I love a mystery - only if I can solve it. Problem solving was my job at the U.MD Extension Service as well as at Garland's Garden Center.

For the last month I've been trying to figure out what caused the leaves of my two containers growing Catharanthus (otherwise known as vinca, periwinkle, or Madagascar periwinkle) to turn yellow. The flowers looked great.
In a January, 1990 Greenhouse Manager article entitled "Pointers on Germinating Periwinkle", it states that the seed needs a temperature of about 80 degrees F and that the growing medium should have a pH of 5.5 to 5.8 Not a very wide margin and a rather low acidic pH. That's when I decided to muddle through the info I hadn't yet downsized and learn more about the significance it may have on my vinca with the yellow leaves.
For as long as I can remember, I just assumed that plants which prefer growing in shade such as azaleas, rhododendrons, hollies, etc. grow their best in an acid soil with a low pH. That's pretty much true. On the other hand, I believed that plants which prefer growing in the sun for 6 or more hours, do their best when growing in a sweet alkaline soil with a pH at or above 7. I was wrong.
In case you forgot, the letters pH stand for potential Hydrogen and is a measure of relative acidity or alkalinity. The pH scale ranges from 0 to 14 with 7 being neutral or basic. As the pH goes down from 7 towards 0 it becomes progressively less basic and more acidic. As the pH rises above 7, it becomes less acidic and more alkaline. A very high pH is termed "highly alkaline." What confuses many people is when a soil test shows a quite low pH, it is said to be "highly acidic." I think the term should be "very acidic." The soil pH directly impacts the availability of nutrients for the plants, especially the micronutrients. So, if a soil's pH is too low (very acidic), most likely calcium, phosphorus, magnesium, and molybdenum will be deficient. If a soils pH is too high, iron, manganese, boron, copper, and zinc will most likely be deficient.
In an article written by Dr. Francis Gouin he talks about transplant shock. That's when I remembered tearing off the bottom brown, crowded, roots and then planting them. He wrote that magnesium is necessary for chlorophyll development. He recommended dissolving a tbsp. of epsom salts into 2 gallons of water and pouring it into the soil. He further stated that it would not alter the pH of the soil.
Since I had an already prepared gallon water jug containing 1 tsp. of epsom salts and 1 tsp. of a water-soluble fertilizer for my scented geraniums, I poured it on. (In Faye Brawner's book -Knowing, Growing, and Enjoying Scented Pelargoniums" - Interweave Press- she recommends the water- soluble fertilizer every other watering. Then she further recommends adding a tsp. of epsom salts to a gallon of the 1 tsp. of water-soluble fertilizer every 4th time you fertilize. That's a complicated solution).
After deciding to stop my search for more info, I want to tell you what I verified. Apparently the mature vinca enjoys a nearly basic pH just around 7.0. When it is planted outside in warm soil, it can

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September 2005

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