One important, but unseen, aspect greatly affecting the performance of a garden is the potential hydrogen, or pH, of the nutrient solution. A basic understanding of the pH scale and how it affects plant growth will only help a horticulturist in his or her quest for better growth in the garden. The biggest impact the pH of a nutrient solution has on plant growth is in the way it affects the availability of nutrients for absorption. In other words, there are ranges of pH values that are most suitable for nutrient absorption and, when the pH level fluctuates away from that range, particular elements can become “locked out” or unavailable to the plants. As can be imagined, if the plant is no longer able to absorb particular essential elements, the plant growth will be negatively affected.
The pH (potential hydrogen) scale is used to measure the concentration of hydrogen ions. For horticultural purposes, the pH scale is used to determine if a soil, water source, or nutrient solution is neutral, acidic, or alkaline (basic). On the pH scale, a reading of 7 is considered neutral, all readings above 7 are considered alkaline, and all readings below 7 are considered acidic. The pH scale is an exponential logarithmic scale. Every number on the pH scale represents an increase or decrease of tenfold. For example, a pH value of 3 is ten times more acidic than a pH value of 4 and 100 times more acidic than a pH value of 5. The same holds true for readings above 7. For example, a pH value of 10 is ten times more alkaline than a pH value of 9 and 100 times more alkaline than a pH value of 8.
Nutrient solution refers to the solution of water mixed with fertilizers and/or additives that will be watered into the planting containers either manually or by a mechanical system. The nutrient solution of a hydroponic system is the solution that is pumped from the reservoir to the plants and, in recirculating systems, returned back to the reservoir. It is always a good idea to make sure the pH of the nutrient solution is balanced before feeding it to the garden. The three biggest influences over the pH of a nutrient solution are the water source, the nutrients and bacteria.
The water source is the heart and soul of any biological system, including soil and hydroponic gardens. The pH of the water is determined by the soluble chemicals it contains. The two most common chemicals which influence the pH of the water are CO2 and bicarbonate. Large amounts of free CO2 found in the water will cause a lower (more acidic) pH. On the contrary, high amounts of bicarbonate found in the water will cause a higher (more alkaline) pH. One habit hydroponic growers should get into is aerating the water before adding nutrients or testing the pH. Most hydroponic systems will require the nutrient solution to be aerated continuously. When the water is aerated (with air stones or recirculation) the chemical concentrations in the water will actually change and affect the pH. Aeration causes an escape of CO2 which will affect the overall pH of the solution. Therefore, testing pH before aeration is not an accurate representation of what the plants will receive. The best solution to this problem is a holding tank (or reservoir) for aerating water before it enters the hydroponic system. In many cases, the grower may wish to filter the water before adding nutrients or testing/adjusting the pH. This valuable step can help avoid a whole bunch of trouble down the road. The two most commonly used water filtration methods for greenhouse horticulturists are carbon adsorption and reverses osmosis.
Carbon adsorption is the most widely used method of water filtration. Carbon adsorption is the best way to remove chlorine and chloramine. Both of these chemicals create an objectionable flavor for humans and are harmful compounds for the microorganisms found around a plant’s roots. For thrifty gardeners who use tap water, a carbon adsorption filter can serve the valuable purpose of removing the chlorine and chloramine at a reasonable price. Carbon adsorption will remove many soluble organic compounds but generally does not affect the total dissolved solids of the solution. This type of filtration will not remove heavy metals or water hardness. In other words, these filters can affect the pH of the water source but probably will not neutralize it completely. Gardeners who want water with a completely neutral pH should use a reverse osmosis filter.
Reverse osmosis is by far the preferred method of water filtration for serious horticulturists. Reverse osmosis filtration effectively removes up to 99% of all contaminates. In addition to removing contaminates, water treated by reverse osmosis has a neutral pH (pH value of 7). In other words, gardeners who use reverse osmosis filtration will have a neutral water source to which they can add the nutrients they wish and then set the pH to the desired level. This enhanced control over the water source is why reverse osmosis filters are so popular in the horticultural industry.
Nutrients is a general term referring to any fertilizers or additives horticulturists mix with the water they feed to the plants. Nutrients used in either a soil or hydroponic system will directly influence the pH levels of the solution. For example, high potency enzyme additives can significantly drop pH levels while potassium-silicate or bicarbonate additives can bring up the pH level. It is a good idea to get into the habit of mixing the base formula (main fertilizer) and then stabilizing the pH before adding any amendments. After adding each amendment, the grower should check his or her pH again. This will show how much the given amendment is affecting the pH and how the concentration should be adjusted to best adapt to the hydroponic system. Many gardeners don’t realize how the plant’s uptake of nutrients can affect the pH level. As a plant takes in nutrients, the concentration of chemicals changes in the solution which also changes the overall pH. Continually monitoring the pH will give the gardener clues as to when and how much the nutrient solution tends to fluctuate. This can also help indicate the optimal time to change the nutrient solution (usually every 7-14 days).
A colony of bacteria can be a cause of pH fluctuation in systems with a nutrient reservoir. If a grower is experiencing a recurring low pH, especially right after (within an hour or so) making adjustments, more than likely, he or she has a bacteria living in the reservoir. Cleanliness is the best preventative measure for bacteria-caused pH fluctuation. Start by removing any dead or dying root matter in and around the grow system. Also, running a diluted bleach solution or a high concentration of hydrogen peroxide through a hydroponic system between crops is a great way to lower the chance of bacteria in the reservoir.
For soil gardening, the ideal pH range falls just below neutral, in the 6-7 pH range; with the maximum absorption around 6.3-6.8. This is because of the complex microscopic world of microorganisms living in and around the rhizosphere (the region of soil affected by a plant’s root secretions). A slightly acidic pH is ideal for the reproduction and functioning of the beneficial microorganisms which help break down organic matter into soluble nutrients.
Hydroponic gardeners will find their ideal pH range for nutrient absorption is slightly more acidic than what is ideal for soil gardening. In hydroponic systems, there is less of a dependency on the microbial life which is normally harbored in the soil. Hydroponic nutrients are broken down into a soluble form which bypasses many of the functions performed by the beneficial microbes. This bypass of microorganisms changes the plant’s optimal pH range for nutrient absorption. The ideal pH range for hydroponic gardening falls in the 5-6 range; with maximum absorption around 5.5-5.8. Unlike soil, which acts as a natural pH buffer, the pH value of a hydroponic solution can fluctuate rapidly. Because of this, it is very important to regularly monitor the pH of the solution in a hydroponic system.
There are many tools available to help horticulturists monitor and measure the nutrient solution’s pH. Litmus paper and liquid pH tester drops are the least expensive options and can be picked up at any local hydroponic retailer and/or fish/aquarium store. These types of measurement tools have a chemical compound that reacts to the solution and will turn a certain color depending on the pH of the solution. For more accurate measurements, there are digital pH monitors available. There are many different models of digital pH monitors. Some offer all the bells and whistles, such as automatic calibration and temperature compensation. However, for most hobbyists, an inexpensive digital “pen” pH monitor is more than adequate.
In many cases, the pH of the solution will not be in the desired range and the horticulturist must make adjustments to correct the pH. The best way to make pH adjustments to a nutrient solution is with liquid pH buffers. These buffers are usually sold under the generic name of pH Up and pH Down. Be sure to follow the manufacturer’s directions before making adjustments. It is also very important to remember the exponential magnitude of the pH scale. In other words, the amount of buffer solution needed to make an adjustment will vary greatly depending on how far away or close the solution is to the neutral point (7).
Gaining a better understanding of the pH scale is important for all horticulturalists because every essential element used by plants has an ideal pH range in which it can be absorbed. If the pH fluctuates too far from the ideal range, a nutrient “lockout” may occur; thereby causing nutrient deficiencies which hinder the vibrant plant growth within a greenhouse. Many times gardeners don’t understand that it is the pH of the nutrient solution, not the lack of essential elements that is causing an issue. Growers, who take the time to understand the pH scale, learn how to monitor the pH of a nutrient solution, and make necessary adjustments to keep the pH of the solution within the desired range will have gardens that reflect their efforts with vibrant growth and prolific yields.
Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability. He is a Garden & Greenhouse contributing editor and may be contacted at Ehop@GardenAndGreenhouse.net.