What do plants eat?

Richard Bogren, Gill, Daniel J.  |  7/28/2009 12:51:42 AM

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For Release On Or After 08/29/09

By Dan Gill
LSU AgCenter Horticulturist

One of the most famous phrases ever written is, “Let there be light.” It is amazing that such a simple statement has such profound implications about creation and life. Light out of darkness evokes a responding concept of life out of death. In fact, light is the very basis of life for virtually all plants and animals.

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 plant in the shade a plant that needs a sunny location, 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 fairly recently, people had no idea what plants eat. Early farmers eventually realized that if they spread manure over a field before planting a crop, the manure 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, now we 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 the energy from light – generally sunlight, but artificial light also works. The energy is used to combine carbon dioxide plants absorb from the air with the water plants absorb from the soil to form molecules of sugar. A byproduct of photosynthesis is oxygen, which plants release into the atmosphere.

The energy the plant absorbed from the sun is stored within the sugar molecules, which we, in turn, use when we eat plants – or the animals that eat the plants.

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

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, however, 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 diets.

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 took this morning – are all we need to keep us healthy.

Plants need only small amounts of fertilizers, and people often put out more than they need. More is not necessarily better. Some vitamins people take can be toxic at high levels, just like you can damage your plants by applying too much fertilizer. And no one ever mistakes the little vitamin pill they take for the food they need.

So the relationship plants have with light is not just about what light levels they prefer. Light is their sustenance. It is what they consume to live. Plants are solar-powered organisms. Light is as important to the African violet 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, the plant has no other way 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 to you. 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 though I’d give you a little reminder. Keep this in mind the next time you are wandering around your yard with a plant in your hand, looking for someplace to plant it.

Rick Bogren

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