How to Find your Soil’s PH

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Sometimes plants grow poorly for no apparent reason. A healthy plant can be set in what seems to be a good soil, can receive plenty of sunlight and water and still look sickly and perform poorly. If you have areas around your home where plants react this way, check for a soil problem, especially the wrong soil reaction or pH.

Now, your next question, what is pH? Well, pH is nothing more than a chemist’s shorthand for describing the amount of hydrogen in the soil. The capital letter “H” is the chemical symbol for hydrogen and pH is a figure describing the concentration of hydrogen in the soil, which in turn determines the acidity of the soil.

Simple explanation of pH: pH is a measure of its ratio of hydrogen atoms to hydroxide radicals, which are molecules composed of one oxygen and one hydrogen atom. If the ratio is one-to-one, the solution is neutral, and its pH is 7. A low-pH solution is acidic and a high-pH solution is basic (alkaline).

A simple numerical scale is used to express soil pH. The scale goes from 0.0 to 14.0. The midpoint, pH 7.0, is the value for pure water, which is called neutral – pure water, is neither acid nor alkaline. Figures below 7 on the scale indicate acid or ‘sour’ soils and the lower the number the more acid the soil. Each whole number drop in pH denotes ten times the acidity. For example, a soil having a pH of 5 is ten times more acid than one having a pH of 6. Numbers above 7 indicate alkaline or ‘sweet’ soil. Again, the higher the number the
more alkaline (less acid) the soil.

soil ph
low soil phpH range preferred by acid-loving plants
high soil phpH range preferred by most garden plants
*Soils nearing extremes require professional intervention to modify pH.

Two good examples of acid and alkaline liquids are orange juice and seawater. Orange juice has a pH of about 3.7 (very acid) while seawater has a pH of 7.9 (slightly alkaline).

Soil pH is very important because it influences several soil properties that directly affect growth of plants. Soil pH effects soil bacteria and nutrient leaching and nutrient availability.

Most plants have a range of tolerance to pH. Plant nutrients are generally most available in the pH range of 5.5 to 6.5. This is also a good range for soil bacteria. And, most important, this is the best range for most plants.

Certain plants thrive best on acid or alkaline soils having pH values out of the usual “best” range. For example, azalea, bougainvillea, croton, dogwood, gardenia, hibiscus, ixora and magnolia grow best in acid soils having a pH below 5.5. These plants grow poorly and many have yellow leaves in areas where soils are alkaline. In contrast, cabbage palms, yucca, sea grape and many other plants grow quite well on alkaline soils.

A pH determination will tell whether your soil is within a range that produces good growth but it is not an indication of fertility. If the pH is not optimum, it can be raised or lowered using chemicals available to all homeowners.

If excess alkalinity exists near the house, the only solution is to remove the soil and replace with a more suitable material. If the soil is too alkaline because of liming or a minimum of natural alkalinity, several acid-forming materials can be used to reduce the soil pH.

croton foliage
Crotons grow best in an acid soil.

Elemental sulfur can be used to acidify a soil. About 10 pounds of sulfur applied over a 1000 square foot area will decrease a sandy soil one unit in pH=like from 7.0 to 6.0. Sulfur must be washed into the soil immediately after application or it will severely burn grass or shallow plant roots. Even with the utmost care some burn may occur after an application of sulfur. The best approach is to apply an acid forming fertilizer such as ammonium sulfate. This material applied at the rate of 5 pounds per 1000 square feet of area will acidify the soil without burning the grass.

Normally, lime or dolomite is used to increase the pH or sweeten the soil. Lime contains calcium carbonate while dolomite contains both calcium and magnesium carbonate. Another material, hydrated lime, can be used for quick change in soil pH but it can severely burn plants and is seldom recommended for home use.

To increase the soil pH one unit, say from 5 to 6, apply about 50 pounds of dolomite or lime over a 1000 square foot area. Lime materials are slow to react so six months may be required before the soil pH rises.

You can buy a pH tester for more accuracy (needs no batteries… a great gardening tool!). But until you have one, there is Do It Yourself (DIY) method that can give you an idea of your soil’s pH. Using Just two ingredients that are probably already in your home: vinegar and baking soda.

This little “quickie” method won’t give you a specific pH number, but you will be able to tell if your soil is more alkaline, acidic, or neutral. Note: Ideally, distilled water is neutral, with a pH of 7.

Vinegar: Take a dry bit of soil (from where you want to test… a 1/4 cup or so) and mix it with distilled water to make mud. Pour white distilled vinegar on top of your mud. If the mixture fizzes, it is more alkaline.

Baking soda: Mix dry soil and distilled water as above and sprinkle baking soda over the top of the mud. If the mixture bubbles, it’s acidic.

If neither test produces a reaction, you have fairly neutral soil.

To sum up the pH problem follow these steps. First, find out the pH value of the soil where planting is planned (kits are available for pH determination). Second, learn the pH preference of plants you choose to grow. Last, if necessary, raise pH or lower pH of the soil to best suit the plant. Or if this isn’t possible, select a plant which will thrive at the natural pH of the soil.

pH Plant Preference Charts

See also…

Identifying Your Soil Type
How pH Affects Plant Foods
Raising Soil pH
Lowering Soil pH

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