The Big Ratchet: How Humanity Thrives in the Face of Natural Crisis (11 page)

Bottleneck from Below

“Life’s bottleneck” was the term that biochemist and science fiction writer Isaac Asimov ascribed to phosphorus in the 1970s. “Life can
multiply until all the phosphorus is gone, and then there is an inexorable
halt which nothing can prevent.”

Like nitrogen, phosphorus is a basic building block for all forms of life. Plants need phosphorus in their cells to transfer energy for photosynthesis. In animals, the nutrient is critical for bones and teeth. In cells, it transfers energy in the food we eat into a form usable for growth and movement. Also, as with nitrogen, humans and other animals can only obtain phosphorus by eating plants that contain it or by eating animals that have eaten phosphorus-containing plants.

Centuries before the life-enabling role of phosphorus was known, the element displayed other intriguing properties. In 1669, the German alchemist Hennig Brandt accidentally discovered glowing phosphorus while he was searching for the legendary Philosopher’s Stone to turn iron, copper, and other base metals into gold.
His recipe:

      
•
   
Boil urine to reduce it to a thick syrup.

      
•
   
Heat until a red oil distills up from it, and draw that off.

      
•
   
Allow the remainder to cool, where it consists of a black spongy upper part and a salty lower part.

      
•
   
Discard the salt, mix the red oil back into the black material.

      
•
   
Heat that mixture strongly for sixteen hours.

      
•
   
First white fumes come off, then an oil, then phosphorus.

      
•
   
The phosphorus may be passed into cold water to solidify.

Brandt followed this recipe to turn fifty buckets of urine into a white, waxy substance, a pure form of phosphorus. The phosphorus was not the magic stone to transform metals to gold, but it did have the intriguing property of glowing in the dark with a faint green light. The mysterious property gave rise to the name, which comes from the Greek
phos
, meaning “light,” and
phorus
, meaning “bearer.” With the prized ability to combust, phosphorus found uses through the centuries
in profitable products, from matchsticks to bombs. Charred animal bones, rather than urine, became the source for commercializing the
unusual element by the mid-eighteenth century.

Despite the vital, life-giving role of phosphorus, the planet’s recycling machinery does not make it easy for human ingenuity to replenish soils with this nutrient. Unlike the case for nitrogen, with phosphorus there are no bacteria to take it from the air and put it back. Indeed, there is no phosphorus in the air at all, unless you count the small amount attached to dust particles and blown by the wind.

But nature does have a recycling mechanism for phosphorus. Consider an atom of phosphorus. Maybe it will be in a plant, carrying out essential functions in the plant’s production of sugar to fuel its growth. The plant will eventually die in situ, or perhaps it will be eaten by some animal. The animal will eventually die. Either way, organisms in the soil will decompose the plant’s or animal’s remains. The atom of phosphorus will return back to the soil, dissolve in water, and be taken up again through another plant’s roots, and the cycle will start again. This cycle occurs quickly, over years or decades. But some phosphorus will
slowly escape from this closed loop. Some will be lost as rain washes away the soil and carries the phosphorus to rivers and eventually the ocean, some when winds blow phosphorus-containing soil particles to faraway places. So unless some process replenishes the phosphorus trickling out through these slow leaks, the soil is ultimately depleted of this essential nutrient.

The only way that leaking phosphorus can get back into the soil is slow, and it occurs over geologic time scales. The luminescent, phosphorus-bearing rock apatite holds the key to this very long-term cycling. Phosphorus leaches out from the soil and eventually travels to the ocean, but some of it sinks to the ocean floor and gets squeezed into rocks from the pressure of the weight above. Phosphorous could remain locked in apatite rocks at the bottom of the ocean, with no prospects to complete the cycle. But plate tectonics keeps the cycle of phosphorus churning, just as it does the long-term cycling of carbon, keeping climate in check. Phosphorus that makes its way to the ocean’s floor creeps across as crustal plates spread. When the crustal plates collide at their edges to form mountains, apatite rocks piggyback along and get lifted high above sea level. The uplift makes apatite rock available for weathering, and the phosphorus released in the process becomes available for plants. This cycle plays out over a span of about a million years. For people growing crops, this time scale is not much help for restoring nutrients to their soil.

The mismatch between the geologically paced time scale of phosphorus’s recycling machinery and civilization-paced demands to recharge the soil has posed a puzzle for human ingenuity since agriculture began. Phosphorus’s slow journey presents a challenge for human ingenuity: how to break into the cycle and speed it up. And we have, indeed, found two basic ways to do this: we can keep the phosphorus in manure and dead plants and animals cycling in a short-term loop, or we can dig up the phosphorus and move it to a place of our
own choosing. The second option didn’t become feasible until people found ways to dig up and transport valuable rocks—a story told several chapters later. The first option is what the Peruvian farmer was pursuing in his maize field.

Slash-and-burn farming breaks into the planetary apparatus that keeps nitrogen cycling through soil, plants, and air and keeps phosphorus cycling through soil, plants, and rocks. When these nutrients are tied up in trunks, branches, and leaves, the trees are nice to look at, but not of much use to the farmer. He needs the nutrients to nourish his crop of maize. After the farmer first clears and burns the forest, the nutrients left in the debris of ashes and half-burnt leaves and branches nourish the soil. Crops grow with the benefit of the nutrients, but the nutrients are carted away in the form of maize or sugarcane, with no way to return. After a few harvest cycles, yields become so low that the proceeds are no longer worth the farmer’s efforts. That’s when it’s time to let the trees grow back. When they have grown good and tall, the cycle can start again, with clearing, burning to release the nutrients, and harvesting until the nutrients once again become depleted.

In the normal course of the planetary machinery, the trees would eventually die, bacteria and fungi would decompose the dead wood and leaves, and the nutrients would enter the soil once again to grow more trees. The process would take decades—if not centuries—and the farmer would have no way to get the nutrients into his crops rather than into trees. Slashing and burning essentially speeds up the process and redirects the nutrients into crops rather than trees and other wild plants. It’s an age-old, popular manipulation of the cycles in forested regions, and it requires only a blade, fire, and know-how. The popularity of this twist on the planetary recycling machinery is reflected in its many names in different cultures—
milpa
in Central America,
jhum
in India,
tavy
in Madagascar,
ladang
in Indonesia, and swidden, shifting cultivation, or slash and burn in English.

A lack of written accounts has kept us from knowing exactly how the practice first emerged. But the strategy was a major part of the repertoire that enabled people to expand outward into forests from the places where farming first began. Today, one associates slash-and-burn agriculture with rainforests in the tropics. But this is precisely the way people cleared forests across the Near East and Europe for millennia. Beginning around 9,000 years ago, clearing for agriculture began to edge its way into the Near East. Over the next few millennia, Mediterranean forests were under the ax, then
forests in southern and middle Europe. As the numbers of people ratcheted up, eventually the forests all but disappeared. Settled agriculture took over where forests once stood.

Life from Rivers

In some parts of the world, rivers have provided a solution to the soil-depleting conundrum of settled life. Water, after all, is an irreplaceable ingredient for growing food, so it’s no surprise that civilizations first arose where water was plentiful. Surplus food was a critical element for complex societies. It allowed them to divide tasks among their members, concentrate authority in the hands of the powerful, and support administrative bureaucracies. People living in the valleys of the Indus, Nile, Tigris and Euphrates, and Yellow rivers all achieved great technological feats to control the rivers’ flows more than 5,000 years ago. All were civilizations where towns grew, bureaucracies burgeoned, and
stratified societies emerged.

But the rivers not only brought water. They also carried the lifeblood of nutrients via the silt and soil washed downstream from the highlands. One of the earliest advances in routing water and nutrients from river to field occurred in the “Land Between the Rivers” in ancient Mesopotamia, located in current-day Iraq and Iran. The Tigris and Euphrates run nearly parallel, carrying rain and melted snow southward
from highlands in Turkey, Syria, and Iraq. The two rivers meet not far upstream from where they empty their waters into the Persian Gulf. The rivers would overflow their banks at least once a year as the water reached the delta, bringing the gift of fertile soil to the Mesopotamians. But the floods brought challenges as well. The floodwaters did not drain readily through the soil, leaving the fields waterlogged. Moreover, the floods came in April or May, when weather was too hot for planting.

The ancient Mesopotamians devised complex systems of earthen canals and levees to channel the flow from the rivers. Canals crisscrossed the landscape to bring water from the rivers to the fields of wheat and barley. The canals brought water and nutrients to the crops at the right time, but the solution gave rise to another problem. The heavy silt load brought by the rivers choked the canals and required constant dredging. Salt built up on the surface as water evaporated from the poorly drained soil, a major problem with irrigated lands that persists to this day.

Over the centuries, invaders and conquerors elaborated on the irrigation system with more canals, dikes, and levees. But the burden and expense of dredging the canals and draining the soil overcame the administrative structures. The waterworks got another blow in the mid-thirteenth century, when Mongol invaders destroyed canals and levees in their ruthless conquest. The land between the rivers never recovered from the clogged canals, salt-encrusted soils, and political collapse to its heyday as one of the
great cradles of civilization. The nutrient-laden silt from the mountains solved the problem of infertile soils but created another problem in its wake.

Ancient Egyptians faced an easier challenge with channeling the mighty Nile to bring water and nutrients to their fields. The ebb and flow of the Nile’s floods set the pace of life in ancient Egypt. Each year, the Nile carried rain from the summer monsoon in the highlands of Ethiopia thousands of miles to the triangular-shaped, fertile delta along the Mediterranean Sea. As in Mesopotamia, the receding floodwaters
left fertile soil and moisture for the season’s crops, but the land drained more readily and catastrophic floods and droughts were rare.

The Egyptians did not know the mysterious source of the water, thinking it came from a great celestial ocean. But they did know the river held the key to their fortunes. The idea to regulate the Nile’s flow with a dam upstream from the delta dates back to the eleventh century. Iraqi mathematician Ibn al-Haytham proposed a hydraulic project,
but the caliph refused. Centuries later, British colonial rulers put the idea into action with the masonry Aswan Low Dam, followed by the massive, concrete Aswan High Dam that the newly independent Egyptians built several decades later a few miles upstream. The dams trapped the rich source of nutrients upstream and severed the annual rhythm that linked the Ethiopian highlands with harvests in the delta far downstream. Ironically, expensive chemical fertilizers now replace what was once a practically free solution to the conundrum of settled life.

Beginning with the great ancient river civilizations along the Tigris and Euphrates, the Nile, the Indus, and China’s Yellow River, nutrient-bearing silt carried by rivers has been part of the story of humanity’s ability to expand its numbers and live in cities. More than 100,000 people lived in Thebes, the capital of ancient Egypt and the largest city in the world 3,000
years ago. These large numbers could not have been sustained without the river’s nutrient-bearing silt to circumvent the first conundrum of settled life.