Horsefeathers in Horticulture
Starve a cold and feed a fever. Don’t cross your eyes too long or they’ll get stuck that way. Always wait thirty minutes after eating before swimming. Drinking coffee in adolescence will stunt your growth. Shaved hairs grow back thicker. Unless you were raised by wolves, you’ve likely heard at least one of these admonitions growing up. Perhaps some of you are even guilty of perpetuating these and other old wives’ tales when raising your own children!
You can rest assured, though, that adherence to old wives’ tales like these is generally harmless. In all likelihood, the persistence of these tales may be due to the lack of scrutiny that comes with being so innocuous. Unfortunately, the same cannot be said for a lot of advice that gets circulated in the world of gardening as there are a number of practices that can cause plant disease or even death.
A prime example of this is the practice of placing gravel or pot shards at the bottom of a pot in order to enhance drainage. While this advice appears to make sense on the surface, it falls apart under closer scrutiny. The reason for this lies in a concept called “capillary action”. In simple terms, capillary action dictates that water will flow from areas of the pot with larger pores towards areas with smaller pores. Therefore, water would be expected to have difficulty moving from the areas with potting soil into the areas with gravel or pot shards—and this is exactly what happens. The irony here is so thick you can just about collect it in a bucket and pour it on some pancakes because by adding these components in an effort to improve drainage, gardeners have been effectively reducing drainage!
And while we’re on the topic of drainage, let’s consider another widely held belief for gardeners cursed with heavy clay soil conditions. Conventional wisdom suggests that because clay and sand occupy opposite ends of the drainage spectrum, then clay’s deficiencies can be remedied with the addition of sand to create Goldilocks conditions. In other words, a soil that is neither too heavy nor too light but just right in retaining the best properties of both clay and sand. This fantasy is far from reality, however, because combining sand and clay is how bricks have been made for millennia. So unless you’re willing to trade in your trowel for a chisel, the best advice would be not to take this advice. Instead, lighten clay soils by adding an amendment made of organic matter, such as compost or manure.
Speaking of organic matter, what about the notion that banana peels, egg shells, and other food scraps can be incorporated into the soil in order to increase soil fertility? While it is true that many of our castoffs from the kitchen are packed with nutrients that can be of great benefit to the plants in our garden, there are some major red flags here. The first is that food scraps have the potential to attract certain, shall we say, “less than desirable” creatures into our gardens, which of course can be problematic. And the second concern is less obvious and requires a deeper look at the decomposition process. When microbes like bacteria and fungi in the soil metabolize carbohydrates in the form of food scraps and begin to break this organic matter down, they require large amounts of nitrogen in order to synthesize amino acids. In garden soils where nitrogen is somewhat scarce, these microbes don’t always play nicely with
others. They will frequently out-compete plant roots for nitrogen, effectively creating a phenomenon known as a “nitrogen draft”. Fortunately though, order is restored to the underground universe once available carbohydrates become limited again and lead to a decline
in microbe populations. But, until this occurs the activity of these microbes can actually undermine soil fertility. With this in mind, always allow food scraps the time necessary to break down properly in a compost bin before incorporating them into your garden.
Your plants will thank you!