The tree
that eats air

A tree eats air, we tend to think. It is not a wrong thing to say — a tree breathes in the carbon dioxide of the air and builds its own body from it. As we saw in an earlier story, half of a tree is carbon that came, just so, from the air.

But carbon is not all a tree needs to grow. Among the rest, one thing it cannot do without is nitrogen — what greens the leaves and fuels the very force of growing. And here a strange thing arises.

All around, yet out of reach

Nitrogen, in truth, lies all around us. Nearly 80% of the air we breathe is nitrogen — far more than oxygen. Quite literally, most of the air is nitrogen.

And yet the tree cannot eat this nitrogen. In the air it comes in pairs locked so tightly together that a plant simply cannot pull them apart to use. It is as if a mountain of food sat on the table, sealed inside a tin that cannot be opened. Plenty before the eyes — and still it starves.

So most trees get their nitrogen from the soil alone, drawing up through their roots the little that lies dissolved there. When the soil's nitrogen runs out, the tree goes hungry. When we feed a field with fertilizer, it is really this nitrogen we are replenishing.

The trees that open the tin

Yet there are trees with the knack of opening that stubborn tin — the trees of the legume family.

Dig at the roots of a legume tree and you find little nodules clustered along them. Inside these nodules live a special bacteria, and tree and bacteria strike a bargain of mutual aid. The tree gives the bacteria a home and food; in return the bacteria pry apart the nitrogen of the air and turn it into a form the tree can use. The tin no one could open, these bacteria open.

This, in fact, is not the tree's problem alone. People, too, cannot eat the nitrogen of the air directly. The nitrogen we take from rice and meat, traced back far enough, is there because someone opened that tin. And the one with the knack of opening it is the legume plant.

So a legume tree draws straight from the air the nitrogen others sought only in the soil — it makes its own fertilizer. This is why farmers of old, when the land grew tired, planted beans to bring its strength back: the nitrogen a legume left behind as it grew made the ground rich again.

The tree that stepped down from the throne

You may recall, from the story 'The Tree That Became King,' the acacia that was king of tropical plantations until disease brought it down. That acacia, in fact, was a legume too — a tree that made its own nitrogen and fattened the soil.

So when the crown passed from acacia to eucalyptus, one thing was quietly lost. Eucalyptus has no knack for opening that tin. Behind the slow drying-out of the soil after the king changed, this was the hidden reason.

So we call the legume back

One thing must not be misread here. This is not to say we will replant the very acacia (mangium) that fell to disease. They share the name acacia, but they are kin at most, and different trees. What we have our eye on is a cousin of it — another acacia, one known to endure wet ground well and to suffer somewhat less from the disease that toppled the king.

This acacia, too, grows every bit as fast as eucalyptus. And while it grows into its own share of timber, its roots draw nitrogen from the air and add it to the soil — growing quickly and tending the ground at once. All the while it keeps, undiminished, that knack peculiar to legumes.

Plant this tree side by side with our mainstay eucalyptus, and the two help one another. The acacia draws nitrogen from the air and adds it to the soil, and in that richer soil the eucalyptus grows better. The tall eucalyptus breaks the wind, and beneath it the acacia tends the ground. Even the disease that came when a single species was planted thick spreads only with difficulty when different trees are mixed like this.

The vow we left at the end of 'The Tree That Became King' — not to stake everything on a single tree — takes shape, in the end, just like this: setting a tree that enriches the soil beside a tree that grows well. And at the beginning of it stands the tree that eats air.