Miron Hlywiak wrote, "After building some walls in the basement, I had several pieces of Sheetrock wallboard left over. Wallboard is made from gypsum (calcium sulfate). Garden lime is calcium carbonate. I wondered if I could till the wallboard into the soil to lower the acidity instead of buying lime?"

Although calcium sulfate and calcium carbonate do seem like similar chemicals, in the soil they act differently. It is the sulfate and the carbonate parts of the molecules that have the potential to decrease the acidity of the soil. It turns out that carbonate combines with positively charged hydrogen ions set loose by calcium to form carbon dioxide and water. Both those substances migrate out of the soil, taking the hydrogen ions with them. It is the loss of hydrogen ions that causes soil pH to rise, i.e., for the soil to become less acid.

Conversely, sulfate does not participate in any soil reactions, nor does it carry any hydrogen ions away. Therefore, adding calcium sulfate (gypsum wallboard) doesn't affect soil pH.

Sorry, Miron!

Fix for blossom end rot

On the other hand, most gardeners have likely heard claims that gypsum can be added to clay soil in order to soften it and make it more tillable. This is perfectly correct if and only if the soil has lots of sodium in it. We have lots of clay in Georgia, but, unfortunately for the gypsum folks, our soil does not have much sodium present. You can add as much gypsum as you like, and it will have little effect on your soil-shoveling labors.

Out in Monroe, Ed Diemmer was curious as to whether the calcium sulfate in wallboard could be used to add calcium to his soil to counteract blossom end rot on his tomatoes. This possibility has a better prognosis!

Blossom end rot is aptly named. Your tomato can appear perfect from above, but when you grasp it for harvest, the bottom end is a black, gooey mess: blossom end rot.

It is caused by a lack of calcium in the fruit just after pollination. Tomatoes, plus squash, watermelon and other fruits, have a hard time moving calcium to their young fruit. Vigorously growing plant leaves steal the calcium away, causing blossom end rot unless there is plenty of calcium to go around in the plant's tissue.

The disease can be thwarted by keeping the soil evenly moist, by spraying the young fruit with calcium chloride or by adding calcium to the soil before planting. That's what Ed wants to do, and for his purpose gypsum wallboard is fine. However, he doesn't need much -- only a couple of tablespoons per plant. I suppose he could bury a piece of wallboard beside each tomato at planting and let the tomato absorb calcium when it needs it this summer.

Hydrangea color change

Carole Danielson says that three years ago she planted some 'Forever Pink' hydrangeas. In that time, the blooms have turned more purple than pink.

Hers is a perfect example of how soil pH affects a plant. The hydrangeas are changing color because her soil has gradually become more acidic. In acid soil, the aluminum that causes hydrangea blooms to be blue is chemically available to the plant. As the soil pH went down, the available aluminum caused pink flowers to become purple. All Carole has to do is to raise the soil pH, making it more alkaline.

How? By mixing a cup of hydrated lime (not garden lime) in 5 gallons of water and sprinkling the area around each plant with this solution. Each plant should receive 3 to 5 gallons. The hydrated lime (available in small bags from garden centers) will raise the pH quickly, perhaps "pinkening" her flowers this summer. Hydrated lime is only a short-term solution, though. Carole should scatter a cup of garden lime around each plant every year to keep the pH raised for the future.

Relate these cautionary tales to your teenagers. Perhaps they will be less likely to sleep through their science classes if they see there is a practical reason to learn about soil chemistry.

You can get more tips from Home Garden Plums at www.ces.uga.edu/pubcd/c743-w.html. To learn more about the care of your other fruit trees, download the latest University of Georgia Fruit Spray Guide at www.ces.uga.edu/pubcd/SB-40.htm.

Q: As you may know, some coffee shops give away free coffee grounds. They say they are good for your garden. How should they be applied? Should I mix them straight into the soil? If so, how much at a time? Or should they be used as part of compost?

-- David Brooks, e-mail

A: You can use the coffee grounds in your garden or in your compost equally well. The grounds are acidic, so mix a tablespoon of garden lime into a 5-pound bag before you dig them into your garden or put them on the compost pile. In the garden, apply at a rate of 5 pounds per 3 square feet. For composting, mix at a ratio of 5 pounds per 3 cubic feet of yard waste.

Q: I have successfully held over three poinsettia plants in a window greenhouse for four years. Every year they get bright red bracts. They are very leggy. When is a good time to cut them back, and how far do I cut them?

-- Catherine Denil, Crabapple

A: Cut off the bracts now, leaving only the green leaves. When it is warm enough in April to set them outside on a shady patio, cut the stems back to 6 inches tall. As the plants re-sprout, move them gradually to even more sunshine. You may need to move them to larger pots. Fertilize monthly. As they grow bigger, shorten the longest branches a couple of times to make the plants nice and compact. On Oct. 1, you can start the "14 hours of darkness, 10 hours of bright light" regimen that will cause them to form colored bracts.

Q: I want to plant some oaks in my landscape. Which ones are fast growing, drought resistant and disease resistant ?

-- Alice Youmans, south DeKalb County

A: DeKalb County Master Gardener Mary Kay Woodworth recommends, for a dry site, blackjack oak, Shumard oak, sawtooth oak, Darlington oak or live oak. For a wet site, swamp white oak, cherrybark oak and overcup oak will do well. Fast growers are overcup oak, pin oak and willow oak. For more information, download Landscape Plants for Georgia at www.ces.uga.edu/pubcd/B625.htm.