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GARDENING

To Get the Best Dirt, Test the Soil and Learn to Level With It

August 24, 1996|From Associated Press

There are some basic things that are good to know about soil--the pH level, the soluble salt level, the presence of fertilizers and the amount of magnesium, calcium and trace elements.

The Perennial Plants Assn. gives this advice mainly to professional plant growers, but it's a good checklist and caution for home gardeners as well.

Soil Solution pH

Soil solution pH will affect how nutrients are taken up, writes Paul A. Thomas of the University of Georgia in the association's quarterly magazine. "If your pH is above 7.0, your plants may not take up manganese or iron and become chlorotic. If the pH is below 5.0 you may have plants that take up too many trace elements."

(Chlorosis is an abnormal condition of plants in which the green parts lose their color or turn yellow as a result of lack of chlorophyll production due to disease or lack of light. Many universities recommend a pH between 5.0 and 6.5.)

Soluble Salts

The more lime and fertilizer in the soil, the higher the soluble salt level, Thomas says. "Excess salts can cause root burn by increasing dehydration of the root cells as the soil becomes dry. Fertilizer salts can actually draw water from root cells under dry conditions."

Soluble salts are calculated in millimhos, which is a measure of the electrical conductivity of the soil solution. For seeds and cuttings, levels above 0.75 mmhos can cause problems. For established plants, above 1.5 mmhos will do the same.

"If you know you have high salts, keeping the plants evenly moist will prevent damage, but you need to know," Thomas adds.

Fertilizers

Thomas recommends testing for both ammoniacal and nitrate forms of nitrogen.

"Just because a fertilizer is 20-20-20 doesn't mean it's suitable for your crop," Thomas says. "Some plants do not grow well in the presence of high NH4. Pansies are a good example. Most plants do very well in high NO3, low NH4. So why do soil producers use high levels of NH4? I don't know."

Trace Elements

Regarding trace elements, Thomas notes that lime is added to most but not all soils to adjust or buffer the pH.

"Plants also require magnesium and calcium, so its presence is welcome in most cases. Excess lime or trace elements can cause serious problems, however, and testing your soil before you use it will verify that no mistake was made at the soil production site," he says.

Indicators

Foliage is a good indicator of overall crop nutrition and root function, Thomas says.

"A common tendency to avoid is to test visually. If the foliage turns chlorotic, it's a deficiency or toxicity, right? But which one is it? You can't tell without testing both soil and foliage."

Water tests are rated essential with changing seasons.

"You may be surprised how much pH and hardness change over that time. In Georgia, I have a client who has water that is pH 6.9 in June and a whopping 8.3 in December," Thomas says.

He also recommends testing on a schedule rather than on a crisis or need-to-know basis.

"Crisis management doesn't work with plant production, never has, never will," he says.

If you are unfamiliar with soil testing procedures, check your local Cooperative Extension Service office or state university for suggestions.

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