pH Demystified

03 February 2010

In The Garden With the Fluvanna Master Gardeners
By Irene C. Burke - Fluvanna Master Gardener
Fluvanna County Extension Office: 434.591.1950

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A thorough technical explanation of pH requires some understanding of mathematics and chemistry. A functional definition tells us how to use pH. Before exploring the usefulness of pH, lets highlight some important points.

TThe p is the math part (p for negative logarithm) and the H is the chemistry part (H for hydrogen ions). The math part tells us that pH is a scale measuring the acidity (the hydrogen part) of a solution whether orange juice or garden soil.

The pH scale, which goes from 0.0 to 14, is most peculiar. Each number below the previous one is ten times more powerful. The higher numbers on the pH scale are not more acid. The higher numbers are less acid, meaning more alkaline.

A soil that has a pH of 4.5 is ten times more acid than one that has a pH of 5.5, while a soil that has a pH of 8.5 is ten times more alkaline than one that has a pH of 7.5.

The acidity of the soil is important because most vegetation thrives when the soil is slightly acid or has a pH of 6.0 to 7.0. There are always outliers like rhododendrons, needing a pH of 4.5 to 5.5 or even 6.0, and lilacs, a pH of 7.5.

If your soil tests at a pH of 6.0 to 7.0 you dont need special pH amendments to grow healthy plants. To lower the pH slightly for acid-loving plants, work in an acid-forming fertilizer like ammonium sulfate. To raise the pH a bit, add ground limestone calcium carbonate or calcium-magnesium carbonate (dolomitic lime).

The application rate for pH amendments is found in soil test results. When you submit the soil sample, tell the soil-testing lab what kind of soil you have (clay or sandy) and what you want to grow in the soil. Pick up a soil test kit at any Cooperative Extension office or the Fluvanna Farmers Market.

What about wood ashes, (never coal ashes)? They can raise the pH, but at twice the rate for limestone and no more than 20 pounds per 1000 square feet. Dont allow the ashes to touch seedlings or plant roots. Spread a -inch layer in the winter and then work the ashes into the soil in the spring. Check pH every year if you have applied wood ashes.

Raising pH for an established landscape requires multi-staged surface applications over time with effects limited to a 4-inch depth. So the springtime ritual of applying lime to your lawn will not reach the deeper roots of your tall fescue or Kentucky bluegrass.

The most important impact pH has in your garden is nutrients and minerals. Acid soil makes some unavailable, while alkaline soil makes others concentrate at toxic levels. Pouring on synthetic fertilizers with beneficial nutrients and minerals wastes your time and money if soil pH blocks absorption.

It is better to let pH dictate your choice of plants rather than battle such a potent natural force. There are trees and shrubs, annuals and perennials for most pH levels from 4.5 to 8. Enter pH in the search window at http://www.ext.vt.edu/ to find and welcome them to the soil you have.

Tip of the Week

2010 is the International Year of Biodiversity. To keep insect pests and disease to a minimum, to reduce your gardening costs, plan for variety. When losses are confined to one or two plants, devastation is limited.