No Phooling Soil Acidity, Alkalinity Determines Nutrient Use

By Lindsay Bond Totten Scripps Howard News Service | St Louis Post-Dispatch (MO), November 25, 1993 | Go to article overview
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No Phooling Soil Acidity, Alkalinity Determines Nutrient Use


Lindsay Bond Totten Scripps Howard News Service, St Louis Post-Dispatch (MO)


Few gardeners are blessed with perfect - or even good - soil. Garden loam is a product of hard work, gardening skill and time.

The major component of any soil - clay, slit, or sand - reflects the geology of the region. Previous uses determine its structure.

Along with its composition, each soil has a pH - the measure of how acid or alkaline a soil is. You can't see it or touch it, but pH is important to gardeners and their plants.

On the pH scale (0 to 14), pH 7.0 is neutral. Soils below 7.0 are acid (sometimes called "sour"); those above 7.0 are alkaline ("sweet"). A pH reading of 9.0 is 10 times more alkaline than one of 8.0. Small increments on the pH scale therefore represent distinct changes in soil pH.

Native soils around the country range from 4.0 to 9.0. Generally, those in arid regions, such as the west and southwest, are alkaline. Where rainfall is plentiful, soil tends to be acid.

While general guidelines are helpful, they're not accurate enough to depend on. Only a soil test can give a baseline reading.

As a service to farmers and gardeners, state universities sponsor soil testing programs through their respective cooperative extension services. The inexpensive tests tend to be more reliable than home testing kits.

What is the optimum pH? Vegetables prefer a range of 6.2 to 6.8, with potatoes on the low end of that scale (mildly acid soil helps prevent scab) and cabbages and their relatives on the high end (neutral soil discourages maggots and club root.)

Evergreens, including spruce, pine, holly and rhododendron, generally prefer acid soil, with a pH in the range of 5.0 to 6.0. In the east, deciduous shrubs and flowers tend to grow best if the pH remains mildly acid, around 6.0 to 6.5. In the West, native plants are accustomed to more basic (alkaline) soils.

A soil's pH is closely related to nutrient availability. Iron, for instance, is most readily available to plants when the pH rests somewhere below 5.5. Conversely most other nutrients, including phosphorus and calcium, are severely limited when the pH is that low.

"Acid-loving" plants such as pin oak and rhododendron don't need acid soil as much as they crave iron. If they can get enough iron (difficult to do in alkaline soil because iron is bound tightly to soil particles), they really wouldn't care so much about the reading on the pH meter.

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