In hydroponics, growers have the opportunity to provide plants with ideal growing conditions. An important aspect of developing and maintaining a working hydroponic system is the pH of the growing solution.
According to research conducted at the University of Florida, the pH of the nutrient solution used in hydroponics should typically be between 5.5 and 6.5. However, some plants prefer a growing environment with a pH outside of this range. For example, blueberries prefer to grow in more acidic conditions with a pH between 4.0 and 5.0.
The optimum pH range for a variety of hydroponic crops are listed in the table below (Sharma et al., 2018).
| Crop | pH |
|---|---|
| Asparagus | 6.0 – 6.8 |
| African Violet | 6.0 – 7.0 |
| Basil | 5.5 – 6.0 |
| Bean | 6.0 |
| Banana | 5.5 – 6.0 |
| Blueberries | 4.0 – 5.0 |
| Broccoli | 6.0 – 6.8 |
| Cabbage | 6.5 – 7.0 |
| Celery | 6.5 |
| Carnation | 6.0 |
| Courgettes | 6.0 |
| Cucumber | 5.0 – 5.5 |
| Eggplant | 6.0 |
| Ficus | 5.5 – 6.0 |
| Leek | 6.5 – 7.0 |
| Lettuce | 6.0 – 7.0 |
| Mint | 7.0 – 8.0 |
| Pak Choi | 7.0 |
| Pea | 6.0 – 7.0 |
| Peppers | 5.5 – 6.0 |
| Parsley | 6.0 – 6.5 |
| Rhubarb | 5.5 – 6.0 |
| Rose | 5.5 – 6.0 |
| Spinach | 6.0 – 7.0 |
| Strawberries | 6.0 |
| Sage | 5.5 – 6.5 |
| Tomato | 6.0 – 6.5 |
Sources:
Sharma, Nisha; Acharya, Somen; Kumar, Kaushal; Singh, Narendra; Chaurasia, Om. (2018). Hydroponics as an advanced technique for vegetable production: An overview. Journal of Soil and Water Conservation. 17. 364-371. 10.5958/2455-7145.2018.00056.5.
Tyson, Simonne, White, Lamb. 2004. Reconciling Water Quality Parameters Impacting Nitrification in Aquaponics: pH Levels. Proc. Fla. State Hort. Soc. 117:79-83. 2004.
What is pH?
pH is a measure of the acidity or alkalinity of a solution. Acidity is a measure of the concentration of hydrogen ions, and alkalinity describes the ability of a solution to neutralize hydrogen ions.
The letters pH stand for power of hydrogen. pH is defined as the negative base 10 logarithm of the concentration of hydrogen ions. This definition is presented in equation form below.
pH = -log10 [H+]
Because pH is logarithmic, a solution with a pH of 6.0 has a hydrogen ion (H+) concentration that is ten times greater than the H+ concentration of water with a pH of 8.0.
pH values range from 0 to 14. A pH value of 7.0 is considered neutral because it represents a condition where the concentration of hydrogen ions ([H+]) is equal to the concentration of hydroxide ions ([OH–]).
Solutions with a pH value of less than 7.0 are acids, and solutions with a pH greater than 7.0 are alkalis (bases). Acids contain a higher concentration of hydrogen ions than hydroxide ions, and bases contain a higher concentration of hydroxide ions than hydrogen ions.
The pH of some common items is listed below.
| Item | pH | Acid or Base? |
|---|---|---|
| Battery Acid | 1.0 | Acid |
| Lemon Juice | 2.2 | Acid |
| Vinegar | 2.8 | Acid |
| Apples | 3.0 | Acid |
| Soft Drinks | 3.0 | Acid |
| Tomatoes | 4.5 | Acid |
| Black Coffee | 5.0 | Acid |
| Human Saliva | 6.4 | Acid |
| Rain Water | 6.5 | Acid |
| Milk | 6.8 | Acid |
| Pure Water | 7.0 | Neutral |
| Human Blood | 7.4 | Base |
| Egg | 7.8 | Base |
| Baking Soda | 8.3 | Base |
| Ammonia | 11.0 | Base |
| Lime (Calcium Hydroxide) | 12.4 | Base |
| Bleach | 13.0 | Base |
Why is pH Important?
It is important to monitor the pH of your hydroponic solution because pH affects a number of factors that influence healthy plant growth in hydroponics such as hardness, solubility and availability of nutrients, algae growth, and germination rate.
Nutrient Availability
pH affects the solubility and availability of nutrients necessary for healthy plant growth. Some nutrients such as phosphorus and boron are more available in solutions with a high pH. In contrast, some nutrients such as aluminum and manganese are more available in acidic soils. Some nutrients such as nitrogen, sulfur, and potassium are available within a wide range of pH.
Every plant needs 16 elements in order to continue growing. Some plants do need additional nutrients (Resh, 2013). These elements can be classified as macronutrients and micronutrients. Macronutrients are required in large quantities, and micronutrients are required in smaller quantities. Macronutrients include carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), sulfur (S), and magnesium (Mg). Micronutrients include iron (Fe), chlorine (Cl), manganese (Mn), boron (B), zinc (Zn), copper (Cu), and molybdenum (Mo).
The table below lists the optimum pH range for some of the essential nutrients described above.
| Nutrient | Optimum pH Range |
|---|---|
| Nitrogen | 5.5 – 8.5 |
| Phosphorus | 6.0 – 7.5 8.5 – 10 |
| Potassium | 6.0 – 10.0 |
| Sulfur | 6.0 – 10.0 |
| Calcium | 7.0 – 8.0 |
| Magnesium | 6.5 – 8.5 |
| Iron | 4.5 – 5.5 |
| Manganese | 5.0 – 6.0 |
| Boron | 5.0 – 6.0 8.75 – 10.0 |
| Copper and Zinc | 5.0 – 6.5 |
| Molybdenum | 7.0 – 10.0 |
Source: Table adapted from chart available in Sprague, H.B., Hunger Signs in Crops. 1964. p. 18. Courtesy of George Barile, Accurate Art, Inc., Holbrook, NY.
There is no physiological difference between plants grown in soil or plants grown hydroponically. The process of the plants uptaking nutrients, which involves water and ions moving across a membrane, is the same in soil and in hydroponics.
Germination Rate
A study conducted by researchers at the University of Extremadura in Spain examined how pH affected seed germination. The results of the study indicated that a high pH adversely impacted the rate of germination. However, pH had no effect on the percent of seeds germinated (Pérez-Fernández et al., 2006).
Source: Pérez-Fernández, Calvo-Magro E, Montanero-Fernández J, Oyola-Velasco JA. 2006. Seed germination in response to chemicals: effect of nitrogen and pH in the media. J Environ Biol. 2006 Jan;27(1):13-20.
Algae Growth
pH also impacts algae growth in a hydroponics system. Algae thrive in water with a pH between 7.0 and 9.0. For most species of algae, optimum growth occurs in water with a pH between 7.2 and 8.2.
Algae can cause pH swings that are detrimental to plant health. These swings are diurnal in nature meaning that pH will be higher during the day and lower during the night. This is because algae consume carbon dioxide, which is a weak acid, when it is photosynthesizing during the day.
To prevent algae growth in your hydroponics system, make sure the nutrient solution is not exposed to light.
Water Hardness
Water hardness describes the amount of dissolved calcium and magnesium in water. If you have ever washed your hands and a residue remained on your hands, you have experienced the effects of hard water.
Hard water contains a lot of bicarbonate ions. This can interfere with the uptake of other nutrients. This makes hard water detrimental to healthy plant growth.
Buffering Capacity
Buffering capacity is a solution’s ability to resist changes in pH. This concept is relevant when you are working with hard water. Hard water contains a lot of carbonates which are a group of chemicals that contain the CO32- molecule. Some examples of carbonates include limestone, calcite, dolomite, and marble.
If you find that your nutrient solution has a consistently high pH and it is difficult to bring down using pH Down, the carbonates in the hard water are probably increasing the buffering capacity. In this situation, you might consider using a reverse osmosis filter.
Reverse Osmosis Filters
If you are using hard water for your hydroponics system and it is causing problems, you could get a reverse osmosis (RO) filter to remove the carbonates from your water. However, RO filters are expensive and hard water scale will build up on the filter membrane over time. If you are building a small-scale hydroponics system at home, using distilled water is probably a much more cost-effective solution to hard water problems.
Changes in pH
There are several factors that influence the pH of a nutrient solution including the concentration of your nutrient solution, the type of growing media, and the presence of bacteria.
Concentration of Nutrient Solution
The nutrients you add to your reservoir make the nutrient solution more acidic. When the plants consume these nutrients, the pH of the nutrient solution will go up. If the pH of your nutrient solution is above the optimum range for the particular species of plant, consider adding more nutrient solution to the reservoir.
Although it is more common for pH to rise as plants uptake nutrients, your nutrient solution can actually become more acidic over time. This is an indication that you put too many nutrients into your nutrient solution. If water is rapidly evaporating from the reservoir, the solution becomes more concentrated with nutrients. This can also result in a more acidic nutrient solution.
If your nutrient solution is becoming more acidic over time, it may also be an indication of bacterial growth.
Growing Media
Although hydroponic growing media is chemically inert, it can influence the pH of your nutrient solution by acting as a buffer. For example, rockwool has a pH of 7.8 which is higher than the optimal range for most plants. Growing media like rockwool has surface-active base site which act as buffers and causes the pH to readjust to their “natural” pH.
To prevent this, pretreat your growing media before using it. Place your growing media in a bucket of water containing one liter of vinegar for every gallon of water. The growing media will begin to neutralize the acetic acid in the nutrient solution and its surface base sits will lose their ability to act as a buffer.
Root Rot
Significant swings in pH can also be an indication of bacterial growth or dying roots.
When dying roots are being decomposed by bacteria, they release acid. This lowers the pH of your nutrient solution. A pH reading of 4.5 or lower indicates root disease.
To treat roots, remove the diseased plant from your hydroponics system. Then trim the dead or sick-looking parts of the roots. Dip the roots into a 3% hydrogen peroxide solution to kill bacteria. Then rinse the roots. Finally, put the plant back and see if the pH stabilizes. If it does not, consider removing the dead or dying plants. It might also be a good idea to disinfect your equipment.
Testing pH
Because there are so many factors that influence the pH of your nutrient solution, you should test the pH of your system daily. This will allow you to correct issues quickly. There are several ways to test the pH of a solution. Some of the most common methods are listed below.
Litmus Test Strips
The most inexpensive and simplest way to test the pH of your nutrient solution is to use litmus strips. These are strips contain dye that is sensitive to changes in pH. You use them by dipping the strip in your nutrient solution and comparing it to a color chart that comes with the strips.
Litmus test strips come in a variety of pH ranges. Choose strips with a pH range of 4.0 – 7.0 in increments of about 0.2 or 0.3 units.
Digital Meter
A digital pH meter is an electronic device that measures the electrical potential between two electrode. The electrical potential is then converted to pH based on the results of solutions with known pH values.
Digital pH meters tend to be more accurate than pH test strips. This is because pH meter results are based on set ranges and standards.
A problem with inexpensive digital meters is that they become less accurate over time, and they must be calibrated often with buffer solutions and distilled water. However, inexpensive digital meters will work well for most hobby hydroponics setups as long as it is calibrated prior to each use.
You can also purchase more expensive pH meters as your hydroponics systems become more complex and warrant such equipment.
More expensive pH meters can even give you a constant reading of pH and will even set off an alarm if the pH of the nutrient solution is above or below a prescribed range. These types of meters are attached to the downstream end of the main line from the nutrient tank or injector system and cost $250 or more.
How to Calibrate a pH Meter
It is important to calibrate pH meters because digital meters measure the electrical potential of a solution and compare it to known solutions. Calibration is performed using buffers which are solutions with a known pH.
Step 1 – Clean the electrodes. Rinse the electrodes with distilled water and wipe the electrodes dry with a lint-free tissue.
Step 2 – Calibrate with a pH Buffer of 7. Submerge the electrodes into a buffer solution with a pH of 7. Press the calibrate button and wait for a reading. If it says 7.0, press accept. Otherwise, edit the entry.
Step 3 – Repeat Step 2 with the other buffers. Repeat step 2 for your other buffers (pH of 4.0 and/or 10.0).
Step 4 – Measure the pH of your nutrient solution.
Liquid pH Test Kits
Liquid pH test kits are another inexpensive way to measure pH. It is also an easy way to measure pH. Like litmus test strips, determining pH simply involves comparing colors to the chart that comes with the test kit.
Use a pH liquid test kit by first adding a small amount of nutrient solution to a clean vial. Then add a few drops of the testing solution. Shake the vial and compare the color of the mixture to the chart that comes with the test kit.
Liquid pH test kits are typically more expensive than litmus test strips. However, they tend to be a little more accurate.
Adjusting pH
If the pH of your nutrient solution is outside of the optimal pH range, it is necessary to adjust the pH.
Keep in mind that plants dislike sudden changes in pH (or anything in their growing environment). Therefore, it is not a good idea to expose your plants to an abrupt change in pH. If the pH of your nutrient solution is far out of the optimal range (greater than 2 units), adjust the pH slowly.
pH Up and pH Down
The most common way to adjust the pH of a nutrient solution is to add pH Up or pH Down. As you may guess, pH Up is used to increase the pH of nutrient solution, and pH Down is used to lower the pH of nutrient solution.
pH Down is made of food-grade phosphoric acid. pH Up is made of potassium hydroxide and potassium carbonate. When adding pH Up or pH Down to your nutrient solution, add a little bit at a time. Typically, you only need 1-2 milliliters of these chemicals per gallon of water to adjust the pH.
You can purchase pH Up and pH Down on Amazon or at hydroponic supply stores.
Be careful not to splash the leaves of your plants with these chemicals. This can burn them.
Lemon Juice
As shown in a table presented in a previous section, lemon juice has a low pH (2.2). As a result, it will definitely lower the pH of your nutrient solution.
However, it is not a good idea to use lemon juice to lower the pH of your nutrient solution because lemon juice will kill the nitrogen fixing bacteria that live in your system. Nitrogen is an essential part of plant growth. For instance, it is an important component of chlorophyll, which makes plants green.
If you have used lemon juice to to lower the pH of your nutrient solution, don’t panic. Lemon juice is not an immediate killer. Just swap out your nutrient solution as soon as possible.
Baking Soda
Baking soda will raise the pH of your nutrient solution. Unfortunately, it will also increase the sodium content which is not desirable. This is because excess sodium diminishes nutrient uptake. For this reason, I do not recommend using baking soda to increase the pH of your nutrient solution.
Related Questions
Does pH impact EC?
Electrical conductivity (EC) descibes the ability of a material to conduct electrical current through it. EC is an indirect measurement of the amount of total dissolved solids (TDS) in an solution.
Basic solutions contain more negatively charged ions, and acidic solutions contain more positively charged ions. Conductivity depends on the concentration of all ions. Therefore, solutions with a high pH or a low pH will have higher EC values.
In hydroponics, adding nutrients tend to increase EC and lower pH. That is because nutrients that are negatively charged will bond with hydrogen ions which in turn raises the pH.