Soil pH and its Effect on Nutrient Availability

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There are 18 elements necessary for plant growth. They are Hydrogen, Oxygen, Carbon, Nitrogen, Phosphorus, Potassium, Calcium, Magnesium, Iron, Manganese, Sulfur, Boron, Copper, Zinc, Molybdenum, Chlorine, Nickel, and Silicon. Soil pH directly affects the growth and quality of many landscape plants by influencing the chemical form of many elements in the soil and soil microbial processes. For example, landscape plants may exhibit nutrient deficiency or toxicity symptoms as a result of highly acidic or alkaline soils pH. In acidic soils, the availability of plant nutrients such as Potassium, Calcium, and Magnesium is reduced, while the availability of potentially toxic elements such as Aluminum, Iron, and Zinc are increased. In alkaline soils, Iron, Manganese, Zinc and Boron are commonly deficient.

So what is soil pH? Soil pH is a measure of the acidity or alkalinity of the soil. On the pH scale, a value of 7 is neutral, a value less than 7 is acidic, and a value greater than 7 is alkaline. Florida soils can vary widely in pH, depending on the “parent material” from which the soil formed or on the management of the soil. For example, soils formed under pine flatwoods can be quite acidic. In contrast, soils formed from calcium carbonate-bearing materials like limestone, marl, or seashells are alkaline. Alkaline conditions are common in coastal soils and the mineral soils of south Florida. It is also common to encounter alkaline soils in the home landscape as a result of calcium carbonate-rich building materials (i.e., concrete, stucco, etc.) that may be left in the soil following construction.

Most common landscape plants are well suited to a wide soil pH range. For example, popular woody shrubs and trees (e.g., pittosporum, viburnum, oak, and pine) grow well in acidic to moderately alkaline soils. However, there are a few acid-loving plants like azalea and gardenia that do not grow well in soils with pH greater than 5.5. St. Augustine, Zoysia, and Bermuda grass prefer a pH range between 6-7 while Centipede prefers a range between 4.5-6.

The best advice about dealing with soil pH is to choose landscape plants suited for the natural pH of your landscape soil. While some soil additives can raise or lower the pH of soils, the effects of these materials are often short-lived. In addition, if your soil pH is within 0.5 of a pH unit of the ideal range, adjusting the pH will probably not improve plant performance. However, if you want to try to change your soil’s natural pH to grow a specific plant, you have the following options.

To raise the pH of acidic soils, add a liming material like calcium carbonate or dolomite. Dolomite has the added benefit of supplying Magnesium, which is often deficient in Florida soils. Have your soil tested before applying any liming materials because many of Florida’s natural and urban soils have an alkaline pH. If a soil pH test indicates that your soil is acidic, it is important to test for the lime requirement before applying any liming materials to the soil. The lime requirement test measures your soil’s natural ability to resist (buffer) changes in pH. This test is part of the standard landscape and garden soil test offered by the UF/IFAS Extension Soil Testing Laboratory. Results of this test will indicate the amount of agricultural limestone you should apply to a specific area to reach a target pH.

In established landscapes, lime can be surface-applied and watered in, but take care not to overwater (e.g., no more than 0.5 inches of water over the treated area).

Unlike liming, lowering the pH of strongly alkaline soils is much more difficult if not impossible. In fact, there is no way to permanently lower the pH of soils formed from high Calcium materials, such as marl or limestone, or soils severely impacted by alkaline construction materials. In these circumstances, it is best to select plants that are tolerant of high pH conditions to avoid chronic plant nutrition problems.

Soil pH can be temporarily lowered by adding elemental sulfur. Bacteria in the soil change elemental sulfur into sulfuric acid, effectively neutralizing soil alkalinity. However, the effect of elemental sulfur is localized to the area that was amended, and the effect is temporary. Soil pH will begin to rise shortly after soil bacteria exhaust the added sulfur supply. This effect prompts repeated applications of sulfur to ensure that the soil remains at the desired pH. Using sulfur to amend a soil is complicated. Adding sulfur at high rates or applying it too frequently can damage your plants. If you decide to apply sulfur, be sure to look for signs of plant response after the application.

Always consider the pH of your soil when selecting new plant material for your home landscape or garden. Take action to correct soil pH only when it is substantially higher or lower than the desired pH for the plants you are growing. To avoid damage to your landscape plants, always have your soil tested for pH and lime requirement (if soil pH is acidic) before adding lime or sulfur to the soil.

 

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