PBIO 100 Lecture Notes
Undergraduate Program in Plant Biology, University of Maryland
LECTURE 39 - AGRICULTURE AND THE ENVIRONMENT
Introduction
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Current agricultural practices are good at maximizing current yields. Newer
varieties of crops provide higher yields and combinations of desirable
traits. Fertilizers provide faster plant growth and greater amounts
of food produced per plant. Pesticides are used to prevent losses to insects
and provide the blemishless food consumers demand. However, sometimes
the practices destroy future capacity to produce food. We will examine six
problems created by present cultivation practices: crop genetics, soil erosion,
soil salinization, desertification, fertilizer problems, and pesticides.
Solution to these problems must be based on the concepts of
sustainable agriculture
- the idea that practices should be based upon renewable resources and long-term
productivity.
Example: Tomato Production
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Location- Major production takes place in California and Florida. Florida
has two winter crops- ships October through May
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Scale of production-Individual growers plant 50-100 acres. 100,000 acres
of tomatoes are planted in Florida each year. Each acre contains about 7,250
tomato plants. Each plant was raised from seed in a greenhouse and hand planted
in the field. Each plant is staked to keep the plant upright.
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Soil preparation- Soil fertility is important- fertilizers are needed to
replenish nitrogen, phosphorus and potassium taken up into the plants previously
grown in the field. Herbicides (chemicals which kill plants) are used to
clear the field of weeds- mechanical weeding is difficult on this large scale.
Plastic is laid on top of the soil to retain moisture and either warm the
soil(black plastic) or cool the soil(light-color plastic).
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Pests
and problems affecting crop- Insects (e.g. Colorado Potato Beetle, fruit
worm), fungi (e.g. Early blight), bacteria (e.g Buckeye Rot), and nematodes
cause crop losses due to their attack on plants. The growth of all the same
plant together enhances the threat of these pests as they can quickly spread
throught a field. Pesticides are applied to plants to reduce losses and produce
fruit with no blemishes (due to minor diseases and pest feeding). Organic
growers must use other controls- one can not sell a blemished crop that one
has not done anything to control these pests.
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Harvest-
Tomatoes are picked green to allow shipment while firm. Treating tomatoes
with ethylene gas at the destination or during shipment will induce ripening.
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Economics- An average harvest is 35,000 pounds per field. The cost of growing
these tomatoes (whether or not there are any to harvest) is about $4,276
per acre- or about 12 cents per pound. Harvesting costs 14 cents per pound.
Together, a tomato crop requires $0.26 per pound. Consumers pay $0.50-$1
per pound, with much of that going to the grocers. Depending upon weather
and growing conditions ( drought, flood, pests, harvest problems), and market
conditions, growers can make money on a crop or wind up with a significant
loss.
Problems associated with Agricultural practices
I. The problem
of declining genetic diversity [REQUIRED READING] in our
food crops [REQUIRED
READING] and agricultural animals. Seed companies focus on providing
seeds of certain new varieties of crops to growers. These varieties have
been bred for combining favorable characteristics, such as high yield, ability
to survive machine picking and packing, and resistance to certain diseases.
However, by providing only a few varieties, we lose the diversity provided
by growing the wide range of varieties historically developed for that crop;
Different varieties are adapted to different environmental factors (drought,
different pests or diseases, flooding) that we may not forsee as a problem
until they happen. Focusing on one variety is similar to the old adage
of "putting all our eggs in one basket".
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A. Wild populations have great genetic
diversity [REQUIRED
READING]
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1. Genetic diversity
[REQUIRED READING] contributes to a species' long-term survival (provides
the variation that enables a population to adapt to changing conditions)
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B. Domesticated crops have much less genetic diversity
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1. Selective breeding of crops results in uniformity because traits that
are not of obvious value to the farmer at that time are lost in selection.
Only a few varieties of a crop are often used by many farmers accross a wide
region.
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C. Lower genetic diversity increases the likelihood that domesticated plants
and animals will succumb to new stresses which were not forseen (e.g. new
strains of disease organisms or insects, drought, flooding, high UV levels).
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1. Disease organisms- bacteria, fungi, viruses- evolve quite rapidly (e.g.
think of the new strains of flu virus that spreads each year).
2. When a new strain of disease breaks out in a uniform domesticated population,
all of the population may be highly susceptible
3. A new strain of disease would probably not hurt a wild population much
because it is more genetically diverse- at least some of the population would
have genes to resist the pathogen
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D. Human in many parts of the world rely upon plants found native in other
parts of the world, especially
North America. Today,
national
and international efforts exist to preserve and maintain genetic diversity
in a variety of plants, especially
domesticated
crops.
II. Soil
erosion: Loss of soil comes from tilling (plowing) the land and the
effects of wind and
water
Soil is a valuable resource. The rich and deep soil of the Midwest
US is what makes our country the breadbasket of the world and is important
to our economy. Soil is made of organic and inorganic matter.
The organic matter is important for giving
texture
to the soil, which allows root growth, and for feeding the
microorganisms
that live in the soil and the plant needs to grow. The topsoil
is the richest in organic matter.
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A. Problems: average soil erosion rate = 18-100 metric tons/hectare/year;
average rate of soil formation = 1 metric ton/hectare/year (hectare=2.54
acres)
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1. The soil on top (topsoil) which is lost contains nutrients and has an
open structure; the soil which is left after erosion (the subsoil) contains
fewer essential minerals, less water-holding capacity, and has a texture
more like concrete.
2. Soil erosion causes
silting of
waterways
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B.
Solutions
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1. Less plowing
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a. Less reliance on annual crops and more reliance on perennials (develop
a perennial form of corn?)
b. No-till agriculture (weeds and certain pathogens are a problem with this
method, however) - helps with soil fertility, remember the soil is a complete
ecosystem
c. Increase use of
crop
rotation and other practices
d. Use
green
manure crops
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2. Prohibit growing crops on marginal land (sloping, dry, or already eroded
land)
3. Use of contour
strip intercropping, a new method used where wind erosion is a problem,
along with crop rotation
4. Gully reclamation; planting shelterbelts (windbreaks)
III.
Salinization
of soil from irrigation: salt built-up in the soil to the point that
crop plants can not grow. Water is necessary for plant growth. Irrigation
is needed to farm areas that do not have sufficient rainfall to water crops.
Many areas of the US could not otherwise be used for the crops that
consumers will buy.
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A. Problem: All fresh water used for irrigating crops contains salts. Always
adding the minimum amount of water required to wet the soil means that when
most of water evaporates from irrigation, the salts in the water are left
behind to accumulate in the soil ( e.g. like salt crystals seen after a glass
of salt water evaporates)
B. Solutions
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1. New irrigation systems such as trickle systems deliver water to roots
so there's less evaporation (these methods also consume much less water);
laser-leveling of field prevents wasted water in low-lying soggy areas
2. Restoration of salt-damaged land- VERY expensive- involves pouring excess
amounts of water on the land to dissolve the salts and letting it run off
IV.
Desertification:
The expansion of desert at the expense of cropland or rangeland.
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A.
Problems
[REQUIRED READING]
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1. 35 % of earth's surface is arid or semiarid desert
2. Deserts are increasing alarmingly due to both natural processes and human
activities
3. Worldwide, 2 billion acres have become deserts in the past 50 years; current
rate of desertification is 15 million acres per year
4. Worst problem in sub-Saharan Africa
5. Causes of desertification
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a. Overgrazing of rangeland (too many livestock on a given area consume plants
faster than they can recover = exposed soil = soil erosion and degradation
= plant life cannot get re-established)
b. Cultivating crops on marginal arid or semiarid land (droughts cause crop
loss) = exposed soil etc.
c. Improper irrigation -> salinization becomes so bad that plants don't
grow in the soil = exposed soil etc.
d. Deforestation (removal of forests) without adequate replanting = exposed
soil etc.
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B. Solutions
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1. Replant area with drought-tolerant plants (grasses with fibrous root systems)
so the soil is covered
2. Retire the land from grazing and crop cultivation for a minimum of 10-20
years (this is currently being done with degraded farmlands in the western
U.S.)
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a. Not feasible in Africa where large numbers of people and their livestock
are "supported" by this land
b. Catch-22 situation for Sahelians (in sub-Sahara)- if they allow the land
to recover, they will starve; if they continue to graze and farm the already
degraded land, they will starve
V. Excessive use of commercial
fertilizer:
Fertilizers must be used because growing and harvesting plants depletes the
soil of essential minerals (fertilizer mainly contains: N - nitrogen; P -
phosphorous; K - potassium) that are taken up into the plant. Some
crops, such as tobacco, use up more of the minerals than others.
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A. Problems
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1. Chemical fertilizers eventually decrease organic matter in soil (from
continuous cropping)= less water-holding capacity
2. Fertilizer in surface and ground water entering lakes and streams allows
overgrowth
of algae, whose overgrowth kill off other plants and animals in the waterway
(example- excessive runoff
into tributaries leading to Chesapeake Bay (Nitrogen levels in
Bay- image at right)-> increased population of algae -> loss of bay
grasses -> decline in rockfish
3. Extreme possibilities- nitrate poisoning of groundwater or of leafy vegetables
grown with excessive nitrate
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B. Solutions
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1. Use organic (slow-release) fertilizers (organic fertilizers also improve
soil texture) - commercial fertilizers are now more expensive since they
are made from oil. A main tenet of
organic gardening.
Composting- turning
garden and yard waste into fertilizer.
2. Crop rotation with a leguminous crop (a nitrogen fixer-supplies nitrogen
to the soil)
3. Water treatment (tertiary treatment) to remove nitrate from runoff (okay
for urban runoff from lawns, but too expensive for farmer) e.g
Florida's
sugarcane farmers
4. Great care by
all
of us
5. One example from our
area
VI. Widespread use of pesticides- chemicals used to control insects
(pictures
of examples), fungi, bacteria, nematodes which injure crop plants.
Overall, pests reduce the productivity by about 15-20%. At some times
or in some places, pests and diseases will become much more of a problem
and can effectively wipe out most of a crop. Pesticides can reduce this
loss and so are valuable for producing
crops.o
keep the plant upright.
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A. Problems:
-
1. Consumers demand unblemished fruit and vegtables - pesticide use is widespread
as a preventative measure to assure unblemished produce.
2.Genetic resistance to pesticides is developing in many insects -thus we
can lose more to pests now than when we didn't use pesticides or try to use
more toxic pesticides.
3. Many agricultural chemicals in use today have never been tested for their
health effects; we don't know is they cause cancer or birth defects. Some
foods have been found to contain residues of pesticides
(see List
of 12 foods that have the highest pesticide residues)
Possible health problems caused by
pesticides
4. pesticides are used excessively (apples may be sprayed as many as 20 times
during a growing season) as a preventative measure rather than being applied
in response to the presence of a pest.
5. Pollution of soil and water (example- widespread use of DDT interfered
with eggshell production in birds at top of the food chain) - pesticides
don't stay put
6. Broad spectrum insecticides kill beneficial insects as well as harmful
ones
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B. Solutions
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1. Trend today is for replacement of chemicals with biological pesticides-
living organisms that work as a pesticide
as long as the organism is alive, e.g.
biological control
using
parasitoids
insects,weed
feeders,predators,pathogens,
and
antagonists
[REQUIRED READINGS - all five sites from Cornell University]. A main tenet
of organic gardening
2. IPM
(Integrated Pest Management), which makes use of a combination of natural,
biological controls (preferred) and chemical controls (only when needed and
only the amounts necessary) to attack pest problems when they occur
VII. Other examples
-
A. Forest fire
B. Deforestation,
especially in the
tropics from salinization,
desertification, fertilizer problems, and pesticides. Solution to these problems
must be based on the concepts of
sustainable agriculture
- the idea that practices should be based upon renewable resources and long-term
productivity.
Other Sites of Interest:
Web of Life
World Wildlife Fund
World Resources Institute:
Index to topics
Agriculture Genome Information
Center
AgriGator: An
agricultural based search engine FAO: Food
and Agriculture Organization of the United Nations
Union of International Associations: Links
to all aspects of human effort
CIESIN: Information for a changing
world
CGIAR: Consultative Group on
International Agricultural Research
Review of soil erosion
Pesticide
residues in food
Review of salinization
and inadequate soil drainage
Review of drought
"Desertification
of arid lands" by H.E. Dregne, a classical paper
A
history of the Chesapeake Bay
Pesticides Consumer
information
Report on Pesticide concerns- Consumer
Reports web site
Pesticides in foods
Technical report on pesticides in
food
National IPM Network
National Biological Control
Institute
Some technical
definitions relating to biodiversity
Growing Green:
Executive Summary
IPM applications
Last revised: November 3, 1998 - Straney