannabis Low pH Correction: Potassium Bicarbonate
- Provides 993 ppm of potassium to correct low pH problems with cannabis
- Mixture: 2 pounds per 100 gallons of water
- Rates exceeding 2 pounds per 100 gallons can cause phytotoxicity in plants
- Increases substrate pH by approximately 0.8 pH units
- Rinse foliage immediately
- Leach considerably the next day (to reduce electrical conductivity and rebalance nutrients)
- Handle carefully due to potency
Why it is Best to Monitor and Manage Your pH Regularly
Your takeaway from all this information should be that plant nutrition and pH are inextricably linked. Nutrients change forms according to pH, and some are more bioavailable to plants than others. Thus, even if there is an abundance of a particular nutrient around a plant’s roots, the plant could still become deficient. If the pH is wrong, it could lose the ability to absorb said nutrient.
For gardeners and cannabis growers using liquid nutrients, it is especially crucial to keep tabs on pH, as it can fluctuate quickly. Remaining constantly in the correct range provides plants access to the most nutrients possible over time. The benefits of this careful management include a reduced likelihood that crops will suffer from deficiencies or become overwhelmed by any one chemical element. Also, you will be able to identify potentially harmful developments as they occur and avoid tragedy. Ultimately, all species monitored and cared for this way will flourish, grow fuller, and yield more prodigiously.
