Regional Gardening Reports :: National Gardening Association

In the Garden:
Coastal and Tropical South
July, 2010
Regional Report

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Bountiful red holly berries later in the year depend on healthy green berries now.

Soil Chemistry 101

Part of the grand diversity of our regions is the vast difference in soils. From gumbo clay soil in Louisiana to silky sand in Florida, there is a wide range of types and textures of soil.

Soil chemistry
The ability of a plant's roots to absorb particular nutritional elements is dependent on the chemical reactions inherent in its soil. These reactions are measured in what is called the pH spectrum. The way the organics and minerals in soil react produces "acidic' or "basic" conditions as indicated by pH values. The pH scale extends from 0 to 14, where 7 is considered neutral. pH numbers higher than 7 are naturally found in soils of the West and Southwest, while soils in the Southeast, East and Pacific Northwest are older, acidic soils. What might seem like esoterica is important to gardeners when plants suffer because the soil pH is too far out of their preferred range.

Most vegetables grow best in soils with a pH between 6 and 7, turf grasses at 6.0 to 7.5. Since many soils in our regions are naturally acidic, it may be necessary to "sweeten" the soil (or raise its pH slightly) by applying lime in order to grow both vegetables and turf grasses successfully. Container soils may also be too acidic because they are made of peat or pine forest byproducts. If the lawn or vegetable patch just does not do well, despite your best efforts, test the soil pH. If lime is needed, use pelleted products (to keep from inhaling dust) following soil test recommendations or at a rate of 5 lbs. per 100 sq. ft for established plantings and lawns. Test again in 6 months to see how the pH has changed.

Acid loving plants
Most plants tolerate a wide range of soils, but a large group much prefers acidic soil conditions. This category of beloved shrubs, trees and perennials is grown across our regions, even where the soils require amendment to adjust pH and improve drainage or enhance water-holding capacity. Shrubs and trees such as azalea, camellia, sasanqua, gardenia, holly and ixora (commonly called Flame of the Woods) are not native here, but are acid lovers. If pH conditions are too alkaline, azaleas cannot take up iron and will develop iron chlorosis. The leaves look yellow with green veins, like the green has been sucked out of them. Many will apply iron, and that will remedy the situation temporarily, but chlorosis will reoccur unless the soil pH is adjusted.

Natives like clethra, fothergilla, dogwood and beech derive from the old, acid soils of our region and, like native blueberries and grapes, will grow best at pH between 4.5 and 5.5. Even some vegetables will be more bountiful at pH slightly lower than 6, including strawberry, radish and sweet potato. Amending the soil with sulfur will lower the pH if needed, but regular additions of compost often accomplish the same goal.

Conversely, if you want to grow asparagus, check to be sure the pH is safely above 6.5. Although the traditional way to raise the pH is to add lime, organic and sustainable practitioners may choose to make a slurry of eggshells to drench the soil. Test before planting and 3-6 months after to determine if additional applications will be needed.

Changeable pH
If you doubt the power of pH to tie up nutrients, or have a penchant for science experiments, play with hydrangeas for a season or two. The color of their flowers depends on the pH of the soil. Sandy, alkaline soils (5.5 and above) produce pink blooms while old, organic-based soils (pH between 4 and 5) turn the flowers blue. Soils with a pH about neutral or in the process of being changed will deliver flowers in odd lavender or purple shades. To see if you can change your French hydrangeas from pink to blue or vice versa, try this: add half cup of garden lime to the soil around blue hydrangeas now and again in the fall, or, in the fall, begin adding half cup of aluminum sulfate to the soil around pink hydrangeas and repeat that again next spring. The flowers will begin to change their colors in the first season if they can, but it usually takes 2 years of treating the shrub for transformation to be completed.

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