Maybe you are at the beginning of your growing journey, or maybe you already have some growing experience behind your back, but either way, you’ve probably heard about the importance of the pH value of plant substrates. The optimal pH value and soil pH amendment are burning subjects of numerous threads on the world wide web. There are various opinions and different solutions to pH problems, and this is not a surprise. Solid plant substrates like soil are complex systems that include many variables. In order to untangle this yarn and understand how pH works, we need to start from the basics.
What is the pH value?
pH stands for “potential of hydrogen”, and its value is a measure that describes acidity (sourness) or basicity (sweetness) of an aqueous solution, i.e., the concentration of H+ ions in the aqueous solution. The pH scale goes from 0 to 14, with the value 7 being neutral. Values below pH 7 are considered acidic, while the ones above pH 7 are considered basic. Sounds pretty simple right? Well, there are some intricacies. For starters, the pH scale is logarithmic. This means that each value is ten times more basic than the previous one, or more acidic, depending on the side of the scale you are starting from.
To describe this practically – pH 2 is ten times more acidic than pH 3, and a hundred times more acidic than pH 4; pH 7 is a thousand times more basic than pH 4; pH 4 is ten thousand times more acidic than pH 8, and so on. This is one of the reasons why it is very important to keep the pH level of the soil solution in favorable ranges, as seemingly small deviations can actually make a huge difference.
Fun fact: The pH value can actually have values below 0 and above 14, as the logarithmic pH scale is open-ended. Some highly concentrated acids (e.g. hydrochloric acid) can have a pH value near -1, while concentrated sodium hydroxide can reach a pH value of 15.
Why is pH important?
Plants are not able to run around and scavenge for food, and a vast majority of them cannot catch prey, so they get most of the necessary food from the soil, and make the rest themselves. However, for the plants to absorb the nutrients from the soil, these nutrients need to be bioavailable. This means that the necessary macro and micronutrients have to be in a certain chemical form to be recognized and absorbed by plants.
The pH value of the soil plays a vital role in nutrient availability and the decomposition of organic matter. An excess or a lack of H+ ions in the soil solution chemically alters the environment, making specific key elements unavailable to plants. So even though the soil may be abundant with micro and macronutrients, the plants cannot use them if the pH value isn’t within a certain range. This happens because hydrogen ions (H+) take up space on negatively charged surfaces of soil particles, and thus displace nutrient ions. How exactly will the pH value affect nutrient availability, depends on the size of soil particles and qualities of the specific nutrient ion (its size and charge).
How does pH value affect plant nutrients?
When dissolved in water, ions of metal nutrients like copper (Cu), zinc (Zn), and manganese (Mn) are quite small and have a considerable charge (2+ or 3+). For this reason, they can stick to soil particles very tightly. If the soil pH is high (basic), they will bind so tightly that they’ll become unavailable to plants. As we reduce the soil’s pH, fewer metal ions will remain attached to soil particles because hydrogen ions will outcompete them. As a result, more metal ions become available for plants to absorb. However, this also makes these nutrients more susceptible to leaching.
Elemental sulfur (S), as well as calcium (Ca2+), magnesium (Mg2+), potassium (K+), and sodium (Na+) ions, are relatively large and don’t stick well to soil particles. So even when the soil pH is high, these molecules can detach from the particles and get released in the soil solution. However, calcium is an exception here, as it tends to bind with other nutrients (e.g. phosphorus) at alkaline pH – its availability starts to drop above pH 7.5. Anyway, if the soil pH is too low, hydrogen ions will outcompete these elements and cause deficiency. Similar to metal ions, low pH also makes these elements more susceptible to leaching.
The bioavailable form of nitrogen (N) includes ammonium (NH4+) and nitrate ions (NO3-). The pH value of the soil solution affects the availability of this element indirectly – by affecting microbial activity. The activity of microorganisms that take part in nitrogen cycling in the soil is significantly reduced at pH levels that are too low or too high.
Phosphorus (P) is absorbed by plants in the form of a phosphate ion (PO3-). When the soil is too acidic, phosphate ions react and bind with other elements, like iron and aluminum, and thus become unavailable to plants. On the other hand, if the pH of the soil is alkaline, phosphates will become unavailable due to fixation by calcium. However, at very high pH levels (>9), phosphorus becomes available again, but this is of little relevance to cultivated plants, as other nutrients are not available.
Iron (Fe) is found in two forms: ferrous (Fe2+) and ferric (Fe3+) iron. Plants can only use ferrous iron, as it is the bioavailable form. Iron doesn’t have a problem with low pH – the more acidic the soil, the more available it is to plants. However, if the pH is too high, ferric iron will be the dominant form in the soil, and the plants can suffer from a deficiency.
Boron (B) is available at pH levels similar to copper and zinc. While its availability drops with the increase of pH, it becomes available again at a pH > 9, just like phosphorus.
The availability of molybdenum (Mo) increases with the increase of pH, and this element is most available at pH > 8.5. Luckily, the plants don’t need a significant amount of this element in order to thrive, so the neutral pH provides enough available ions.
Aluminum (Al) becomes more soluble as the pH of the soil solution decreases. At pH 5 or lower, this element can reach toxic concentrations in the soil solution and inhibit root growth and water uptake. It can also reduce the bioavailability of phosphorus, causing deficiency of this nutrient in plants.
What is the optimal pH for nutrient availability?
The pH range in which the previously mentioned nutrients are available to plants depends on the specific nutrient ion and environmental conditions, but all of them are usually available at pH 6,5-7. This is one of the reasons why this is considered the most favorable soil pH range for plant growth. Of course, there are differences between specific plant species, but almost all commonly cultivated plants thrive in slightly acidic to neutral soils. Another reason why plants prefer pH 6.5-7 is because many species of beneficial soil microorganisms are most active in this range.
Remember: If the pH of the soil solution drops too low (<5), it will become toxic to plants.
The most common classes of soil pH:
- 3.5 – 4.4 – extremely acidic;
- 4.5 – 5.0 – very strongly acidic;
- 5.1 – 5.5 – strongly acidic;
- 5.6 – 6.0 – moderately acidic;
- 6.1 – 6.5 – slightly acidic;
- 6.6 – 7.3 – neutral;
- 7.4 – 7.8 – slightly alkaline;
- 7.9 – 8.4 – moderately alkaline;
- 8.5 – 9.0 – strongly alkaline.
Acidic (Sour) Soil
Generally speaking, any soil that has a pH value below 6.6 is considered acidic. There are three main factors that contribute to soil acidity:
1. Amount of organic matter – Soils with a significant amount of organic matter tend to be more acidic. This is because the end products of the microbial degradation of organic matter are fatty acids and CO2. The presence of these organic acids increases the pH value of the soil solution.
2. Irrigation – Intense irrigation can cause leaching, and thus carry away the nutrients that don’t bind well to soil particles – calcium, potassium, magnesium, and sodium. All of these elements contribute to soil’s alkalinity, so removing them from the solution will cause the pH to drop. Decreased soil acidity caused by irrigation is highly unlikely to happen in indoor plant cultivation, considering that the plants are grown in containers.
3. Use of fertilizers – Fertilizers that are based on ammonia (e.g., ammonium nitrate, urea, and their combination – UAN) increase soil acidity. Overfertilizing with nitrogen leads to soil acidification, which has become one of the biggest problems in modern agriculture.
Some plant species happily grow in moderately and strongly acidic soils, but most cultivated crops are better adapted to grow in soils that have a pH value above 6. If the soil pH has a value lower than 6, it creates two problems for the plant – most nutrients become unavailable and fungal diseases are more likely to occur. Fungi thrive in acidic conditions, so low soil pH increases their activity and makes it easier for pathogenic species to infect plants.
A soil that has a pH between 6.6 and 7.3 is considered neutral. Many plants grow best in this pH range, as the microbial activity is very high and all the necessary nutrients are bioavailable.
Alkaline (Sweet) Soil
Soils with pH values above 7.3 are considered alkaline. Most naturally alkaline soils are alkaline because of high calcium carbonate (CaCO3) and sodium carbonate (Na2CO3) content. Compared to the previous two types, alkaline soils are the least favorable for plant growth. When the pH value of the soil is above 8, very few plant species are able to grow and reproduce successfully. Phosphorus and metal ions become mostly unavailable at this point, causing deficiencies and poor growth. Microbial activity is also significantly reduced, which results in delayed decomposition of organic matter and slower nutrient cycling. The three most common causes of increased pH in garden soils are:
1. Naturally high sodium carbonate content
2. Irrigation with water that has high sodium bicarbonate content
3. Over-liming of acidic soils
Limestone is a carbonate sedimentary rock rich in calcium carbonate and magnesium carbonate. Ground limestone (lime) is the most common soil amendment for increasing the pH value of acidic soils. However, this simple rock dust is not to be taken lightly. Over-liming is a common mistake in soil pH amendment that can have potentially devastating effects on plants. When you are increasing the pH of your soil make sure to add the exact amount of lime as advised on the product label. If you are not certain about the parameters, always add less. It is better to add more lime later than to subject the plants to toxic pH levels.
A list of commonly cultivated plants and optimal pH values for their growth:
1. Acidic (sour) soils (pH < 6):
– Alpine strawberry (5.5 – 6.5)
– Apple (5.0 – 6.5)
– Dill (5.5 – 6.5)
– Ginger (5.5 – 6.5)
– Microgreens (5.5 – 6.5)
– Paprika (5.5 – 6.5)
– Parsley (5.0 – 7.0)
– Peanut (5.5 – 6.5)
– Pepper (spice) (4.5 – 6.6)
– Potato (4.5 – 6.0)
– Sweet potato (5.6 – 6.5)
– Various berries:
- blackberries (5.0 – 6.0)
- blueberries (4.5 – 5.0)
- cranberries (4.0 – 5.5)
- currants (5.5 – 7.0)
- elderberries (5.5 – 6.5)
- gooseberries (5.5 – 7.0)
- raspberries (5.5 – 6.5)
– Many ornamental plants
- camellias (5.8 – 6.5)
- evergreen trees (5.5 – 6.5),
- magnolias (5.0 – 6.0)
- holly (5.0 – 6.0)
- hydrangeas (5.2 – 5.5), etc.
2. Slightly acidic to neutral soils (pH 6.0 – 7.3):
– Apricot (6.7 – 7.5)
– Artichoke (6.5 – 7.5)
– Basil (5.5 – 7.5)
– Beet (6.0 – 7.0)
– Broccoli (6.0 – 7.0)
– Broccoli rabe (6.5 – 7.5)
– Brussels sprouts (6.0 – 7.5)
– Cabbage (6.0 – 7.5)
– Cauliflower (6.0 – 7.5)
– Carrot (5.5 – 7.0)
– Celery (5.8 – 6.8)
– Chili peppers (6.0 – 6.8)
– Chive (6.0 – 7.0)
– Collard (6.0 – 6.5)
– Coriander (6.5 – 7.5)
– Cress (6.0 – 6.7)
– Cucumber (5.5 – 7.0)
– Fennel (5.5 – 6.8)
– Garlic (5.5 – 7.5)
– Hemp (6.0 – 7.0)
– Hops (6.0 – 7.5)
– Kale (6.0 – 7.5)
– Kohlrabi (6.0 – 7.5)
– Lettuce (6.0 – 7.0)
– Melon (6.0 – 6.5)
– Mizuna (6.0 – 7.5)
– Mustard (6.0 – 7.5)
– Okra (6.0 – 6.8)
– Onions (6.0 – 7.0)
– Oregano (6.0 – 7.0)
– Parsnip (6.0 – 6.8)
– Pea (6.0 – 7.5)
– Peach (6.5 – 7.0)
– Radish (6.0 – 7.0)
– Rhubarb (6.0 – 6.8)
– Rosemary (6.0 – 7.0)
– Sage (6.0 – 6.7)
– Spinach (6.0 – 7.5)
– Squash (6.0 – 6.8)
– Strawberry (6.0 – 7.0)
– Sunflower (6.0 – 7.5)
– Tarragon (6.0 – 7.5)
– Tomato (6.0 – 6.8)
– Turnip (6.0 – 7.5)
– Watermelon (6.0 – 7.0)
3. Alkaline (sweet) soils (pH > 7.3):
– Asparagus (6.0 – 8.0)
– Leek (6.0 – 8.0)
– Marjoram (6.0 – 8.0)
– Radicchio (7.5 – 8.0)
– Thyme (6.0 – 8.0)
Measuring soil pH
If you want to optimize nutrient absorption and give your plants what they need while using less fertilizer, measuring the soil pH is going to be a fundamental part of the process. Testing soil pH is especially important before fertilizing. If the soil is too acidic or alkaline, adding more nutrients won’t have any significant effect on plants as they won’t be available to them anyway.
You can test and measure the pH of your soil in a number of ways – using pH test strips, pH testing kits, or a pH meter. The strips are the least accurate method, suitable only if you want to find out the approximate pH of the soil solution. Testing kits are somewhat more precise, but considering that both pH testing strips and kits use universal indicators for measuring pH, they cannot give a precise result. Universal indicators are complex compounds that display a gradual change of color after exposure to certain pH values. The gradation of colors is the same as the rainbow color palette – from red (very acidic, usually pH<4) to purple (very alkaline, usually pH>11).
To explain this practically – after you dipped the pH testing strip into the sample solution, the universal indicator on its surface will change the color of the strip according to the pH value of the sample. The resulting color of the strip is then compared to a color-coded scale to determine the pH value of the sample. However, the scale usually includes whole values only. This will allow you to determine if the pH value of your sample is, for example, close to 7, but not if that value is 6.8, 7.2, or something in between. Testing kits are usually more precise because their color-coded scales include .5 values (5.5, 6.5, 7.5, etc.), but the result is still an approximation.
The easiest, fastest, and most precise way to measure pH is to use a pH meter. This gadget is an absolute necessity for hydroponic growers, but also for all growers that want to optimize fertilization and control the pH value of their substrates throughout the whole growing cycle. The pH meters are quite inexpensive, yet their utility can be priceless.
Remember: If your plants are displaying symptoms of nutrient deficiency, always test the pH of the soil before adding fertilizers.
Amending soil pH
If you had your soil tested, and found that its pH value is on one of those unfavorable ends of the scale, the next step would be to add some alkalizing or acidifying agents to adjust it. Here are the most common soil pH amendments used for that purpose:
Increasing soil pH
- Agricultural lime – The most common type of lime. It provides a slow release of calcium carbonate over a long period of time, which serves well for maintaining pH at favorable levels in the long term.
- Dolomitic lime – Besides calcium carbonate, dolomitic lime also contains magnesium carbonate. For this reason, we use dolomitic lime when there is a need to increase the pH of the soil and add more magnesium.
- Hydrated lime – This type of lime is significantly stronger and faster-acting than the previous two types, so we use it when the soil pH needs to be increased quickly. Its effects are short-lasting, and as such, it is not good for maintaining pH in the long run.
Lowering soil pH
- Organic matter – Adding compost or other organic fertilizer will decrease the pH of the soil, but similar to agricultural lime, it will do so slowly and over a long period of time. This is because hydrogen ions are released from the organic matter through biological processes, which are significantly slower compared to chemical reactions that occur almost instantly.
- Sulfur – The most commonly used amendment for the high pH of the soil. Elemental sulfur is transformed into sulfuric acid by soil bacteria, and the acid is actually the agent that decreases the pH value of the soil. How long will the effects last, depends on the activity and abundance of soil microorganisms. Because this is a biological process, the pH tends to change slowly and over a long period of time.
- Aluminum sulfate – Adding this compound to the soil will decrease its pH value almost instantly. Many growers who have the need to decrease the pH of the soil quickly use aluminum sulfate to achieve this.
We cannot end the story about decreasing soil pH without saying a word or two about ammonium-based fertilizers. Similar to sulfur, ammonium-based fertilizers need the help of microorganisms to perform their primary function. The soil bacteria convert ammonium to nitrate in the process of nitrification and produce hydrogen ions (H+) as byproducts. When the plants take up the nitrates, they release hydroxide ions (OH-) which react with free hydrogen ions to form water. However, if too much ammonium fertilizer is applied, the plants won’t absorb all the nitrates produced by the bacteria. Consequently, there won’t be enough OH- to react with H+, and the soil pH will decrease.
Because fertilizer leaching is minimal in indoor plant cultivation, overfertilization and accumulation of hydrogen ions can occur very easily. Too little is always better than too much when it comes to plants, but just enough is the best. So always think twice before deciding how much fertilizer you’ll apply. And, of course, measure the pH of the soil beforehand.
If you were to send a sample of your soil to a certified laboratory, you would get a detailed report that includes concentrations of all essential nutrients and, to make things a bit more complicated, two pH values. One would be simply titled “pH” – that is the active pH that we discussed in the text. The other would be titled “buffer pH”, and it can have a different value from the “active” pH. The buffer pH is an indicator of the buffering capacity of the soil, i.e., its ability to resist pH change.
What is the difference between buffer pH and soil pH?
While the active pH tells us the concentration of hydrogen ions in the soil, buffer pH talks about the number of sites on the soil particle surfaces that can accommodate hydrogen ions. Soils that have a lot of these sites have the ability to resist a change of the pH value. How does this work exactly? Well, if the pH starts to decrease in soils with good buffering capacity, the excess hydrogen ions will attach to the available sites on the soil particle surfaces. This will stabilize the pH value. On the other hand, if the pH increases due to the neutralization of hydrogen ions in the soil solution, the ions attached to soil particle surfaces will be released into the soil solution, thus decreasing and stabilizing the active pH value.
Buffer pH depends on the physical, chemical, and biological properties of the soil. Clay has the highest buffering capacity, while sand has the lowest. As loam is a mix of the two, so its buffering capacity depends on the amount of clay in the mix. Soils with diverse microbiological communities and a good amount of organic matter have a greater buffering capacity compared to poor soils. This is one of the reasons why rich, loamy soils are optimal substrates for many popular crops and herbs.
Hydroponics & the pH value
Hydroponic systems are a different gig compared to growing plants in solid substrates. Soil is a complex, active system that includes various living and non-living agents that affect the pH value and nutrient availability in different ways. It is also the natural habitat of many cultivated plants, so they are already well adapted to grow in it. The soil requires care and nutrient replenishing from time to time, but it is a far more independent system than hydroponics. This is because instead of diverse solid structures and a rich microbiological community, we have water in which all the necessary nutrients are dissolved and mixed. The amount of nutrients in the hydroponic solution depends on the grower only. Although this allows the grower to precisely give the plant what it needs and get incredible yields, it also very unforgiving when it comes to mistakes.
Considering the nature of hydroponic plant cultivation and the fact that nutrients must be dissolved to be available to plants, the pH of the aqueous solution is crucial for nutrient bioavailability in these systems. The optimal pH range of a hydroponic nutrient solution is 5.5 – 6.3, which is considerably more acidic than the optimal pH of soil. If you are the more inquisitive type, you are probably wondering why. Well, beneficial soil microbes operate best in slightly acidic to neutral pH and many nutrients are available in this range. However, as we don’t need these microbes in hydroponics, we can lower the pH to better accommodate nutrient availability.
The pH value in hydroponics deserves its own story, so we won’t go deeper into this subject in this piece. Keep that thumb green and stay tuned for more.