Institute of Food and Agricultural Sciences
Hendry County Horticulture News
pH Problems Produce Peaked Plants
Are your trees or bushes looking peaked, vegetables and flowers performing poorly despite your having watered, fertilized and provided tender loving care? You may have a pH problem. A which problem, you counter? What is all the fuss about pH? Soil pH is an index of soil acidity or alkalinity. It is important to know what soil pH you are dealing with as it has a great deal of influence on plant growth.
Soil pH is significant because it has a great deal of effect on which nutrients are available to your plants. When the pH is too high or too low, your plants may get too much of some nutrients and too little of others. At high pH (alkaline), certain minerals, such as iron and manganese, react chemically in the soil and are "tied up" in forms that are unavailable to plants. This is why many plants including gardenia, crape myrtle, some palms and azalea display chlorosis or yellowing of their leaves when growing on alkaline soils. The availability of boron, copper, and zinc is also limited at high pH and may cause deficiency symptoms on plants grown on alkaline soils.
Phosphorus availability is limited at either high or low pH and is most available to plants at soil pH ranges from 6.0 - 7.5. On certain low pH (acid) soils, the solubility of some minerals (e.g. aluminum) may increase to levels that are actually toxic to plants. In addition to the effects of soil pH on plant nutrition, it also influences the balance and health of the soil microbes and other organisms. Most beneficial microorganisms are prefer a soil pH of between 6.0 and 7.0. Earthworms do not thrive in very acid soils. Given the far reaching effects of soil pH on plant performance, it is important that gardeners are aware of the pH of their soils. The pH is the single most important characteristic of the soil that gardeners can influence. Maintaining proper pH will ensure nutrient availability, good growth, and resistance to pests and diseases.
The acidity or alkalinity of soils is measured on the pH scale. A pH of 7 is neutral, while a pH above 7 is alkaline and below 7 is acid. The pH of Florida soils may ranges from 4 (very acid) to 9 (very alkaline), although such extremes are not commonly encountered.
The pH of a soil is influenced by several factors including, rainfall, vegetation and parent material. Sandy soils under high rainfall that are subject to leaching and covered by thick mats of leaves or pine needles tend to be acid in nature. Many of our soils have alkaline pH readings of between 7.0 and 8.2 due to naturally occurring lime (lime rock, marl, or shell) in the soil. The same is often true of soils near concrete structures and marl or shell driveways or roads, where excess lime has raised the soil pH to alkaline levels. The subsoil fill material used in our area often has a high lime content and causes pH related problems to the gardener. Irrigation water with high lime content can also cause a rise in pH.
The optimum pH for the majority of plants is 6.5 to 7.0. Plants adapted to a lower pH are called "acid loving." Plants that do well under acid conditions (pH 5.5 -6.0) include azalea, bahia grass, blueberry, gardenia, hibiscus, holly, ixora, and podocarpus to name a few. Other plants are tolerant of alkaline soils. These include ash, butterfly bush, elm, red cedar, sycamore, and yucca. Soil pH tests can be arranged at the Hendry County Extension Office and local garden centers.
A soil pH that is not optimum for plant growth may be corrected by adding materials to the soil to change the pH. Consider correcting the pH of your soil only when it is appreciably higher or lower than ideal. If your soil pH is within 0.4 pH units of the ideal range, adjusting pH will have little effect on plant performance. Liming acid soils will raise pH. The amount of lime applied should be based on a soil test, which will determine the pH and buffering capacity of the soil. Addition of lime without a soil test may result in over liming and increased problems.
High pH soil may be acidified by adding elemental sulfur. Sulfur is transformed by soil bacteria to sulfuric acid which will neutralize soil alkalinity. Like lime, sulfur should not be applied indiscriminately. Due to the total amount of lime present in alkaline soils, this is a never ending battle. As soon as the sulfur is "used up" pH will begin to return to original levels. Sulfur is useful for reversing the effects of over liming or for changing soil pH in a small area for specimen trees or shrubs. Addition of organic matter and use of organic mulches can also help acidify soils and lower pH levels.
Many plants are tolerant of a wide range of pH values and should be considered for use on soils where pH levels are high or low. Other plants thrive under high or low pH conditions and should be selected for use on sites according to the pH of the soil. Other techniques for coping with high pH soils include use of foliar nutritional sprays to provide micro nutrients and the application of smaller amounts of soil applied fertilizer more frequently to compensate for the "binding" effect of high pH soils.
Soil testing and an understanding of the effect of soil pH and how to compensate for less than optimum conditions can make the difference between gardening success and failure. Good luck and good gardening.
Gene McAvoy is the horticulture agent with the Hendry County Extension Service. Direct your horticulture questions to PO Box 68, LaBelle, FL 33975, e-mail - firstname.lastname@example.org or phone 863-674-4092 or 863-983-1598. You are also welcome to visit the Hendry County Extension Office at 225 Pratt Blvd., LaBelle. Office hours are from 8:00 - 5:00.
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