Looking Over Clover

To any person who has spent enough time sitting on a patch of lawn to get a little bored, the leaf in the picture below will be instantly familiar. 

One clover leaf.

It's a leaf of the white clover plant, and there is plenty of it at out outdoor classroom right now.  One clover leaf has three parts, which is reflected in the Latin name of the plant: Trifolium repens.  Tri- means three, and folium means leaf.  Repens means reclining, which this plant does well, as it spends its entire life within about 3 inches of the ground.  Clover leaves usually have faint white lines in them, and they are never heart-shaped.  Another common lawn plant called oxalis has leaves divided into three heart shapes, and sometimes people get confused about it.

A patch of clover.  See any lucky ones?

White clover has several claims to fame.  First, they are tough little plants, and they survive well on lawns even under heavy foot traffic, so they grow everywhere there is a lawn.  People who are sticklers for uniform-looking lawns consider the plant a weed, but many people value the plant for its ability to grow in harsh conditions where grass can't grow.  Second, clovers are known and loved for their sweet-smelling flowers, which are white or pinkish clusters on stems.  I bet you have made a flower chain from clovers before.  Bees love the flowers even more than humans do, and they make great honey from it.  The sight of clover flowers on a lawn should serve as a warning to wear shoes, since bees are likely to be on the flowers, and stepping on a bee will get you stung.  Clover's third well-known benefit is its value as a food source for animals.  Clover seed is often included in the mix of seeds farmers plant for growing cattle forage (the plants cattle eat).

A small clover plant with leaves, stems and roots.

Clover also has a secret.  Most people don't know much about the hidden power that makes clover so important in nature and explains some of its better-known characteristics.  If you dig up a small clover plant and look at the roots, you see something that is not usually present on plant roots: tiny lumps.  Those lumps are called nodules, and they are actually little areas where clover keeps its own pet bacteria.  The clover provides food and housing (and maybe even affection) to the bacteria in return for the services the bacteria provides to the clover.  The bacteria produce an otherwise almost unobtainable nutrient called nitrogen.  Nitrogen is a nutrient used to make protein - the microscopic parts of organisms that provide much of their actual structure as well as much of the machinery to conduct life's processes.  Other plants can only get nitrogen by absorbing leftover nitrogen from dead clover-type plants or from decomposing animals or animal manure, but clover has its own constant supply.

Root nodules on a clover - where nitrogen is fixed.

Clover's source of nitrogen means it can grow on poor soil where grass can't.  It also means it has a high nitrogen content, making it more nutritious than grass for animals to eat.  When clovers die, they leave behind richer soil with more nitrogen, where other plants can now grow.

Clover is related to bean-type plants (pinto beans, Limas, black beans, lentils, peas), and all types of bean plants have the same pet bacteria for making nitrogen (actually called fixing nitrogen).  This type of interaction between two organisms that live in close contact and help each other is called mutualism.   Can you think of other examples of mutualisms?

Have you ever found a four-leaf clover?  Sometimes the plant makes an error when growing its leaves, resulting in our four-leaf symbol of good luck.  If you look at a patch of clover for long enough, you will probably find a four-leaf clover.  If you do, press it flat between pages of a book for a week or so, then you can glue it to paper or press it between clear tape to preserve it.

Weeds: The Superhero Gang of the Plant World

The lawn in our outdoor classroom is lush, thick and inviting.  It looks like a perfect sea of even, green grass.  Just look at it!  If you stop and really look, though, you'll start to see weeds.  They are stealthy and hidden, but they are everywhere!

A clover plant thriving in a nutrient-depleted patch.

Weeds, by definition, are plants that humans consider to be growing in the wrong place.  They annoy us in our lawns, we spend time removing them from our gardens, and when they grow amongst our crop plants, they reduce the amount of food that is produced, so they cost us food, time and money.

Still, I rather admire weeds.  If you look at them from the plants' perspective, weeds are the ones that manage to survive even after people have done everything they can to get rid of them.  To make our outdoor classroom, humans removed all the vegetation and reseeded with very thick grass to completely out-compete the weeds for sunlight and nutrients, but the weeds found a way.

Spring cress, false-strawberry and a dandelion battling their way into our lawn.

Weeds usually have some unique 'special power' (well, growing ability) that lets them grow in hostile habitats.  Some weeds, like the spring cress in the picture above, can grow when it's too cold for other plants, so they take off while the grass pauses for winter (plus they have exploding seed pods!).  Clover's super power is to produce a nutrient called nitrogen that other plants can't make, so it can grow in nutrient-depleted habitats.  Dandelions, are shape-shifters: generalists that can adapt to just about any condition (plus their seeds fly on the wind).   The spurge's power (seen below) is speed: the ability to grow and make seeds so fast they can live their lives before people notice them and kill them.

A spurge weed with milky sap - tear the stems and notice it oozes a white liquid.

Some conditions are too harsh even for weeds.  Notice the worn pathways in the grass where students walk.  There don't seem to be any grass plants or weed plants there.  Now we just need to find a weed whose special powers are to grow despite dozens of people walking on them every day!

Another reason I admire weeds is that they provide variety to the types of habitats available for other organisms.  The more types of plants that grow in an area, then the more types of insects and birds and mammals and other species you can have.  Variety of types of living organisms is called biodiversity.  A pure, uniform lawn is like a desert in terms of biodiversity, because it only has one type of organism.  Weeds increase the biodiversity of our outdoor classroom.

Do you think weeds are more likely to be found in the middle of the lawn or at the edges of it?  You can experiment to find the answer.  Use a small hula hoop as your measuring device.  Throw the hula hoop randomly onto the grass in the center of the lawn and count how many weeds are present in the circle.  Then randomly toss the hoop on the grass at the edge and count weeds again.  Do this a couple more times, and you should have your answer.  Now you just have to figure out an explanation for why you think weeds prefer one habitat over the other.

A Love Letter to A. P. Environmental Science Students

It's getting close to Halloween, and the scariest thing for most APES students is the nitrogen cycle, so why don't we tackle that today.  The dreaded nitrogen cycle strikes fear into the hearts of Environmental Science students everywhere because it's so difficult and so necessary to know.  But just like vampires, mummies and werewolves, it gets more interesting the more you understand it (though it still might scare the you-know-what out of you!).  Don't fear - you can do this!

Do you know why I put a picture of this Chicago bean tree (actually a honey locust) here?

First of all: what is nitrogen and why do we care so much about it?  Living things on earth are mostly made of the atoms carbon, hydrogen and oxygen, but proteins need a little nitrogen.  No living thing can live without proteins, therefore nitrogen is necessary for life.  Nitrogen, however, is in short supply, at least in a usable form.  If you think of atoms like Legos, each atom would be a different type of Lego.  Atoms can bond together to make larger structures like proteins, carbohydrates, DNA and oils, which then add up to whole organisms, just like Legos can be put together to make bigger things.  Nitrogen atoms are the special Lego pieces that are rare but that you can't build anything without.  Animals get all their nitrogen from eating protein in food.  When you eat food, your body takes apart the food's Legos (atoms) so that it can put them back together again into the molecules you need (or your body burns the Legos for energy, but the analogy kind of breaks down there).

Usable form?  Take a big, deep breath - it will help calm you down, plus it demonstrates the next concept.  What you just inhaled (air, hopefully) is 78% nitrogen.  OK, exhale.  You just exhaled every bit of nitrogen that you had inhaled, absorbing none of it.  Nitrogen in the air is in the form of N2 (I don't have subscript), or two nitrogen atoms double-bonded to each other.  There is actually a triple covalent bond holding two nitrogen atoms together in N2, which makes them nearly impossible to get apart.  Think of identical Lego pieces so hopelessly stuck together that you have to go get a kitchen knife to separate them.  Your lungs don't have the biological equivalent kitchen knives, so they can't break apart the nitrogen atoms to use them, and all the nitrogen you inhale is useless to you (except that it makes our atmosphere very stable).

The Kitchen Knife Monopoly:  Nature runs a tight kitchen, and it won't let you play with knives.  Only specific soil bacteria are allowed to have knives.  By which I mean only bacteria can fix nitrogen from it's stuck form to separate, useful atoms.  The separate atoms are quickly bonded to hydrogens to make ammonia (NH3).  The name of this process is nitrogen fixation.  It is thought that the ability to fix nitrogen evolved once on this planet when bacteria were pretty much the only critters here.  Everyone else after that found that it was easier to trade with bacteria than to evolve their own way of fixing nitrogen.  It is extremely difficult to fix nitrogen because a lot of energy is required to break the triple bond between two nitrogen atoms.  Bacteria expend a lot of ATP to break the nitrogen, but no one else is going to do it for them, so they just get on with it.

Wheeling and Dealing:  Anyone else who wants fixed nitrogen has to get it from bacteria.  They can wait for the bacteria's waste and absorb that straight from the soil in the form of ammonia, but bacteria aren't very wasteful, and there isn't much nitrogen available this way.  Some organisms have found a better way to get nitrogen: they can strike a deal with the soil bacteria.  Many plants, especially legume-type plants, have a mutualistic symbiosis with the soil bacteria.  Plants provide the ATP and some oxygen in exchange for lots of ammonia.  Legumes even provide specialized little lumps (nodules) on their roots to house the bacteria. 

What About Us?  As I said earlier, we get all our nitrogen from food.  We need protein in our diet because of the nitrogen in protein.  Think of foods that are high in protein.  Did you say meat and eggs?  You're right, but think of vegetarian foods that are high in protein.  Hopefully you said beans, peanut butter and tofu.  When you think about it, you realize that all these foods are legume-type plants, which makes sense because legumes are able to get more nitrogen from housing nitrogen-fixing bacteria in their roots.

Random picture of black beans!

But that's not all is it?  Sorry, no.  When we learn about Environmental Science, we always learn about the full cycle of things - externalities, life-cycle costs, recycling, etc.  We have to finish the story.

Energy From Nitrogen?  Since it takes so much energy to break N2 apart (a reduction reaction for you chemistry folks), it would follow that energy can be had from getting those two nitrogens back together.  In fact, that's just what TNT and fertilizer bombs do is allow a large quantity of nitrogens to reunite - boom!  Bacteria in the soil can take advantage of single-nitrogen molecules and get some energy from oxidizing them in two different ways.  The first way is called nitrification, in which bacteria take ammonia molecules and make nitrite, then nitrate ions for energy.  Extra nitrate ions left in the soil from this process are very easy for plants to absorb and use.  You can think of this as half-oxidizing the nitrogens (sorry, chemistry teachers), and releasing some of the available energy.  Fully oxidizing two nitrogen compounds results in the production of N2, releasing more energy and returning the nitrogen to that useless gas molecule in the atmosphere.  (We just came full circle!)

Still Not Done:  It helps to know the following trivia about the nitrogen cycle too.  (1) Animal waste and dead organisms contain a lot of nitrogen.  Soil bacteria break down that waste and release the nitrogen into the soil as ammonia (in a process called ammonification), which is available for plants to use.  You may have noticed this if you have a dog that pees in spots around the yard that result in greener grass patches.  If your dog always pees in the same spot, you no doubt notice that too much ammonia is toxic.  (2) Agriculture results in nitrogen being absorbed from the soil into plants, then plants being carted away to markets, along with the soil nitrogen (where did all the good Legos go?).  Soil nitrogen must be replenished with chemical fertilizer, manure, growth of legumes or other fertilizer.  (3) Too much nitrogen in a body of water, say from fertilizer runoff or sewage overflows, can cause the plants to overgrow, choke out the sunlight and cause lower-down organisms to die, decompose and use up all the oxygen.  Then everything else in the water suffocates and dies.  This is called eutrophication and is worth understanding very solidly.  (4) Some single-nitrogen compounds can be made from the energy of lightning separating N2 molecules.  Also, humans use the Haber Process the break the N2 bonds to make ammonia for making bombs and fertilizer.  The Haber process uses lots of energy to split the N2.  It uses N2 from the air but gets hydrogens from natural gas (CH4), so making fertilizer is a very fossil-fuel-intensive process.