David Robson Extension Educator, Springfield Extension Center, University of Illinois
Yard & Garden
Sweet and Sour Soils?
Mix up the right recipe for your garden
Just say the words “soil chemistry” and some people will quickly look away. Many are probably already turning the page, looking for the recipes or some ad.
The word “chemistry” can conjure up these horrible images associated with high school or college. Some gleefully dream of combining Compound A with Compound B, and then blowing up the lab. Fortunately, it doesn’t work that way in our gardens.
Soil chemistry refers to all the big-three nutrients such as nitrogen, phosphorus and potassium. It also involves the 10 others: calcium, magnesium, sulfur, molybdenum, manganese, iron, copper, boron, zinc and chlorine.
Aluminum isn’t one of the top 10, but it’s important for hydrangeas if you want blue flowers. On the other hand, aluminum is quite toxic to many other plants including blueberries.
Sometimes cobalt is added to the group, but it’s not in the top 13.
But the biggie involving soil chemistry is soil pH.
Most gardeners know a little bit about pH, probably enough to scare themselves. You’ll hear them throw words around like “sweet” and “sour” soil, or acidic, basic, neutral or alkaline. Old timers will say “sweet soils” are what’s needed for plant growth, and then talk about yearly applications of lime to the soil to make plants grow better.
The technical definition of pH is the negative logarithm of the hydrogen ion content. Whew! A mouthful. Essentially, it’s a mathematical relationship based on multiples of 10. That last part is important.
A pH of 7 is neutral. Less than 7, and we say the soil is acidic or “sour.” Higher than 7, and the soil is alkaline, basic or “sweet.”
Using “sour” and “sweet” can get you in trouble. When we think of things being acidic, we think of vinegars, which definitely are “sour.” However, honey has a pH of 3.0 to 4.0, which can be more acidic than vinegar. And most of us would probably think of honey as “sweet.”
The pH scale runs from 0 to 14. If you find something with a pH on either end, and pour it over your body, you’ll probably dissolve quickly. Fortunately, most soil is hovering between a pH of 5.0 and 8.0, depending on where you live.
That’s just a difference of 3 pH points. But in reality, since it’s based on a multiple of 10. There is a 1,000 times difference between the two. So, a pH of 5 is 1,000 more acidic than a pH of 8. Even a one-point difference, say pH 5.0 to 6.0, is a difference of 10 times.
There are some Illinois soils with a more acidic pH than 5.0. Those are usually strip-mined lands. Some Chicago area soils are higher than 8.0. Soil with a strong limestone base or near limestone concentrations, such as gravel driveways or concrete, may have a higher pH.
The vast majority of Illinois plants like a pH of 5.5 to 7.0. There are exceptions such as oaks, rhododendrons, azaleas, blueberries, dogwoods, and sweet gums. If the soil is too alkaline, you’ll usually find yellowing, or chlorosis of the leaf usually between the veins. Iron is one of the elements that is more available at lower pHs, which is why some folks in northern areas have problems with oak trees.
The only way to tell the soil pH is with a soil test. You can buy kits that will give you a general idea, provided the kit is fresh. Soil test labs give a more accurate reading.
Limestone is used to raise the soil pH if the readings are too acidic. But indiscriminately adding limestone year after year may raise the soil pH where nutrients aren’t available.
Or it may be a waste of time and money. Some soils are buffered, meaning they resist any significant pH change. You might get by with changing the pH one point, but not two or three.
Garden sulfur (don’t use dusting sulfur which is a fungicide) can lower the soil pH if it’s too acidic. Ammonium sulfate, iron sulfate and aluminum sulfate can be used, though watch out for the latter. Remember that aluminum can be toxic.
David Robson is an Extension Educator, Horticulture, at the Springfield Extension Center, University of Illinois Extension, P.O. Box 8199, Springfield, IL 62791. Telephone: 217-782-6515.
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