The nitrogen cycle

If you're going to win the jumping competition today you have to eat carrots, so you'll get a lot of energy! In order to grow, the carrots need sunlight, carbon dioxide and water. But they also need nutrients. Two of the most important nutrients are Nitrogen and Phosphorous. Michael has some plant food that he can use as a fertilizer for the carrot plants.

The fertilizer contains nitrogen, but why would the plants need this to be added, when the air is mostly composed of nitrogen gas? We're surrounded by lots and lots of nitrogen, but it's usually a lack of that very substance that limits how large plants can get! How can that be? That is because the nitrogen in the air can't be used directly by plants nor animals. But there is one way that it can become part of a biological cycle.

In order to be useful to plants and animals, the nitrogen needs to be built into different nitrogen compounds. This process, called nitrogen fixation, requires a lot of energy. In the soil and in the water, we find a certain type of bacteria. These bacteria can fix the nitrogen from the air, by forming ammonium ions. After that, the ammonium ions can be converted to other nitrogen compounds, such as nitrates.

That is also done by bacteria. The nitrogen-fixing bacteria in the soil can often be found together with the roots of legume plants. The bacteria in the sea that fix nitrogen are called cyanobacteria. Nitrogen can also be fixed through lightning that makes the nitrogen in the air react with oxygen. This produces nitrous oxides, that can be turned into nitrates.

Once the nitrogen is in the form of different compounds, it can be absorbed by plants and be built into their DNA and proteins. Animals feed on the plants. Nitrogen is returned to the soil through the urine and feces of the animals... Or when the plant or animal dies. Dead plants and animals are decomposed by fungi and bacteria.

The nitrogen that was bound in the plants and in the animals is eventually converted to ammonium. This ammonium can either be absorbed by another plant... ... or be converted to nitrate. Other bacteria can convert nitrate ions back to gaseous nitrogen and oxygen. That's the only way nitrogen can be released into the air again.

This process is called denitrification, and is a way for bacteria to generate energy in places like swamps, where there is a lack of oxygen. The nitrogen cycle looks like this. The nitrogen in the air is fixed in ammonium or nitrate compounds that can be built into plants and animals. Through the droppings from the animals, or when animals or plants die, the nitrogen compounds come back to the soil where they can be absorbed again. Or some of the nitrate ions can be converted back to gaseous nitrogen again, by other bacteria.

So Michael, now you see why the carrots might need extra nitrogen to grow well.