Thirsty: Maize

In August the roads of Iowa run between walls. The maize stands two and a half metres high on both sides, so close to the tarmac that a car at the crossroads cannot see what is coming, and it goes on, field after field, county after county, across a state the size of England in which it is the one thing growing. Iowa and Illinois between them grow more maize than any country except the United States itself, and the Corn Belt around them, from Ohio to Nebraska, grows about a third of the world's crop. Hardly any of it is eaten as maize. It is fed to animals, fermented into fuel, milled into starch and sweetener, and shipped, and the green walls of August are the start of most of the meat, milk, eggs and soft drinks in an American life.

A kilogram of maize carries about 1,200 litres of water, and this article is about where it comes from, why it is a smaller number than the other grains', and the two places where the small number hides a large one: the aquifer under the sprinkler circles, and the sea at the end of the river.

Rain

Maize is a grass from Mexico that was bred, over nine thousand years, into the most productive grain plant there is, and in the Corn Belt it grows where the summer rain falls: eight or nine hundred millimetres a year on deep black soil, enough for a crop that transpires five or six hundred in a season. Almost all of the water in a kilogram of American maize is that rain, drawn up through the roots and let out through the leaves, which is the green water of the method this site uses, and the method counts it because it is water the land would otherwise have given to something else. The yields, ten or eleven tonnes a hectare on the best land, are twice what the plant managed in 1970 and three times what it does in most of Africa, and a high yield spreads the field's water over more grain, which is why maize comes out below wheat and well below rice per kilogram.

The global average hides the places. Maize in the Corn Belt is almost all green; maize in Egypt is almost all blue, from the Nile; maize in the Sahel is rain and a prayer, with yields of a tonne a hectare and a crop that fails one year in four. The number this site quotes is the world's; the water story is where the field is.

The circles

West of the hundredth meridian the rain gives out, and the maize does not. On the High Plains of Nebraska, Kansas, Colorado and the Texas panhandle, the land that was shortgrass prairie and the Dust Bowl is maize country now, and from the air it is the circles that the irrigation article on this site describes: centre pivots, half a mile across, each drawing on a well into the aquifer beneath. Nebraska irrigates more land than any state in the country and nearly all of it is maize for cattle feed and ethanol, and the aquifer, which filled over tens of thousands of years, is drawn down by a metre a year in the dry south and has fallen by thirty metres and more in the Texas panhandle, where the circles are going brown as the wells fail. The blue water in a kilogram of High Plains maize is several hundred litres rather than the world's eighty, and it comes from a stock that is not refilling.

Where it goes

The American crop, which sets the world price, goes three ways. About a third, in a normal year a little more, is fermented into ethanol and blended into petrol, which is the law since 2005 and the reason a fifth of the world's maize is grown to be burned in a car. A similar share is fed to animals: the chicken, the pig and the dairy cow on this site eat maize before anything else, and the feedlot steer finishes on it. The rest is exported, to Mexico, Japan, China and Colombia, or wet milled into starch, oil and the high fructose corn syrup that sweetens most of the soft drinks in North America. Maize eaten by people as maize, as tortillas, polenta, grits and popcorn, is a small share of the American crop and a large share of the Mexican, Guatemalan and African ones, where it is the staple and the water in it is the water of a meal rather than of a fuel.

The grain people eat

The other maize is the one on a plate. In Mexico, where the plant was bred, it is the tortilla, made from grain soaked in lime water and ground wet, which is why the country's cooking is built on a water step; Mexico grows white maize for its own tables and imports yellow maize from the United States for its animals, more than any other country, so that a Mexican chicken is in part an Iowa field. In eastern and southern Africa maize is the staple of a few hundred million people, grown on rain by smallholders on a hectare or two, with yields of a tonne or two a hectare, and its water is the season's rain and nothing else: when the rain fails, as it did across Zambia, Zimbabwe and Malawi in the El Niño drought of 2024, the crop fails with it, the governments declare emergencies and the grain is imported, by ship, from the Americas, where the same plant grew on rain that did not fail. The water in a kilogram of African maize is, in a good year, the same rain as Iowa's at a fifth of the yield, which makes it, per kilogram, several times thirstier and, per field, the only thing the household has.

The ethanol plant

An ethanol plant is a brewery without the beer. The grain is ground, cooked, treated with enzymes that turn its starch to sugar, fermented with yeast, and distilled, and the alcohol is dried and sent to the blending terminal while the spent grain goes to the cattle. The plant uses about three litres of water for every litre of ethanol, mostly for cooling and for the cooking, which is a modest figure that the industry has cut by half since the 1990s, and it discharges little, because most of the water leaves as vapour from the cooling towers. The water that matters is behind the plant. A litre of ethanol needs about two and a half kilograms of maize, and two and a half kilograms of maize carries about three thousand litres of water in the global average and, in the irrigated plains where many of the plants were built, a few hundred litres of aquifer. Counted to the field, a litre of corn ethanol is on the order of two thousand litres of water, and a tank of fuel with a tenth of it ethanol has drunk more than the car's own manufacture, which the car article on this site worked out.

The dead zone

The third water in maize is the grey water, and for maize it is the largest of any grain, because maize is a hungry crop. A Corn Belt field gets about 150 kilograms of nitrogen fertiliser a hectare a year, more than wheat or soy, and a share of it, a fifth or a quarter in a wet spring, leaves the field: down through the soil to the tile drains that lie under most of Iowa and Illinois, into the ditches, the creeks and the Mississippi, which the nitrogen article on this site follows, and out into the Gulf of Mexico. There, every summer, the nitrogen feeds a bloom of algae that dies and sinks and is eaten by bacteria that use up the oxygen in the bottom water, and a zone of sea off Louisiana, on average about fourteen thousand square kilometres and in the record year of 2017 about twenty three thousand, has too little oxygen for fish or shrimp to live in. The grey water in the footprint, nearly two hundred litres a kilogram, is the water that would be needed to dilute that nitrogen to a safe level, and the Gulf is where it was not diluted.

The same nitrogen is in the rivers before it reaches the sea. The water works of Des Moines, which draws from two rivers that run through the maize, has spent decades running one of the largest nitrate removal plants in the world so that the city's tap water meets the limit the nitrate article on this site describes, and in 2015 it sued the drainage districts upstream for the cost, and lost, because farm drainage is exempt from the clean water laws. The fertiliser is on the field so the yield is ten tonnes. The river carries the rest.

What it teaches

Maize is the product on this site that shows how a small footprint can carry a large consequence. Twelve hundred litres a kilogram is a modest number, made modest by the rain of the Corn Belt and the yields of a plant bred for seventy years to turn sun and nitrogen into starch. The consequence is in the parts the average hides: the aquifer under the circles, drawn down to grow feed and fuel where the rain does not reach, and the nitrogen in the river, which the Gulf receives every summer as a sea without oxygen. The kilogram of maize became a kilogram of chicken, a litre of fuel or a can of soda, and the water it used stayed in Iowa, in the ground under Kansas, and in the Gulf.

Twelve hundred litres a kilogram, a metre a year off the aquifer, and fourteen thousand square kilometres of sea with no oxygen in it.