pH – The Power of Hydrogen
pH stands for the power of hydrogen, although many refer to the meaning of pH as ‘potential hydrogen’. Either way, potential hydrogen or power of hydrogen, pH is a parameter that measures the acidity or alkalinity of a solution by measuring the hydrogen ion concentration in solution. A pH value indicates the relationship between the concentration of free ions H+ (hydrogen) and OH- (hydroxide) present in a solution. Put simply, if a solution is very acidic, there will be lots of active hydrogen ions and hardly any hydroxide ions. If a solution is very alkaline, the opposite is true. In pure water, the concentrations of hydrogen and hydroxide ions are about the same. Therefore, pure water has a pH that is neutral at pH 7.0.
The pH scale is a logarithmic scale that typically runs from 1 to 14. Each whole pH value below 7 (the pH of pure water) is ten times more acidic than the higher value and each whole pH value above 7 is ten times less acidic than the one below it. For example, a pH of 5 is ten times more acidic than a pH of 6 and 100 times (10 times 10) more acidic than a pH value of 7. So, a strong acid may have a pH of 1-2, while a strong base may have a pH of 13-14. See following illustration of the pH scale.
Why does pH change in nutrient solutions?
This comes back to understanding the power of hydrogen, otherwise known as potential hydrogen in solution and some of our earlier material on EC where we touched on positively charged cations and negatively charged anions in hydroponic solutions. Other than this, pH changes largely occur due to the principle of electroneutrality where chemical reactions take place on an equivalent basis. The law of electroneutrality states that in any single ionic solution (e.g. a hydroponic nutrient solution) a sum of negative electrical charges attracts an equal sum of positive electrical charges. Therefore, according to the principle of electroneutrality, the total charge of an aqueous solution must be zero. For this to occur, the number of positive charges contributed by cations must be equal to the number of negative charges contributed by anions.
Based on this, in very simple terms, when a plant removes a positively charged cation from the nutrient reservoir/tank it leaves a negatively charged anion in its place and when a plant removes an anion from the nutrient reservoir/tank it leaves a cation in its place. See following images.
Since every macro and micro element ion in solution has an electrical charge plants can’t just take them, otherwise the electrical equilibrium would be out of balance. What plants do is swap them with equivalent amounts of H+ and OH- ions.
Ammonium N (NH4+) is swapped with 1xH+
Nitrate N (NO3-) is swapped with 1xOH-
Potassium (K+) is swapped with 1xH+
Calcium (Ca++) is swapped with 2xH+
Magnesium (Mg++) is swapped with 2xH+
Iron (Fe++) is swapped with 2xH+
Manganese (Mn++) is swapped with 2xH+
Zinc (Zn++) is swapped with 2xH+
phosphates (HPO4–) is swapped with 2xOH-
In this way electrical charge equilibrium remains the same.
The ratio in uptake of anions and cations by plants may cause substantial shifts in pH. In general, an excess of cation over anion leads to a decrease in pH, whereas an excess of anion over cation uptake leads to an increase in pH. That is, when the anions are uptaken in higher concentrations than cations the plant excretes OH- or HCO3- anions to balance the electrical charges inside, which increases the pH value. For example, if a plant absorbs the negatively charged nitrate nitrogen (NO3-) heavily it will start to contribute more OH – than H3O + ions into the solution and the result will be an increase in pH. On the other hand, if the plant absorbs high levels of the positively charged potassium (K+) it will contribute more H3O + than OH – ions and the result will be a decrease in pH.
This phenomenom is frequently seen where plants are grown with a full spectrum nutrient solution that contains nitrogen either as ammonium nitrogen (NH4+) or nitrate (NO3– ) nitrogen. When plants are fed only with NH4+, cation uptake generally exceeds anion uptake and the pH of the substrate decreases. On the other hand, when the plant is fed only with NO3– the uptake of anion to cation ratio is typically higher and as a result the pH of the substrate increases. This becomes important in understanding that a well formulated hydroponic nutrient contains an ideal ratio of ammonium nitrogen to nitrate nitrogen in order to minimize this situation and better maintain pH stability in the root zone and nutrient solution.
As a general rule, daylight photosynthesis (when the plant is taking up high degrees of mineral nutrition) produces hydrogen ions which can cause the nutrient acidity to increase (lowering the pH). When the lights switch off photosynthesis stops and the plants increase their rate of respiration. This coupled with the respiration of microorganisms (the release of CO2 by microorganisms) uses up the hydrogen ions so the acidity of the solution tends to decrease (pH rises). Additionally, plants are known to release organic acids through their roots (root exudates), reducing pH.
Nutrient Availability and pH
This is an area that tends to be misunderstood and/or oversimplified by many hydroponic industry interests who express optimum pH as 5.5 – 5.8.
In fact, from a scientific perspective, provided that adequate nutrients are available in solution, the acceptable pH range can be expressed as somewhat wider.
That is, the recommended pH for hydroponic growing is specified by many hydroponic nutrient manufacturers/suppliers at 5.5 to 5.8 because overall availability of nutrients is optimized at a slightly acid pH. The availability of Mn, Cu, Zn and especially Fe are reduced at higher pH, and there is a small decrease in availability of P, K, Ca, Mg at lower pH. Reduced availability means reduced nutrient uptake, but not necessarily a nutrient deficiency.
As such, there is some tolerance regarding pH where nutrients don’t become a limiting factor. This is because the direct effects of pH on root growth are small… the problem is reduced nutrient availability at high and low pH.
The pH of a solution can influence the availability of the individual ions within that solution. As pH changes one particular nutrient ion may gradually become more insoluble, leaving less of that ion available to act as a nutrient. pH is of little influence over a range, but if it goes too far, especially too high, then problems can result. Therefore, pH where nutrients are present at adequate levels is less critical than many think.
For example, where hydroponic techniques are used to study the growth of various species apparently preferring different pH levels, researchers usually find that they do reasonably well over a fairly wide pH range (approx. pH 5.2 to 7.5 provided a chelated form of iron is used).  The real issue is in ensuring that enough of any particular ion is in solution at a given pH to cater for the plants nutritional requirements. For example, indoor plants tend to grow equally well between pH 5.2 and 6.3 if nutrients in solution do not become a limiting factor.
Put simply, there is some tolerance to pH where adequate nutrients are available. Therefore, while pH 5.5 – 5.8 is expressed by some as the ideal there is some tolerance with regards to pH and you will typically find that a pH of between 5.2 and 6.3 will perform equally well in indoor settings where nutrients are supplied at adequate levels.
Given the rather confusing scientific understanding surrounding pH you can perhaps understand why many hydroponic and nutrient manufacturers simplify the subject and inform their consumers that they should maintain pH between 5.5 – 5.8. However, it is also necessary to raise the point that this is a simplified version of understanding pH because you will find that some express optimum pH between 5.5 – 5.8 while others express a wider range (e.g. 5.2 – 6.3 or 5.2 – 6.5 etc). This can lead to confusion amongst hydroponic retail consumers because the information provided by one nutrient supplier may seem contradictory to information being provided by another supplier. Additionally, you will find growers give what appears to be conflicting advice on forums with some stating that their plants grow best at e.g. pH 5.5 to 5.8 while others may state that the ideal pH range is wider. However, both versions of optimal/ideal/acceptable pH ranges are, in fact, correct. It really comes down to the fact that nutrient status, nutrient availability and pH are interrelated.
To put things simply, for novice growers, if you strive to maintain pH between the ideals of 5.5 – 5.8 this caters more adequately in situations where nutrients may be a limiting factor. This said, pH tolerance is wider than this in situations where adequate nutrient levels are maintained in solution at all times.
See following image that shows each nutrient’s pH range in hydroponics.
The Essential Nutrients and their pH Ranges
Nitrogen is the plant nutrient which most influences growth and development of agricultural crops. Yield is closely related to N nutrition. Plants are surrounded by nitrogen in the atmosphere, but because atmospheric gaseous nitrogen is present as inert nitrogen (N2) molecules, this nitrogen is not directly available to the plants. Plant available forms of nitrogen in hydroponics are typically inorganic and include nitrate (NO3), and ammonium (NH4). Organic forms of nitrogen that are plant available, which are found in some nutrients and additives are N containing amino acids (e.g. glycine) and the organic chemically pure, albeit, synthetic urea. Nitrogen is available across a wide range of pH values from 2 – 7.
There is an extremely important relationship between potassium and nitrogen in flowering/fruiting crops and although potassium is not a constituent of any plant structures or compounds it plays a part in many important regulatory roles in the plant. These include osmotic regulation, regulation of plant stomata and water use, translocation of sugars and formation of carbohydrates, energy status of the plant, the regulation of enzyme activities, protein synthesis and many other processes needed to sustain plant growth and reproduction. Additionally, potassium plays a very important role in plant tolerance of biotic and abiotic stresses.
Potassium is also known as the quality nutrient because of its important effects on quality factors (e.g. essential oils, flavonoids). With the exception of nitrogen, potassium is required by plants in much greater amounts than all the other nutrients. Increasing plant vegetative growth, yield as well as fruit quality and chemical composition due to increasing potassium fertilization levels has been reported by many researchers on different crops. The most prevalent nutrient found in the developed tomato plant and fruit is potassium, followed by nitrogen (N) and calcium (Ca). See graphs 1 and 2.
Potassium is almost completely present as a free ion (K+) in a nutrient solution and is available over a wide range of pH values from 2 to 9.
Graph 1: Element composition of a tomato plant (Atherton and Rudich, 1986)
Graph 2: Element composition of a tomato fruit (Atherton and Rudich, 1986)
Calcium and Magnesium (Ca and Mg)
Like nitrogen and potassium, calcium and magnesium are available to plants across a wide range of pH; however, the presence of other ions can interfere with their availability due to the formation of compounds with different grade of solubility. For example, when the pH of the nutrient solution increases, the HPO42– (hydrogen phosphate) ion predominates, which precipitates with Ca2+when the product of the concentration of these ions is greater than 2.2, expressed in mol m-3 . Sulphate also forms relatively strong complexes with Ca2+ and Mg2+. As pH increases from 2 to 9, the amount of SO42-, forming soluble complexes with Mg2+as MgSO4 and with K+ as KSO4– increases. Practically speaking, where hydroponics is concerned, both calcium and magnesium tend to be reasonably plant available between pH 5.5 – 6.0.
Phosphorus (P) is an important plant macronutrient, making up about 0.2% of a plant’s dry weight. It is a component of key molecules such as nucleic acids, phospholipids, and ATP, and consequently plants cannot grow without a reliable supply of this nutrient. Phosphorus is also involved in controlling key enzyme reactions and in the regulation of metabolic pathways.
Phosphorus is an element which occurs in forms that are strongly dependent on pH. In the root zone phosphorus can be found as PO43-, HPO42, and H2PO4- ions; the last two ions are the main forms of P taken by plants. In inert substrates, the largest amount of P available in a nutrient solution is presented when its pH is slightly acidic (pH 5). In alkaline and highly acidic solutions the concentration of P decreases in a significant way. Namely, with pH 5, 100% of P is present as H2PO4-; this form converts into HPO4-2 at pH 7.3, reaching 100% at pH 10. The pH range that dominates the ion H2PO4-2 on HPO4- is between 5 and 6. In research surrounding P availability by Jacek Dysko et al (2008) with tomatoes grown in various hydroponic organic and inorganic substrates it was shown that regardless of the substrate type, optimum yields were gained at pH 5.5.
“The marketable yield obtained with a pH of 5.5 was significantly higher in relation to the yield obtained at pH 6.5, but it did not differ significantly from the yields obtained at pH 4.5, 5.0 and 6.0. “ Similar findings were made by Chohura et al (2004) while studying the effects of pH in tomato culture grown in rockwool. 
Therefore, optimum phosphorus availability in solution and substrates falls within the range of pH 5.0 – 6.0, with pH 5.5 being ideal.
Sulphur is used mainly in sulphur-containing proteins using the amino acids cysteine and methionine. The vitamins thiamine and biotin, as well as the cofactor Coenzyme A, all use sulphur, and so this element also plays a key role in plant metabolism. Sulphur is most available to plants grown hydroponically over a range of 6.0 to 9.5; however because of the availability of other nutrient elements and their pH ranges, sulphur in hydroponics is absorbed reasonably well between pH 5.5 – 6.0.
The Microelements (Fe, Cu, Zn, B, Mn and Mo)
The microelements, iron (Fe), copper (Cu), zinc (Zn), boron (B), molybdenum (Mo) and manganese (Mn), become unavailable in most cases at pH higher than 6.5 and are most available in hydroponic solution at an acidic pH of 4.0-5.5, although where chelated micronutrients are used tolerance levels are higher.
Boron is an exception and is mainly uptaken by plants as boric acid, which is not dissociated until pH is close to 7; at greater pH values, boric acid accepts hydroxide ions to form anionic species. In simple terms, putting aside the scientific jargon, boron has a wider pH range than the other microelements.
Optimum pH, to accommodate for all of microelements is, therefore, typically expressed in hydroponic solutions, factoring in the availability of other nutrients, at pH 5.5.
However, where chelated or complexed microelements are used in hydroponic solutions the pH range of iron, copper, zinc, boron, and manganese is increased.
It would be remiss of me to talk about pH, nutrient availability and microelements without highlighting that chelation increases the acceptable pH range for microelements.
The word chelate is derived from the Greek word chelé, which refers to a lobster’s claw. Hence, chelate refers to the pincer-like manner in which a metal nutrient ion is encircled by the larger organic molecule (the claw), usually called a ligand or chelator.
Chelators, when combined with a microelement, can form a chelated fertilizer. Chelated microelements are protected from oxidation, precipitation, and immobilization in certain conditions because the organic molecule (the ligand) can combine and form a ring encircling the micronutrient. The pincer-like manner in which the micronutrient is bonded to the ligand changes the micronutrient’s surface property and favors the uptake of microelements found in hydroponic solutions. This acts to increase the acceptable/ideal pH range for microelements in solution.
The common forms of chelates used by many ‘hydro’ nutrient manufacturers are the synthetic chelates, EDTA (ethylenediaminetetraacetic acid) and to a lesser extent DTPA (Diethylene triamine pentaacetic acid). Chelates such as EDTA and DTPA have a high affinity for e.g. iron and generally form stable complexes with the metal across a pH range from 4 to 7.
Chelates have several points of attachment with which they “grasp” the trace element. EDTA has four connecting points while DTPA has five. Higher numbers of connection points isn’t always an advantage. In some cases the four connection points may hold the element too tightly, while in a different situation these may not hold it tight enough. For this reason, various chelates may prove better than others based on the ion that is chelated and the conditions in which the chelate is present.
The effectiveness of a chelating agent can depend on pH. In the case of iron Fe EDTA is best suited to slightly lower than neutral pH levels while Fe DTPA is most effective at higher pH values. DTPA is more costly than EDTA and less soluble and is usually found in higher quality fertilizers. DTPA is stable up to a pH of 7.5 while EDTA is stable up to a pH of approximately 6.5.
The most effective of the synthetic chelating agents is ethylenediaminedihydroxy-phenylaceticacid (EDDHA). It is important to note that EDDHA can be formed only with iron and not with other essential microelements such as Cu, Zn, Mn.
Fe EDDHA is the most stable of all the commonly available iron chelates. This synthetic chelate is held in a bond up to 100 times tighter than DTPA because it has six molecular bonds rather than five bonds. Typically EDDHA is only found in premium fertilizers because of its higher cost. EDDHA is stable up to pH 9.0 (pH range = 4- 9).
In most cases combinations of chelating agents can improve stability and broaden effectiveness. That is, a mix/blend of EDTA, DTPA, EDDHA or EDTA and DTPA in formulation best ensures nutrient availability over a wide range of conditions, including those above or below optimal. For this reason, even in hydroponic growing environments where optimum pH (water temperatures etc) can be monitored and maintained, there are benefits potentially gained from using a blend of chelated elements in solution.
Conclusion re Optimum pH in Hydroponics
pH is an extremely important factor in Hydroponic gardening. It makes your nutrients available to your plants. An unusually high pH will decrease the availability of iron, manganese, boron, copper, zinc and phosphorus. A pH that is too low will reduce the availability of potassium, sulphur, calcium, magnesium and phosphorus.
Based on the information that we have covered surrounding nutrient pH ranges in hydroponic settings it is possible to see that where optimum availability of all the nutrients is concerned, pH 5.5 – 5.8 offers the ideal range to work within. As previously noted, some tolerance to pH is present where adequate nutrients are in solution.
Optimum pH for hydroponics = 5.5 – 5.8
Tolerance range where adequate nutrients are in solution = pH 5.2 – 6.3
 Bugbee, B (2003) Nutrient Management in Recirculating Hydroponic Culture
 Donnan, R Nutrient Management – Part 3 in Practical Hydroponics and Greenhouses Magazine , Issue 16 (May/June 1994)
 Dysko et al. (2008) The Effect of Nutrient pH on Phosphorous Availability in Soilless Culture of Tomato