1. What's under your feet?
2. Past: What not to do with dirt
3. Present: On thin ground?
4. Future: Dig in
Cover crops, like this one at the USDA's
17-acre certified organic research plot in Salinas, Calif.,
are a popular way to fight erosion. Photo
by Scott Bauer, USDA
Ray Weil, a soil scientist from the University
of Maryland, took clumps of soil from this experimental farm in
Ethiopia: some from the forest and some from the cultivated field.
In the lab, a simple experiment -- dropping the clods in beakers
of water -- shows the effects of erosion on soil. The sample on the
left in each picture is from the field, and the sample on the right
from the forest. The sponge-like qualities of healthy soils are
absent in the farmed soil, which means less water for the plants
and more to wash nutrients away as runoff. Photos:
Four elements -- carbon (C), oxygen (O),
hydrogen (H), and nitrogen (N) -- make up 96 percent of living matter. For
ecosystems to work, these nutrients must be continuously recycled.
Since intensive agriculture leaves soil short on nitrogen, farmers
typically replenish their fields with fertilizer. Between 1945 and
1985, the use of nitrogen fertilizer increased twenty-fold. Is there
a way to take better advantage of nature's natural nitrogen cycle?
Soil erosion is a major contributor of phosphorus
to streams. This photo shows bank erosion during a flash flood on
the Rio Chama near Chamita, New Mexico. Photo
by Lisa Carter, USGS.
speaking, people have been about as easy on the land as they have
been on each
other. But there is this: Between 1950 and 1984, the Green Revolution
of global agriculture, farmers used fertilizers, pesticides and irrigation
to dramatically improve crop yields and lower costs. Working in parallel,
researchers bred crops to tolerate salty soils, more fertilizer, and
crowded fields. Farmers began to carefully select crop varieties to
match local conditions. As a result, world grain production nearly
tripled during that period. In 1945, American farmers produced 30
bushels of corn per acre. By 2002, that figure stretched to 140 bushels
World hunger rates fell in
that period, but hundreds of millions remain underfed. The global
population is now 6.2 billion and rising. And, scientists say, Green
Revolution methods -- hot issues themselves -- aren't likely to increase
yields further. In other words, more of the same won't do. "The
global agricultural enterprise...has gone from being a minor source
of off-site environmental degradation 35 years ago to becoming the
major source of nitrogen and phosphorous loading to terrestrial,
freshwater, and marine ecosystems," wrote ecologist David Tilman
in 1999 (see "Global environmental impacts" in the bibliography).
That leaves two options: cultivate more land
or find better ways to use existing cropland. More than a third
of the world's land is already occupied by agriculture, so most
scientists favor the latter choice. The biggest obstacle within
it, says David Pimentel, is still erosion.
"When soil is severely eroded," Pimentel explains,
"there's very little organic matter. There are few earthworms and
insects, and few holes that allow water to percolate through. The
sponge is gone. More water runs off the land." When that water runs
off soils teeming with fertilizers and pesticides, pollution happens.
Since two-thirds of erosion is caused by water (the rest by wind),
runoff pollution is a serious concern.
In the United States, great pains have been
taken to reduce erosion since the 1930s. Some policies remove lands
from production permanently through conservation easements. Others,
like the Conservation
Reserve Program, offer incentives for farmers to give fields
a breather. In addition to land set aside, an additional 1.3 million
hectares of land was treated with erosion control measures in 2001,
according to the U.S. Department of Agriculture. But how successful
these efforts have been is under question. In 1995, David Pimentel
and his colleagues published a paper in Science estimating
erosion rates worldwide. The numbers were staggering.
Each year, water and wind clear 75 billion tons of soil from the
land, most of which comes from agricultural land.
That loss degrades farmland, eventually leaving it useless.
Thirty percent of the world's farmland has become unusable in
the last 40 years due to erosion.
Each year, about 10 million hectares of eroded cropland are abandoned
Erosion rates are highest in Asia, Africa and South America, averaging
30 to 40 tons per year, per hectare, of land.
"Since publishing our 1995 paper, the USDA
has reduced the erosion rate on cropland to 10 tons per hectare,
per year. This is still 10 times the sustainability level," Pimentel
notes. What's more, he says, the soil erosion rate on pasture land
has held steady at six times the rate at which soils naturally replenish.
Erosion is still "neglected from the point
of view of farmers and scientists and, especially, decision makers"
"Agriculture does not improve soils," says Ward
Chesworth of the University of Guelph in Ontario. "For more than
10,000 years we have conducted a long agricultural experiment with
the biosphere by using modified annual grasses to mine the soil
for plant nutrients.
"Like all mining operations, this one will
eventually fail unless we can find a way of conserving the fertility
of soil that does not depend on the diminishing natural resources
that we currently use for that purpose."
By necessity, crops sop up nutrients from
the soil. Historically, these nutrients have been returned by plowing
harvested crop residue back into the ground and with fertilizers.
But intensive plowing causes erosion, and some nutrients -- nitrogen,
most notably -- are difficult to replenish.
Erosion and losses in soil fertility are inevitable
products of conventional agriculture, together constituting the
"problem of agriculture," says Wes Jackson, director of The
Land Institute in Salinia, Kan.
Annual grasses like wheat, rice, corn, rye,
and barley account for 70 percent of the human diet, Jackson says.
Historically, these annuals have been planted in monocultures --
one crop per field. It's a bad model, Jackson says, if preserving
the land is an ultimate goal. Monocultures leach more nutrients,
are invaded by more weeds, and attract more disease and pests more
than fields planted with more than one crop.
At the Land Institute, Jackson and his colleagues
plant many crops on a field to maximize nutrient cycling without
relying on fertilizers.
"There are 16 or 18 elements necessary for life,"
he explains. "And the most important thing that manages these elements
are the roots of plants. In nature, many different kinds of root
architectures exist in one place so that specialization is possible
within the soil ecosystem. Some long tap roots may be good at cycling
one element, and some fibrous roots may be good at something else.
It's a division of labor at the ecosystem level."
Jackson hopes mimicking nature's methods will
help farmers worldwide. Soil fertility is a particular problem in
Africa, where small-scale farmers have removed large quantities
of nutrients from soils without using enough manure or fertilizer
to replenish them. On average during last 30 years, Africa has lost
22 kilograms of nitrogen, two kilograms of phosphorous, and 15 kilograms
of potassium annually, per hectare of cultivated land (See
"Soil Fertility and Hunger in Africa" in the bibliography).
Jackson is part of a growing group of scientists
and farmers who are shunning the old agriculture model. Consider,
for instance, the growing popularity of "sustainable agriculture,"
a catchall phrase including any technique that maintains yields
and natural resources, like soil, over time. It's
a tricky science, but some of the methods used -- old and new -- appear
Windbreaks: Planting trees along field borders
of their fields can reduce wind erosion, especially in flat regions.
Cover crops: Planting winter cover crops like fall
rye or winter wheat act as a ground cover and protect vulnerable
soil from eroding.
Grassed waterways: In some fields the land forms
natural depressions that collects excess water. To keep the soil
in these depressions from being washed away, farmers plant strips
of grass that absorb the water.
Contour cultivation: Planting crops to follow the
contours of a field provides furrows at an angle to the slope of
the field. The irregular surface interrupts the flow of water, reducing
cropping: Alternating a field with strips of different
crops can do the same thing, as well as reducing the pests that
Conservation tillage: Leaving stalks and leaves
of harvested crops on fields protects the underlying soil from wind
and rain until a new crop is planted in the spring. It include no-till
farming (leaving all crop residue on the soil) and ridge-tillage
(molding the soil into ridges, planting seeds between them).
Where do we go from