Pennies—Good for Thoughts, but Not Tomato Blight

Tomatoes - cherry Sunsweet @home LAH-001I recently ran across an article claiming that a penny can help your tomato plants fight off blight. Apparently, the reasoning goes like this:

  • Copper is known to kill molds, algae, fungi, and microbes.
  • Pennies are made out of copper.
  • Therefore, inserting a penny into a tomato stem (or burying it at the roots) will keep your plant from succumbing to diseases caused by molds, algae, fungi, or microbes.

Since I’ve lost tomato plants to Early Blight, a common problem in Colorado, I’d love for this idea to work. I have some pennies lying around; let’s put them to good use, right?

Well, not really.

First of all, let’s learn a bit about copper and how it relates to soils and plant growth.

Soil fertility primarily depends on the big three: nitrogen, phosphorus, and potassium. However, there are a number of other elements also necessary for plant growth, just not in such large amounts. Copper is one of these essential micronutrients. Copper deficiencies are actually quite rare, as soils naturally contain between 2 to 100 parts per million copper, with an average of 30 ppm. In comparison, plants contain about 8 to 20 ppm. However, copper availability can be affected by both pH and organic matter.

Plants can’t use elemental copper; it has to be dissolved in an acid to be taken up by roots. Therefore, the pH of your soil matters—too alkaline (over 7.5 or so) and the copper is unavailable, even if it’s present. Similarly, organic matter interferes with the available of copper. Plants grown in soils too high in organic matter* can show signs of copper deficiency. Look for poor growth, delayed flowering, and plant sterility. The leaves may wilt and turn a bluish-green color. (If this describes your garden, don’t just assume you have a copper deficiency, as other factors can cause similar symptoms.)

If a soil test determines that available copper is lacking, you can apply more in the form of copper sulfate, copper chelate, or copper oxide. Be very careful to precisely follow the directions. While a lack of copper is problematic, too much is toxic. Even worse, copper is very slow to leach from soil, so once it’s there, you’re pretty much stuck with it.

Symptoms of copper toxicity are surprisingly similar to those of copper deficiency: stunted growth, reduced plant vigor, reduced seed germination, and insufficient iron uptake. The foliage will acquire a bluish tinge before turning yellow, then brown.

It’s true that copper has anti-microbial properties. For example, copper sulfate sprays have long been used to control powdery mildew and other fungal diseases. In fact, the most common cause of copper toxicity is the overuse of copper-containing fungicides.

Now, what about using a penny to prevent tomato blight?

584px-2005-Penny-Uncirculated-Obverse-croppedFirst of all, modern pennies don’t contain much copper. Before 1982, they were 95% copper, but since that time they’ve been made of 97.5% zinc, with only a thin copper coating. If you can’t read the date on your coin, try weighing it—copper is heavier than zinc.

But let’s say you have an older penny you’re willing to donate to your garden. Will that work? Consider:

  • Copper is largely insoluble. The copper in the penny stays in the penny.
  • The copper in pennies is elemental copper, not a copper salt that a plant can absorb.
  • Soils naturally have plenty of copper.
  • If plants show a copper deficiency, it’s because the copper is unavailable. A penny won’t solve that problem.
  • And one more thought: Soils are full of microbes that play an important role in decomposition and nutrient availability. Do we really want to indiscriminately kill all of them?

I think I’ll be spending my pennies, rather than burying them or sticking them into a tomato stem. I can save my plants from Early Blight by choosing resistant cultivars, rotating crops, and keeping soil (the source of the fungal spores) off the leaves by staking (or caging) and mulching.

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