pH is a familiar term. It measures the acidity or alkalinity of a solution. The pH scale runs from 0 to 14, with 7 as the neutral point. The pH decreases from 7 as a solution becomes increasingly acidic. It increases from 7 as the solution becomes increasingly alkaline.
Battery acid is very acidic at a pH of 0.0. Common household vinegar is quite acidic at 2.0. Coffee is around 5.0. Pure water is neutral at 7.0. Household ammonia is alkaline at 10.5. Drain cleaner is extremely alkaline at 14.0.
Soil’s pH is one of the invisible factors that determines plant health. The symptoms of unhealthy pH are serious, but their cause isn’t always obvious.
Soil acts as a reservoir for water and minerals. Water forms a film on the inside surfaces of soil pores. Mineral ions, dissolved in that water, stick to charged sites on soil particles. There, they can be taken up by roots. But the solubility of these ions depends on the soil water’s pH. At acidic pH’s, their solubility decreases, and metal ions like iron and aluminum leach from minerals and take up space that could otherwise be occupied by nutrients like magnesium, calcium, potassium, nitrogen, and phosphorus. Phosphorus is particularly troublesome. Its solubility is low even under ideal conditions, but it’s also a critical nutrient. At over-acidic pH’s, it forms insoluble complexes with iron, which are no use to plants.
For most plants, the ideal pH is slightly acidic. At a slightly acidic pH, most nutrients reach their maximum solubility. This includes key micronutrients like boron, iron, and molybdenum. However, when the pH drops too low, these nutrients (as well as aluminum, which isn’t a plant nutrient) become too soluble, and can rise to toxic levels.
The issues that arise from acidified soil aren’t limited to plants themselves. Soil microbes, which are essential to liberating nutrients from soil and pulling nitrogen from the air—are much less efficient at extreme pH’s, or they can die off altogether.
(It should also be noted that too high a pH (a soil that’s too alkaline) is harmful, too.)
This all leaves two obvious questions: What makes a soil acidic? And what can be done about it?
Soils can acidify naturally. When water passes through them, it carries away some of the mineral ions. And plants consume mineral ions as they grow. Mineral ions tend to be alkaline, so their loss drives the pH down. On top of that, root and microbe metabolism produces carbon dioxide, which is also acidifying.
And, unfortunately, nitrogen fertilizers, useful though they are, tend to acidify soil over the long term, as they decompose into usable nitrate and nitrite. In urban and polluted areas, there’s also the problem of acid rain and acidic runoff.
Some soil problems are difficult to treat. It’s hard, for instance, to remove lead from contaminated soil. It’s labor-intensive to remedy soil compaction. But acidic soil is relatively easy to treat. It’s usually done by liming: applying alkaline minerals like limestone, chalk, dolomite, or slaked lime, to push the pH back up. This has the side-benefit of supplying calcium and magnesium, which are essential plant nutrients. Care must be taken not to over-lime, of course. That requires testing the soil’s pH, which can be done with inexpensive soil pH testers, with test strips, or with test kits.
It’s also important to adjust planting and management practices, where needed. Care must be taken not to over-water, and to limit the application of acidifying nitrogen fertilizers where possible. And, in ornamental plantings, it helps to choose plants that don’t mind acidic soils. Some plants even prefer their soils acidic. These include ferns, azaleas, some pines, oleanders, and blueberries. (For more detailed and specific information, the US Department of Agriculture has a fairly comprehensive database available here.)
pH problems are easily missed, since, without testing, the symptoms are vague and easy to misinterpret. But when the problem is found, it’s also a relatively easy fix.
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“Renewable energy” gets said so often that the words have all but lost their meaning. Like the word “sustainable” or “globalization, “renewable energy” tends to go in one ear and right out the other. But concern is rising over the environmental impact of fossil fuels, their supply, and their unpredictable prices.
The geopolitics of energy are far beyond the scope of this post. This post is just an overview of the renewable energy resources–the ones that are already deployed and the ones in development.
Of all the renewable resources available to us, solar is one of the most promising. After all, we’ll always have the sun. And, under ideal conditions, a single square meter of Earth’s surface receives 1,000 Watts of sunlight on average. That’s enough to run a computer or two, or a kitchen appliance. And that’s falling on every square meter.
The trouble is that harvesting solar energy requires harvesting light, and light is hard to hold onto. But there are lots of interesting technologies in the works.
For me, solar power conjures up images of fields of blue-black solar panels. These are photovoltaic panels: sheets of specially-prepared silicon that produce usable current when light shines on them. This is the best-developed solar technology, growing in step with the semiconductor industry.
But photovoltaic panels are only one type of solar generator. Solar-thermal plants use mirrors to focus sunlight on heat exchangers, boiling water and driving steam turbines.
There are downsides. (Aren’t there always?) Solar cells are fragile. Their efficiency falls if they get dirty. As for solar-thermal plants, their production drops off dramatically if it’s cloudy. And neither type can produce power at night.
This last issue is bigger than it sounds. These days, the power grid never switches off, so in order to compete, solar power requires an efficient way to store and release large quantities of energy. We’ll cover that a little later.
Hydroelectric power might be humanity’s oldest means for generating mechanical power. Water wheels go back over two thousand years ago, to a time when they were used to power water pumps, ore crushers, and grain mills. Electric generators driven by hydroelectric turbines date back to the 1880s and 1890s–the dawn of modern electricity.
Hydropower has a lot of attractive features. Nature pumps the water, in the form of rivers. All humans need to do is build a dam and a turbine. It helps that, for historical reasons, the largest and most power-hungry cities tend to be located near rivers.
But the need to build a dam and create a reservoir is hydropower’s main disadvantage. Dams cause flooding upstream, and dams and reservoirs take up a lot of space, and disrupt ecosystems.
Windmills have been found dating back at least a thousand years. Like water wheels, they were first used to grind grain and pump water. And, like water wheels, their use as power generators dates back to the dawn of electricity.
Wind power is attractive for a lot of the same reasons as hydro and solar: all that’s needed is a decent collector. But wind, like sun, isn’t constant, so energy storage is still a necessity for wind farms to compete. And modern windmills are big and need a lot of space.
I started the last two sections talking about how old wind and water power are. Biofuel, in the form of fire, is much older. Evidence of fire’s use by humans goes back no less than forty thousand years, and we might have been using it as far back as a million. The use of fire for industry dates back five thousand years, when it was used to smelt ores and fire pottery.
Of course, when people hear “biofuel,” they don’t usually think of kilns. They think of plant-derived fuels like ethanol, methanol, and biodiesel. But whether it’s one of these, wood, wood-gas, or something else, biofuels have the same set of advantages and disadvantages.
One of biofuel’s most attractive advantages is that it’s carbon-neutral. All the energy-storing carbon compounds in plants are built using carbon dioxide pulled from the air. When biofuel is burned, it can, at worst, return that same amount of CO2.
But biofuel isn’t quite as straightforward to produce as traditional fuels like diesel and gasoline. The latter can be distilled straight from crude oil. The latter have to be fermented from feedstock. And while fermentation is hardly a new technology, scaling it up to meet the world’s fuel demands has proved a challenge. On top of that, since it’s plant-based, biofuel feedstock either has to be diverted from the existing supply of crops like corn, soy, and canola, or grown on fertile land, which is a precious resource.
The earth’s interior is warm. Some of this heat is left over from the Earth’s formation. The rest is produced by the decay of radioactive elements in the mantle. In the upper crust, the temperature of the rock rises about 72 degrees Fahrenheit for every mile of depth, on average. By extracting naturally-heated groundwater, this heat can be tapped and either used to heat houses, or to boil water and drive turbine generators.
We can go a step further than that. There are proposals to produce artificial geothermal wells by injecting high-pressure water into bedrock, creating mazes of fractures that turn the rock into a natural heat exchanger. Of course, creating geothermal wells this way has the same issues as hydraulic fracturing (fracking) for petroleum: the fracturing fluid can contaminate groundwater, and disrupting bedrock can trigger seismic activity.
A major confounding issue is that many renewable energy sources are intermittent. The wind isn’t always blowing. The sun isn’t always shining. And even with steadier energy sources like hydroelectric or geothermal, it’s not always easy to step up production to keep up with surges in demand.
A big requirement for making renewable energy competitive is developing a decent way to store it in large quantities so that it can be released on demand, even when the elements aren’t cooperating.
Batteries are the most obvious option. Lead-acid batteries are mechanically sturdy and electrically robust, tolerating a fair bit of punishment (which is why they’re used in cars). Their cousins, nickel-iron batteries, are even tougher, and can tolerate outright abuse without losing capacity. But both types require corrosive electrolytes, and don’t store very much energy for their weight, compared to other battery types.
Lithium-ion batteries have a much higher energy density, but they also have to be treated a lot more gently. Traditional lithium batteries can fail quite dramatically if they’re mechanically damaged or electrically abused (sometimes dramatically enough to cause a fire or explosion). On top of that, lithium is a relatively scarce and expensive metal. Even so, the energy density of lithium-ion cells is so high that some companies are commercializing them for large-scale energy storage anyway. Plus, far more durable technologies like lithium-ceramic batteries might solve the problem altogether. But there are other interesting options in the works.
The energy input required to move a mass against gravity is substantial. Raising a 2,000-pound concrete block 500 feet (the height of a medium skyscraper) consumes 1.356 million Joules (0.375 kilowatt-hours). That energy isn’t lost–it’s stored as gravitational potential energy. A system like this is already in development. It uses a sloping railroad track, along which railroad cars carry enormous concrete weights. The railcars are driven up the slope by electric motors, storing energy in the elevation of the weights. When that energy is needed, the railcars are allowed to roll downhill with the weights, using their motors as generators. A similar proposal uses computer-operated cranes to stack concrete weights.
A related concept is already in widespread use for storing surplus energy. Rather than using weighted railcars, this system works a lot like a traditional hydroelectric dam. But in this case, the reservoir is filled not by a river, but by a turbine which pumps water into it. That stores potential energy, and when energy is needed, the water is allowed to flow backwards through the turbine, retrieving the energy. (Well, most of it.)
Yet another technology in the same vein describes a floating platform (essentially a large, square zeppelin) covered with solar panels, which proposes using cables to haul smaller weights to an altitude of almost 12.5 miles.
Compressed air offers another possible means of storing energy. It’s already used in some data centers, where maintaining customer data requires instant power in an outage, until a traditional generator can start. Larger-scale plans propose pumping air into abandoned mines, limestone caverns, or air-bags submerged on the seafloor, and storing the energy that way.
Another technology uses large, fast-rotating flywheels, housed inside vacuum tanks and suspended by magnetic bearings to reduce friction. A spinning flywheel can store a surprising amount of energy, can hold it for a long time, and can deliver it almost instantly when it’s needed.
There are other options, of course, but in the interest of brevity, I won’t cover them here.
The human population is growing scarily fast. Our hunger for energy is growing even faster. The Earth’s deposits of coal, oil, natural gas, and nuclear fuel are finite. Environmental concerns are going to continue to drive interest toward renewable alternatives. There are certainly plenty available. They all have their own sets of issues to overcome, but even so, renewable power stands poised to take over a substantial portion of the energy market. With energy-storage technology finally catching up, there may soon come a time when we can provide secure, reliable, locally-produced power to everybody, with minimal environmental impact.
There’s weather underground. I know that’s a weird thing to say, but in a sense, it’s true. As a matter of fact, there’s a whole underground water cycle, with precipitation, lakes, seas, and rivers. And the majority of the world’s fresh water is caught up in that cycle. Let me explain.
Most soils contain a fraction empty space, even dense soils like silts and clays. And most soils are, to a lesser or greater degree, water-permeable.
Surface water (rainwater, lakes, rivers, etc.) takes the place of clouds. Water from the surface seeps down into the soil like slow-motion rain. How fast this water descends depends on how permeable the soil and rock are. Eventually, this groundwater encounters a completely impermeable surface, be it dense clay, thick limestone, or solid bedrock. Then, as water tends to do, the water pools. It doesn’t quite pool in the traditional sense. Apart from in caves, there aren’t a lot of underground lakes: the pools of water exist in the pores and cracks in the ground. The surface of these underground pools of water (the “water table”) is defined by the point where all the spaces are full. And though it’s definitely not a traditional water surface, it behaves a lot like one: the water table tries to reach equilibrium under gravity and flatten out.
But because of the drag provided by the surfaces of all those tiny pores, groundwater flows pretty slowly. Imagine replacing a six-inch pipe with a six-inch bundle of coffee stirrers: they might have the same total cross-section, but capillary drag means the coffee stirrers are going to resist the flow a lot more. So groundwater behaves like a very thick, viscous liquid. Sometimes, it can take thousands of years to flow from one point to another. And this leads to problems.
Water has weight. Quite a lot of it, actually: every gallon weighs 8.3 pounds. A body of water exerts a lot of pressure. That’s true of groundwater, too: the weight of a pool of groundwater, along with the weight of the overlying rock and soil, puts the soil pores under pressure. This pressure is helpful. When a person drills a well, that well fills up with water, usually to the height of the water table. When the water is pumped out, pore pressure forces water out of the walls of the hole and refills it.
Sometimes, you don’t even need a pump. Imagine, if you will, a long slope. The ground’s surface is pretty much impermeable to water. Below that is a permeable layer, and below that, another impermeable layer. Now, say that, at a higher altitude (up in the mountains, perhaps), that top impermeable layer disappears. Water enters the soil there and flows down-slope through the permeable layer until it inevitably fills that layer up. If you drill a well near the bottom of the slope, where the pressure is high, you might end up with an artesian well: the equivalent height of the water table is actually above the surface, not below. The water spews out on its own, under pressure. You might not even need a pump.
Humans need fresh water for pretty much everything. We (and our livestock) can’t drink salt water. Crops can’t tolerate salt water. Agriculture (which is one of the pillars of civilization) wouldn’t be possible without freshwater irrigation. This all presents a problem: Earth is covered in water, but the vast majority of that is salty seawater: 96.5%. That only leaves 3.5% freshwater. Of that, 68.7% is frozen in glaciers and ice-caps. Surface water only makes up 1.2% of the freshwater, and over two-thirds of surface water is trapped as permafrost. Groundwater makes up 30.1% of the freshwater, and it’s the only source that’s easy to extract.  After all, all you have to do is dig a well.
For many thousands of years, there was nothing to worry about. A good well could provide water for a whole village or a whole farm. Now, though, with our population booming, the groundwater supply is under stress. The Ogallala Aquifer is an enormous underground sea (of sorts) buried under the American Midwest. By some estimates, it supplies irrigation water for 30% of U.S. agriculture.  Wells have been drilled into the Ogallala for a long time. But now, in the era of high-intensity industrial farming, there are places where we’re pumping from the aquifer faster than it can refill. And groundwater moves slowly, so when a well pumps water out, it takes a while for more water to flow in and replace it. If the pumping rate is fast enough, it creates a divot in the surface of the water table, called a drawdown cone. If there are too many wells too close together, or a few very large wells, these depressions can add up, dropping the water table low enough that shallower wells go dry. What’s more, with groundwater now flowing towards the low spot, the flow patterns change. If there is, for example, a leaking underground storage tank or sewer or oil well, contaminants will flow towards the wells, dragging plumes of contamination through the ground.
But perhaps a more serious consequence of the overuse of groundwater is subsidence. As I said before, a column of water (ground or otherwise) produces pressure. Quite a lot of pressure, as it turns out. The ancient Romans mined using a method called ruina montium: when rocks contained gold that was too deep to mine, they would dig narrow shafts down behind the rock face, then fill those shafts with water. The pressure of the water blew out the rock face, reducing it to rubble and allowing the gold to be sifted out.
A similar process is at work underground: pore pressure presses on everything. But if enough groundwater is removed, the pore pressure falls. In some cases, when the pressure is removed, the water-bearing rocks collapse (the severity depending on the type of rock.) At this point, even if the groundwater is replaced, the ground has permanently lost some of its water capacity. This collapse of dry aquifers is called subsidence. In some places in California, the effect is quite dramatic, with the ground subsiding as much as ten or twenty or thirty feet.  The problem is especially visible in places like California, Florida, and Mexico, since these places are hot and largely irrigation-dependent, and because the local geography favors subsidence. Florida, for instance, is plagued by sinkholes.
The population keeps growing and growing. Not so long ago, our population surpassed seven billion. And all seven-billion-plus of us need water, and need food produced using water. Groundwater is an attractive source, especially in dry, landlocked parts of the world. But groundwater is also the kind of natural resource that’s easy to neglect or abuse. After all, it’s invisible, and because it moves slowly, the effects of over-use take a while to manifest. And as with so many natural resources, we’re entering an era where we can’t afford to take it for granted. We need to watch our consumption, and we need to minimize waste. Then again, that goes for pretty much all of our resources.
Many people dream of having a home of their own, with a yard of their own, where they can relax and their children can play. And with a yard comes a lawn.
Theres’ nothing inherently bad about a lawn. The problems arise because people want their lawns to look like putting greens, or carpet, or Astroturf. The problems come from everything it takes to sustain a lawn like that.
A flawless lawn is uniform: millions of identical plants growing shoulder-to-shoulder. Monocultures like this are dangerously prone to disease: a disease that strikes one plant can easily spread to the others. For the homeowner, this either means toxic and expensive pesticides, or worse yet, the work and expensive of reseeding or replacing the lawn.
An over-manicured lawn is self-defeating. A blade of grass is a leaf, and like any leaf, it’s built from nutrients drawn from the soil. The grass has a lot invested in that blade. And then, some well-meaning homeowner comes along and mows it down. All that nitrogen and phosphorus and sulfur and magnesium is lost.
In nature, the grass would grow tall, die, decompose, and return those nutrients to the soil. But homeowners who want flawless lawns also don’t want piles of grass clippings lying around, so they rake them or vacuum them or collect them with bag mowers. (They often get rid of fallen leaves, too, another source of soil nutrients.)
All of this puts an enormous strain on the plants and the soil. With nutrients and water constantly being pulled out, converted into grass, and then mowed and discarded, lawn soils easily become depleted. Modern grass is a pretty demanding crop, so when the soil can’t provide nutrients, the grass becomes dependent on high-potency chemical fertilizer.
This just makes the situation worse: not only is the soil depleted, but all the chemicals have killed off many of the beneficial microbes needed to mobilize those nutrients.
Worse, when applied incorrectly, fertilizers run off of lawns, often into storm drains, some of which drain into bodies of water. There, the fertilizers encourage blooms of harmful algae. None of this is helped by the fact that depleted, microbe-deficient soil is already more prone to runoff and erosion anyway. Depleted soils like this don’t have as much organic matter or sticky microbial secretions to hold water, so they dry out easily, meaning, on top of everything, the lawn also needs watering, which can be a problem in an era of widespread droughts and frequent summer water restrictions.
The solution to all this is simple, in theory. First: leave dead leaves and lawn clippings where they lie, or put them in a compost heap and turn them into natural fertilizer. The nutrients feed the grass, and the organic matter improves soil structure and erosion resistance.
The second part is the sticking point: convincing homeowners to plant less-demanding grasses which are often less uniform and less pretty. Or harder still, convincing them not to plant grass at all and to let nature take over. Just because a lawn has a lot of dandelions and crabgrass doesn’t mean it can’t be mowed and taken care of. And the results speak for themselves: less time and money spent maintaining the lawn. Less soil and water pollution. And healthier soil, which might mean the lawn can flourish without watering or fertilizer.
While it might not be as pretty as a fairway, in the end, a natural lawn is a much better option. Less pollution. Less hassle. Less time and money spent keeping it up. And still a good place to have a barbecue or a pool party or to sit in the shade.
We’re made of dirt. We eat plants that grow in dirt. Foods like eggs, meat, and dairy come from animals that eat plants. The iron in our blood, the calcium in our bones, and the phosphorus in our DNA all come from soil.
Getting those nutrients from the soil into our bodies, though, isn’t a trivial matter. Some nutrients, like water, potassium, and nitrogen move easily through the soil. Nutrients like iron and phosphorus, on the other hand, are prone to oxidation and mineralization, and are a lot harder. Phosphorus is vital for all life, and especially for plants, but it’s not very mobile. Once a root has absorbed all the phosphorus around it, the zone of depletion remains depleted for a long while, since the phosphors is slow to diffuse in and replenish it.
And that brings us to the point of this post: mycorrhizae. Mycorrhizae are fungal symbiotes that colonize plants’ roots. Their relationship with plants is ancient: fossil mycorrhizae have been found dating back at least 400 million years.  The symbiosis is very intimate. Plants allow mycorrhizae to penetrate their root cells, where they form structures called arbuscules and vesicles, which allow them to exchange water and nutrients with the host plant. And plants, in turn, willingly exude substances for mycorrhizae to feed on, and accept chemical signals from mycorrhizae that alter their root growth to better suit the mycorrhizae.
Mycorrhizae, like many fungi, grow filaments called hyphae. Mycorrhizal hyphae act as an extension of the host plant’s root system, dramatically increasing its reach and its surface area. In the case of phosphorus, mycorrhizal hyphae increase the volume of soil roots can draw from. They also excrete organic acids and enzymes that convert phosphorus into a mobile form.  In some cases, mycorrhizae alone can supply as much of 80% of a plant’s phosphorus demand.
Phosphorus, though, is only one of the vital nutrients. Mycorrhizae have been documented to help absorb a slew of others, including nitrate, ammonia, ammonium, calcium, iron, potassium, and zinc. 
All other things being equal, mycorrhizae-colonized plants tend to be larger, healthier, more productive, and more resistant to stress and disease than plants grown in sterile soil.
This last point is important: mycorrhizae can provide significant protection against disease.  First of all, since they grow so close to the roots, they create a physical barrier. Second, fungi (mycorrhizae included) are constantly at war with other microbes, and so they’ve got powerful chemical defenses. They’re also in chemical communication with their hosts, so they can mobilize those defenses when the host sends out a distress signal.
Unfortunately, the modern world isn’t always kind to mycorrhizae. Soils see a fair bit of abuse in urban and agricultural soils. Construction, foot traffic, vehicle traffic, and heavy equipment compact the soil. This crushes the soil’s pore spaces, which makes the soil more prone to water-logging and oxygen deficiency, and more difficult for roots and hyphae to penetrate.
In agriculture, soil also sees very intensive use. That often leaves it dry and depleted, and so, to keep their plants healthy, farmers must resort to chemical fertilizers and pesticides. When over-applied or mis-applied, chemical fertilizers can chemically damage the soil, which can kill beneficial microbes. And broad-spectrum fungicides often kill mycorrhizae just as easily as disease-causing fungi. The result is a soil that might have all the nutrients a plant needs, but lacks the mycorrhizae that help plants absorb them.
But the news isn’t too bad: In the last few decades, understanding of the role and importance of mycorrhizae has grown rapidly. And with the rising interest in organic food, and with the increasing cost and regulation of chemical fertilizers, farmers, landscapers, and plant-care professionals are looking for ways to help make plants more resilient and self-sufficient. Mycorrhizae allow plants to grow and be productive with less labor and chemicals. With our growing population and growing demand for food, that’s more important than ever.