The potential hydrogen, or pH, scale is a system of measurement that determines the concentration of hydrogen ions. In the garden, the pH scale is used to measure the acidity or alkalinity of a growing medium, water source, or nutrient solution. The pH scale consists of number values from 0-14 (with 7 being pH neutral). A neutral pH means the substance being measured has neither an acidic nor an alkaline pH but instead sits directly between the two. Numbers on the pH scale below 7 represent acidity while numbers above 7 represent alkalinity.
It is important to remember that the pH scale is an exponential logarithmic scale. In other words, the intensity of the acidity or alkalinity increases exponentially as the value gets farther away from the 7 neutral point. In fact,
each number on the pH scale represents an increase or decrease in potential hydrogen by ten-fold. This means that a pH value of 4 is ten times more acidic than a pH value of 5 and is 100 times more acidic than a pH value of 6. As pH values increase above 7, the same holds true for alkalinity. For example, a pH value of 10 is ten times more alkaline than a pH value of 9 and is 100 times more alkaline than a pH value of 8.
Although gardeners do not need to have a degree in chemistry to be successful hobbyists, a good understanding of the pH scale and how it affects plant growth can only help a horticulturist become better. The pH value’s biggest influence over plant growth is in the way it affects the availability of nutrients for absorption. In other words, there is a pH value range that is most suitable for nutrient absorption and when the pH level moves too far from that range certain elements can become “locked out” and are unavailable to the plants. When the plant is no longer able to absorb particular essential elements, the plant growth will be negatively affected.
When discussing the ideal range of the pH in soil, it is important to remember that the pH value affects each element’s ability to be absorbed a little differently. In order to find the best pH range for nutrient absorption, a grower must try to maintain a pH value that best suits all the essential elements. This ensures the plant will have access to everything it needs for healthy growth. For most plant varieties, the ideal pH range for soil lies just below the neutral range, more specifically, in the 6-7 pH range. The maximum absorption rate of nutrients for most plants growing in soil occurs in the 6.3-6.8 pH range.
The actual reason why this is the ideal range is not because of the plant or the medium but instead because of the beneficial microorganisms found in the soil. Beneficial microorganisms that live in and around a plant’s root mass break down organic matter into a soluble form for the plant to absorb. As it turns out, a slightly acidic pH is ideal for the reproduction and livelihood of the imperative microorganisms. Most potting soils are built with a variety of ingredients which act as pH buffers and help to maintain a slightly acidic range. The ingredients in the soil help provide a consistent pH level and make it difficult for the pH range to fluctuate rapidly. In other words, soils have a heightened stability when it comes to the pH values. Most soil gardeners who experience problems with the pH have allowed the soil to become too acidic. If the pH of a soil drops below a value of 6, there is a significant reduction in the availability of nitrogen, phosphorus, potassium, sulfur, calcium and magnesium.
The ideal pH range in a hydroponic garden differs slightly from that of a soil garden. This is mainly due to the fact that most hydroponic nutrients are already broken down into a form which is ready for plant absorption and do not rely as heavily on the microorganisms which live in and around the plant’s root mass. The ideal pH range for nutrient absorption for most plants in a hydroponic system falls in the 5-6 pH range. The maximum absorption range for many plant species falls in the 5.5-5.8 pH range. Hydroponic systems do not have media that will buffer the pH like soil and, because of this, more rapid fluctuation of the pH value can occur. This is one of the reasons why it is so important for hydroponic gardeners to monitor the pH levels of their nutrient solutions on a regular basis.
To measure the pH of a soil, a grower can use either a soil specific pH device or create a liquid soil solution for further testing. Soil specific pH meters have specialized probes for inserting into a moist soil. These meters are very handy for hobbyists and will give a grower a good idea of what the pH value of the soil is. Another option for the gardener is to create a liquid soil solution for further pH testing. In order to create a soil solution, the grower should take a sample of soil and let it dry out completely. Next, take 20g of the dried soil and place it in a small jar with a lid. Add 100ml of distilled water to the jar with the soil. Please remember, distilled water must be used otherwise the water’s pH will affect the end results. Place the lid on the jar and shake vigorously for three to five minutes. Allow the mixture to settle overnight and then shake it again the next day. Let the solution settle and then strain the solution. The pH of the strained liquid can be tested with litmus paper, liquid pH test drops, or a digital pH meter. Although using a soil specific device is less time consuming, some horticulturists like the soil solution method better as they believe it gives a more accurate measurement.
There are three components that affect the pH of a hydroponic nutrient solution: the water source, the nutrients and bacteria. A water filtration system, like a reverse osmosis machine, is a great tool for hydroponic gardeners as it reduces the likelihood of pH problems that stem from the water source. In most cases, a hydroponic gardener with a pH balanced water source can mix the nutrients as directed and then measure the solution’s pH with litmus paper, liquid pH test drops, or a digital pH meter. A hydroponic gardener who is serious about his or her hobby should invest in a digital pH meter as this will provide the most accurate readings.
There will be times when the pH of a soil or hydroponic solution will need adjustment in order to bring the value back into the desired range. Most soil gardeners, who experience pH issues, have a pH range that has become too acidic. Dolomite lime is a great, fast working additive that can raise the pH of a soil. Dolomite lime is a good choice for beginners because it has a neutral pH which means it can never over-correct the pH of an acidic soil. Another great choice for raising the pH of an acidic soil is oyster shell. Oyster shell will take a little bit longer to work than dolomite lime but will also add a healthy dose of calcium to the soil. In case a gardener has experienced the rare instance when the soil’s pH has become too alkaline, he or she can rectify this problem by adding fresh sphagnum peat moss or compost to the soil. Both sphagnum peat moss and compost will naturally lower the pH of a soil. However, sphagnum peat moss and compost will not solve a pH issue instantly. For alkaline soils where immediate pH adjustment is necessary, a horticulturist can use aluminum sulfate which turns the soil more acidic as soon as it dissolves into the soil.
The need to adjust the pH of a hydroponic solution is much more common than the need to adjust the pH of a soil. In fact, many hydroponic systems will need daily adjustment in order to maintain a value in the desired range. The easiest and fastest way to make a pH adjustment to a hydroponic solution is to use liquid pH buffers. These pH buffers are generally sold under the generic names of “pH Up” and “pH Down”.
As their names imply, “pH Up” is designed to raise the pH of the solution and the “pH Down” is designed to lower the pH of the solution. When making a pH adjustment, don’t forget that the pH scale is exponential. It will take much more “pH Up” buffer to raise a pH value from 4 to 5 than it would to raise it from 5 to 6. Keeping this important fact in mind while making adjustments can help the horticulturist incrementally adjust the amount of “pH Up” or “pH Down” as it affects the solution’s overall pH value. After a little practice, most horticulturists get a good feel for the adjustment process. When first starting out, it may be wise to do all adjustments slowly and test the pH value frequently throughout the process. This will prevent the gardener from having to adjust the solution multiple times due to overshooting his or her target pH.
All gardeners can benefit from testing and adjusting (if necessary) the pH value of the soil or hydroponic solution. In many cases, gardeners who have never tested the pH before will discover that the pH of the garden may not be in the ideal range for optimal plant performance. A slight adjustment to the pH could be all a horticulturist needs to correct nutrient uptake problems in the garden. Measuring the pH is just one more way a horticulturist can take control over one of the variables which influence plant growth. A horticulturist who tests and maintains a consistent pH value will be helping his or her plants efficiently uptake all the essential nutrients they need which, in turn, allows the plants to produce vigorous growth and generous 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.