"Wow! Look at that beautiful soil." That's not something
you hear very often. Most people take soil for granted and
few give it much thought. Yet we benefit from it many times
each day—whenever we eat. Soil plays a crucial role
in our ability to grow food.
Montgomery County, Iowa. Source: USDA |
Healthy soil is a living entity, a rich mixture of organic
matter (also called humus)
and sand, silt, and clay. The layer closest to the surface
is called topsoil.
Beneath the topsoil are subsoils,
layers of unweathered soil that are lower in organic matter.
Below all that lies bedrock.
For farming, topsoil is the key layer. It's home to a host
of living things: ants, mites,
nematodes,
slugs, snails, spiders, worms, and much more, including tiny
dirt dwellers, known as microorganisms.
Topsoil also includes the nutrient-rich
remains of organisms that have died. You can learn about one
prominent resident—the earthworm—at the University
of Wisconsin's Urban
Horticulture website.
SOIL CITY
The microbe population in a single teaspoon of topsoil can be larger than the human population on the entire planet!
Making themselves at home in their crowded world, these soil
residents create a 3-D maze of minute tunnels called pores.
These channels allow water and air to penetrate and move through
the soil. As you can imagine, water's ability to reach plant
roots is very important for raising crops.
Microorganisms also benefit soil in several other ways:
- keeping nutrients in the soil,
- killing pests that attack plants,
- binding with soil in ways that make it less likely to
wash away, and
- making pollutants less harmful to the soil, sometimes
even removing them completely.
THE SOIL FOOD WEB
Like computers connected to the World Wide Web, the jam-packed
residents of the topsoil constantly interact with one another.
But the action doesn't stop there. Plants growing in the soil
also interact with those underground inhabitants. Add all
those relationships together, and you get the soil
food web.
The plant-soil relationship, scientists say, is one of the
most extraordinary interactions in all of nature. For instance,
most plant roots have cells that act as sensors. They can
actually detect vital nutrients in the soil and direct the
roots to grow toward them.
As plants grow, they remove some nutrients from the soil
and return others. Different plants take and return different
nutrients. These exchanges help soil renew itself over and
over again. Understanding the importance of this process,
farmers practiced crop
rotation for thousands of years. They planted each crop
in a different place every year, allowing these plants time
to enrich the soil. Sometimes, farmers also let a field lie
fallow,
or empty. That gave the soil a chance to regenerate itself.
WHEN SOIL GOES "MONO"
Nowadays, however, many farmers do things differently. They
practice monoculture,
planting the same crop year after year. Doing so may help
the farmer survive economically, but it takes a heavy toll
on the soil. Soil
degradation may result.
Without natural ways of replenishing soil nutrients, farmers
grow increasingly dependent on chemical fertilizers.
These fertilizers can increase crop yields, but they affect
the soil food web. Over time, the microorganisms become depleted
and soil loses its vitality and is no longer full of life.
Something else happens too. Lacking microbes to create tiny
waterways and organic matter that binds soil together, the
soil becomes less porous,
and more water runs off the surface. So some farmers need
to use more irrigation
water for their crops.
Problem is, irrigated land is often prone to salinization.
Unlike rainwater, irrigation water contains salts and leaves
salt deposits behind when it evaporates. The amount of salt
that is left on the soil may be modest at first, but it builds.
Once the land becomes too salty, growing anything on it is
almost impossible. Ancient civilizations such as Mesopotamia
disappeared in large part because their farmland contained
too much salt.
RUNAWAY SOIL
What could be worse than damaging one's valuable topsoil?
How about losing it altogether? Erosion
is a major problem in many agricultural areas. The United
Nations estimates that one percent of Earth's topsoil is lost
to wind and water erosion each year.
Water can also carry off nutrients and other soluble matter
that are vital to crop plants. This is called leaching.
Source: Natural Resources Conservation Service |
Erosion happens for a variety of reasons:
- Degraded soil is less porous, so water from irrigation
or rain sometimes races over the land, into streams or rivers.
As the water moves, it carries bits of precious topsoil
along with it.
- Grazing too many animals in a field can strip away plants
that help hold soil together. And the animals' hooves pack
down the soil, making it less porous.
- Without the root systems of living plants to help hold
dirt in place, air movements can easily sweep topsoil away.
Countless acres of topsoil have truly gone with the wind.
- Population growth has forced subsistence
farmers in developing nations to try to raise crops
on steep hillsides and in other areas prone to erosion.
This makes erosion worse.
- Clearing rainforests, primarily to grow commerical crops
for export, has led to massive erosion.
- Deforestation
and poor farming practices in some places have caused an
increase in desertification,
the spread of deserts into surrounding areas. Scientists
are still trying to determine whether that process can be
reversed.
FIGHTING FOR HEALTHY SOIL
Soil degradation and erosion have serious consequences. Topsoil
loss, particularly in developing countries, can lead to food
shortages and even famines. The UN calculates that soil degradation
afflicts 25 percent of farmland in the developing world, and
the percentage is rising. Studies show that poverty and health
problems are most prevalent in places with the worst soil.
To combat erosion, more and more farmers are turning to conservation
tillage. That's a broad name for a variety of planting
techniques designed to minimize erosion. One technique is
to reduce the amount of plowing, or not plow at all (no-till).
Another is drip
irrigation, in which small plastic tubes slowly bring
water to the base of each plant. This method cuts down on
water usage, salinization, and erosion.
Soybeans grow amid "stubble" from earlier crops. Leaving old plants in place and not plowing them underground is an example of conservation tillage. Source: Natural Resources Conservation Service |
The Core 4 Conservation site offers pictures and diagrams
that explain conservation tillage.
Farmers are also creating set-asides.
Set-asides are pieces of highly erodible land on which people
no longer grow food. Instead, they plant cover
crops, whose sole purpose is to help hold soil in place
and build soil fertility. . In some areas, laws force farmers
to take such erodible land out of farm use completely.
Do these anti-erosion methods work? Yes! To take one awesome
example, erosion has dropped by 90 percent in some parts of
Latin America. If we're determined, we really can stop treating
our soil like dirt. |
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The first U.S. President was also an innovative
farmer. He grew more than 60 crops on his
3,000-acre plantation, Mount Vernon. Washington's
pioneering methods made him part of the
sustainable-agriculture movement of his
day, known as the "new husbandry."
