For Release On Or After 08/09/13

By Dan Gill
LSU AgCenter Horticulturist

Most gardeners are aware of the important relationship plants have with light. We are forever talking about the light preferences of plants. And every garden reference stresses how important it is to provide the proper light for different plants – indoors or outside. If you place a plant that needs a sunny location in the shade, it will languish and do poorly.

Yet, I’m not sure gardeners really understand why light is so essential to plants – and why a sun-loving plant will do poorly if it does not get enough light. And for that I blame the term "plant food."

Until the modern era, people really had no idea what plants “ate.”

In ancient times, early farmers eventually realized that if they spread manure over a field before planting a crop, it made the plants grow better. It was easy for them to assume that plants grew better because they ate what was in the manure. It made sense that, just like providing abundant food to a person makes them strong and healthy, the food provided by the manure made the plants grow better. So the idea that we are actually feeding plants when we add manure (or fertilizers) to the soil became entrenched and survives to this day. Am I right? Don’t we all "feed" our plants “plant food”?

The trouble is, we now know better. In the 20th century we discovered that plants carry out a remarkable chemical process called photosynthesis – meaning "to create from light." And photosynthesis, as it turns out, is the basis of life as we know it.

In photosynthesis, plants use a green pigment called chlorophyll to absorb energy from light. This is generally from sunlight, but artificial light also works. Plants use the energy to combine carbon dioxide they absorb from the air with water they absorb from the soil to form molecules of sugar. That energy is eventually stored within the sugar molecules, which people and animals in turn use when they eat the plants.

A byproduct of photosynthesis is oxygen, which plants release into the atmosphere. When photosynthetic bacteria and plants first evolved, the atmosphere had very little oxygen. Today, our atmosphere is rich in oxygen, and the world is full of animals that need oxygen to live. The photosynthetic process and the plants that carried it out literally transformed the world, allowing oxygen-breathing animals to exist.

Plants use energy along with carbon, hydrogen and oxygen from the sugar created in photosynthesis and other elements they absorb from the soil to build tissue and make everything they need to run their metabolisms. That’s right; the body of a plant is literally created from water and carbon dioxide in the air.

So, plants do not eat the soil. Soil does not provide them the energy they need to live and grow. So why did early farmers find that plants grew better when manure was spread over the fields? Along with the water they need, plants’ roots also absorb various mineral elements from the soil. The minerals a plant absorbs from the soil only contribute a tiny fraction to its body, but they are critical to a plant’s health.

This is where fertilizers come in. They are not food. We know now that plants create their own food; we don’t have to "feed" them. What fertilizers do is provide some of the minerals – such as iron, nitrogen, magnesium, potassium and calcium – plants need to absorb from the soil to be healthy. If the soil doesn’t contain enough, adding these minerals makes plants grow better – just as we might take vitamins to supplement our diet.

Indeed, the better analogy for what fertilizers are and do for plants is to compare them to vitamins rather than food. It would much closer to what we are actually doing when we fertilize plants to say, “I need to give my plants some vitamins,” rather than saying, “I need to feed my plants.”

Very tiny amounts of vitamins – think if that little pill you take in the morning – are all that are needed to make us healthy. Only small amounts of fertilizers are needed to keep plants healthy. But people often put out more than needed. And more is not necessarily better. Some vitamins can be toxic at high levels, just as you can damage your plants by applying too much fertilizer. And people never mistake the little vitamin pill they take with the food they need.

So the relationship plants have with light is not just about what levels plants prefer. Light is their sustenance. It is what they consume to live. Plants are solar-powered organisms. Light is as important to the houseplant sitting on your windowsill or the live oak in your front yard as the food on your plate each day is to you.

If you provide a plant less than the amount of light it needs to be healthy, it will become weak and anemic, just as you would if you were deprived of most of the food you need to eat. Nothing else we can do – extra water, fertilizer or begging – will make a plant healthy if it is not getting enough light.

Without sufficient light, there is no other way for the plant to create the food it needs, and it will slowly starve. Indoors, where light is especially limited, it is not unheard of for plants to literally starve to death while their owners worry about everything from watering to fertilizing.

I am probably not revealing anything earthshaking here. We were all taught about photosynthesis in school at some time in our lives. But I think gardeners often lose sight of what the relationship plants have with light is based on, so I thought I’d give you a little reminder. Keep in mind the importance of providing the proper light the next time you’re wandering around the yard with a plant in your hand looking for someplace to put it.

Rick Bogren