What is fertilizer, and why do plants need it?

by | Oct 12, 2008 08:00 PM ET

Welcome to Brain Stuff from howstuffworks.com, where smart happens.

Marshall Brain

Hi. I'm Marshall Brain with today's question. What is fertilizer and why do plants need it?

In order for a plant to grow and thrive, it needs a number of different chemical elements. The most important break into four categories.

First, there's carbon, hydrogen and oxygen, which are available from air and water, and they're in plentiful supply. Nitrogen, phosphorus, and potassium are next. These three macronutrients are what you find in packaged fertilizer. Then there's sulfur, calcium and magnesium. These are called secondary nutrients. And finally, there's boron, cobalt, copper, and things like that that are called micronutrients.

The most important of these, the ones that are needed in the largest quantity by a plant are nitrogen, phosphorus and potassium. These three chemicals are important because they're necessary as basic building blocks like amino acids, cell membranes and ATP. For example, every amino acid contains nitrogen. Every molecule making up every cell's membrane contains phosphorus. The membrane molecules are called phospholipids, and so does every molecule of ATP, which is the main energy source for all cells.

Potassium makes up 1 to 2 percent of the weight of any plant, and as in ion and cells, it's essential to metabolism. Without nitrogen, phosphorus, and potassium, the plant simply cannot grow because it can't make the pieces it needs. It's like a car factory running out of steel or a road crew running out of asphalt.

To make plants grow faster, what you need to do is supply the elements that the plants need in readily available forms. That's the goal of fertilizer. Most fertilizers supply just nitrogen, phosphorus and potassium because the other chemicals are needed in much lower quantities and are generally available in most soils. Nitrogen, phosphorus, and potassium availability is the big limit to growth for plants.

So why don't people need fertilizers to grow? Because we get everything we need from the plants we eat or from the meat of animals that ate plants. Plants are factories that do all the work to process the basic elements of life and make them available to us.

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