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Section C: Horticulture Crops in the agro-ecosystem
Projected
Outcomes
- Students will begin to apply ecological analysis
to vegetable and fruit production systems
- Students will learn about some key agro-ecological
management practices, including soil and fertility
management, crop rotation, and pest management.
- Students will gain an appreciation of the
complexity and variety of the agroecology of
horticultural crops.
Background
/ Lessons
Introduction
Like natural ecosystems, agro-ecosystems are
characterized by nutrient flows and cycles, energy
flows, and the interactions of living organisms
with each other and the physical environment.
However, agro-ecosystems differ from natural ecosystems
in two key ways:
- First, we expect them to export particular
biological goods for our use.
- Second, we deliberately manipulate them to
get them to produce those goods in abundance.
These two special qualities of agro-ecosystems
in turn affect their key ecological processes.
Sustainable agriculture seeks to take advantage
of ecosystem processes by designing an agricultural
system that works with them rather than against
them to achieve its production goals.
This section begins with a quick look at the
role of the soil in the agro-ecosystem and then
encourages students to think about the ecology
of horticultural production by posing four ecological
questions.
- What are the nutrient and water flows in
the system?
- What are the sources and sinks of pollutants
in the system
- What are the interactions of living organisms
in the system
- What are the energy flows in the system?
Horticultural crops vary widely in their ecological
characteristics, from row-cropped annual vegetables
to fruit trees that may be productive for more
than 30 years. Moreover, while some fruit and
vegetable farms specialize in producing one or
two crops, others produce dozens of different
kinds of plants. This unit will provide some indication
of the variety of sustainable horticultural systems
in Iowa and Wisconsin, but it cannot cover it
all. Classes are encouraged to apply the four
agro-ecological questions to crops grown in their
areas.
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The
Ground Beneath Our Feet
| "The nation that destroys
its soil, destroys itself." - Franklin
Delano Roosevelt
Essentially, all life depends upon the
soil .... There can be no life without soil
and no soil without life; they have evolved
together." - Charles E. Kellogg, USDA
Yearbook of Agriculture, 1938
Vist the Natural
Resources Conservation Service site
for more soils quotes
|
Every farmer knows that soils are important.
But different farmers (and scientists) think about
soils in different ways.
One way to look at soils is as a physical medium.
It needs to serve a variety of mechanical functions:
provide a substrate for plant roots to grow in,
allow water to drain so plant roots have access
to oxygen, but hold on to enough water that roots
have access to water. The soil is also where plant
nutrients are stored, transferred to roots, and
sometimes lost.
Many sustainable farmers think about soils as
a living system. They value the mechanical functions
of soil, but they look beyond those properties
to biological and ecological services. These farmers
seek to build and maintain good soil health, rather
than simply avoiding damaging the physical structure
of the soil.
Activity
1: Trial by Water
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Our understanding of soil biology is still quite
rudimentary, but we are learning more and more
about how the multitude of living organisms in
the soil affect soil quality and processes and
about how our actions in turn affect the life
of the soil.
Key groups of soil organisms include:
Bacteria (single-celled organisms that are neither
plants nor animals)
Fungi (neither plants nor animals, typically grow
in long chains of cells called hyphae)
Protozoa (single-celled animals such as amoebae)
Nematodes (tiny non-segmented worms)
Arthropods (invertebrates such as insects, spiders,
millipedes, etc.)
Earthworms
(Plant roots)
Each of these groups contains a wide variety
of species, and the different species do very
different things. For example, one gram of soil
may contain 11,000 different species of bacteria.
Some bacteria help decompose organic matter, some
fix nitrogen, some prey on living organisms causing
disease, and a few bacteria photosynthesize.
Together, soil organisms perform critical ecological
functions such as decomposing organic matter,
changing soil structure, moving, stabilizing,
and transforming nutrients, altering chemicals
such as pesticides, and eating or helping each
other.
Soil
ecology introduction powerpoint (Microsoft PPT)
Soil ecology introuction powerpoint (Adobe PDF)
Scientists are just beginning to explore life in the soil and how it interacts with agricultural management. As you move through the four ecological questions below, keep the role of the soil and of soil organisms in mind.
What
are the nutrient and water flows in the system?
Several factors influence the nutrient and water
flows in horticultural production.
- All plants require nutrients to grow. Because
they take nutrients away from a site when they
harvest crops, farmers have to replace those
nutrients to keep that land productive.
- However, different crops have different nutrient
needs. For example, spinach and sweet corn require
lots of nitrogen to yield well, while beans,
peas, and many fruits do well with little added
nitrogen but may have special water or micronutrient
requirements.
- The growth habit of the crops is also important.
A “crop” such as turfgrass, with
year-round ground-cover, requires different
nutrient and water management than for example
tomatoes, which only grow in summer and are
generally surrounded by bare soil or possibly
a non-living mulch.
- Even within a crop type, different management
approaches can have significant impacts on nutrient
and water flows.
Let’s take a look at nutrient and water
impacts of two crops: sweet corn and cranberries.
Sweet corn
Nothing symbolizes the taste of summer quite
like fresh
early,
sweet corn. Customers seek out the
varieties they like – early, sweet “Sugar
Buns” or elegant white “Silver Queen.”
But most don’t think beyond appearance and
flavor to how the corn was raised.
Sweet corn grows best with lots of sunshine,
enough but not too much water, and lots of nutrients
in the soil, especially nitrogen. Because corn
responds well to high fertility, it can be tempting
to add generous amounts of fertilizer. But excess
nitrogen can leach into groundwater, and phosphorus
can run off into surface water, damaging aquatic
communities and drinking water supplies. (Visit
the Ohio
State University Extension Fact Sheet online,
or The
University of Missouri State Extension) In
addition, money spent on unneeded fertilizer cuts
into farm profits.
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Farmers can use a variety of practices to reduce
the need for purchased fertilizer in sweet corn
production:
- Recognize that sweet corn needs less nitrogen
than grain corn, because it is harvested earlier.
On most soils, a rate of 130 lbs of N per acre
is recommended for sweet corn. On soils with
less than 2% organic matter a rate of 150 lbs/acre
is optimal.
- Use realistic yield goals when calculating
nutrient needs.
- Supply N through crop rotation. If the corn
follows a good stand of alfalfa or clover, it
will need no supplemental nitrogen. If it follows
soybeans, potatoes, or a small grain, N applications
can usually be reduced by about 30 to 40 lbs
per acre. (Crop rotation also benefits sweet
corn by reducing some pests.)
- Use a late spring soil nitrate test to measure
available nitrogen already in the soil, rather
than assuming the crop will need all its nitrogen
supplied by fertilizer applications. (See “Nitrogen
Fertilizer Recommendations for Corn in Iowa,”
p. 2, “Using
the Late Spring Nitrate Test to Reduce Nitrate
Loss Within a Watershed”, “The
Presidedress Soil Nitrate Test”, and
“Nitrogen
$ Rate of Return Calculator.” Remember
to use sweet corn rather than grain corn rates
for the starting point.)
- Side-dress N applications while the crop is
growing and can use the nutrients right away,
rather than relying on fall or pre-plant applications.
Fall N applications are likely to leach significant
amounts of N.
- Use composted manures
to supply the phosphorus, potassium, and part
of the nitrogen needs of the corn.
- Plant a cover crop such as winter wheat or
rye after harvest to prevent runoff and leaching
of nutrients.
- Use conservation tillage to minimize runoff
and erosion .
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Cranberries
Compared to most crops, cranberries require very
little fertilizer, in part because as perennial
plants they do not have to grow new stems every
year and can store nutrients over the winter.
The recommended rates are from 20 to 40 lbs of
N and 20 to 45 lbs of P2O5 per acre per year.
Compare these guidelines to recommended rates
of 80 to 150 lbs/acre N and 0 to 100 lbs/acre
P2O5 per year for sweet corn and 40 to 100 lbs/acre
N for broccoli.
Moreover, unlike most crops, cranberry yields
decline quickly when too much nitrogen fertilizer
is applied. So you might think that should make
sustainable nutrient management for cranberries
easy. But it is not so simple. The way cranberries
grow provides some special challenges.
Cranberry plants are very picky about what kinds
of nutrients they will take up. Whereas most plants
will take up nitrogen as ammonium and/or nitrate,
cranberries only use ammonium. And while most
food crops grow in nearly neutral mineral soils,
cranberries grow in acid soils that are high in
iron and aluminum. These special soil characteristics
change the behavior of phosphorus in the soil.
On top of that, cranberry bogs are regularly flooded
to help with harvest and prevent frost damage
and winter kill. This flooding means that nutrients
that are not held in the plant are more likely
to run off to surface waters (phosphorus) or leach
to ground water (nitrogen).
These features mean that many of the usual tools
for sustainable nutrient management (use of manure,
compost, legumes, and cover crops) don’t
work for cranberry production. The main approach
that most cranberry growers have for improving
the sustainability of nutrient use is to fine
tune the amount and timing of nutrients applied,
using:
- plant tissue testing to determine nutrient
needs over the next 15 months,
- fertilizer selection to target P and N ratio
and type to cranberry needs,
- careful monitoring of developmental stages
and appearance of the plants to determine
optimum timing for fertilizer application,
and
- accurate fertilizer delivery equipment.
These tools can help maximize the amount of
fertilizer taken up by the plants and minimize
loss to surface or ground water.
In recent years, with the growth of the organic
market, some growers have started producing cranberries
organically. Organic growers rely on compost teas
and fish-based fertilizers to add nutrients. These
materials do not provide the rapid release of
plant available N that synthetic ammonium and
urea do, and the difference in nitrogen availability
is thought to be the main reason why organic cranberry
yields are typically much lower than conventional
yields (Visit UW
Madison Center for Integrated Agricultural System's
Overview of Cranberry Production.) Nevertheless,
with good management and the price premium for
organic products, organic production is proving
viable for a number of growers. Research, breeding,
and experimentation with organic management may
improve yields considerably.
Because irrigation and flooding are such important
practices in commercial cranberry bogs, cranberry
production also has special impacts on water cycling.
Cranberry growers store water in natural and artificial
wetlands. On average, in Wisconsin each acre of
producing cranberry bog has about ten acres of
support lands to store water. Cranberry growers
point out that these support lands provide habitat
for a variety of wetland plants and animals (Wisconsin
State Cranberry Growers Association) and that:
… growers now use sophisticated
systems to conserve and recycle the large
amounts of water necessary for cranberry growing.
All flooding and sprinkling involves “borrowing”
water from within the same system—it
is just temporarily moved from one area to
another. Because cranberry vines and berries
absorb little water and the soil is saturated
enough that little water goes into the groundwater,
the water is returned to its original location,
unchanged. The government considers cranberry
growers’ use of water to be predominantly
“non-consumptive” because the
water does not degrade in quality or quantity.
(From “Wetlands
& Cranberry Growing: Environmental Partners”by
the Wisconsin State Cranberry Growers Association)
Critics note that cranberry irrigation and water
storage for harvest can exacerbate the impacts
of drought on natural wetlands by withholding
water that normally would contribute to stream
recharge. They also worry that water that has
cycled through a cranberry bog can pick up nutrients
and pesticides that may then harm aquatic communities.
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Compost
Compost is one of the most important nutrient
management tools for sustainable fruit, vegetable,
and flower growers. Composting is the controlled
decomposition of biological materials. (Visit
The
Art and Science of Composting.)
Compost offers several advantages:
- Nutrients tend to be stable and bound to
organic matter, so they are less likely to
run off or leach than nutrients in raw manure
or synthetic fertilizers.
- Compost is much more homogeneous than raw manure, making it easier to spread on cropland.
- Compost has a mild “earthy”
odor, unlike the unpleasant odor of manure.
- Compost acts as a soil amendment as well
as a nutrient source, adding organic matter
and improving soil tilth.
- The composting process greatly reduces
or eliminates pathogens that are harmful to
human health.
- Many types of compost contain far fewer
viable weed seeds than
uncomposted
manure.
- Compost can help suppress certain plant
diseases.
Compost also offers some challenges:
- Up to half of the nitrogen contained in
the material being composted is lost to the
atmosphere during the composting process.
However in many trials, plants gown with compost
yield more than expected at low levels of
soil N.
- Compost can be highly variable, depending
on the materials composted and on the composting
process. Growers need to either analyze composts or request an analyis to be sure the compost they have will meet their needs.
- Compost is much bulkier and therefore harder
to transport, store, and apply to large areas
than synthetic fertilizers.
- Composting requires careful management,
and depending on the composting system chosen,
may require expensive equipment.
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Activity
2. Compost Recipes
Activity
3. Compost Happens
How do soils relate to nutrient and water flows?
- Water and nutrients move easily through sandy soils. Nitrogen can quickly leach to groundwater. Much of Wisconsin 's sweet corn is grown on sandy soils, which makes sustainable nutrient management both more critical and more challenging.
- Organic matter improves the ability of soils to retain both water and nutrients. Composted manures and incorporation of green manures can help raise soil organic matter over time.
- Nitrogen fertilizers such as urea and anhydrous ammonia can change the composition of soil organisms.
What
are the sources and sinks of pollutants in the
system?
A sustainable system will minimize the amount
of pollutants introduced into the environment.
What is a pollutant? It is a
chemical that is damaging to human health or the
environment.
Just as a weed is a “plant out of place,”
so whether something is a pollutant depends in
part on context. For example, soil particles are
a valuable resource in the crop field. However,
if those same soil particles are carried from
the field to a stream or river by erosion, they
become sediment—a pollutant that can severely
damage aquatic communities.
Suggested
activity: Resource or Pollutant
Thus, the source of many agricultural
pollutants is deliberate application of inputs
such as fertilizers and pesticides. Another source
is beneficial resources such as soil or manure,
which turn into pollutants when mismanaged and
displaced. A sink is where the
pollutant winds up. Surface waters, including
rivers, lakes, and the ocean, are a common sink
for agricultural pollutants.
The broad categories of pollutants in horticultural
crops include:
- excess nutrients running off or leaching to
surface and ground waters
- soil erosion degrading wetlands
- pesticides harming non-target organisms
Apples
What are the pollution sources in apple production,
and what sustainable practices can reduce pollution?
The main pollution concern for apple growers
is pesticides.
Key apple pests in the Upper Midwest include
plum curculio, codling moth, and apple maggot.
Leafrollers, aphids, leafminers, leafhoppers,
scale insects, and spider mites can also cause
damage in apple orchards. And diseases such as
fire blight, apple scab, and powdery mildew can
destroy the crop or make it unmarketable. (Visit
Urban
Phytonarian Series Common Fruit Insects,or
Apple
Pest Managment for Home Gardeners.)
For decades, apple growers have used a wide array
of pesticides to control these insects, diseases,
and weeds. But since the 1970s scientists, farmers,
and consumers have all become increasingly aware
that pesticides can cause both environmental and
human health problems. As a result, some apple
growers are seeking ways to grow apples using
fewer and less toxic pesticides.
The array of techniques and tools farmers use
to get a good crop with minimal use of pesticides
is called Integrated Pest Management, or IPM.
IPM powerpoint
IPM powerpoint notes
Trapping guidelines
for apple pests (PDF)
Activity
4: Economic threshold calculation
Activity
5: Apple IPM video and discussion
Fertilizer recommendations for tree fruits are
low in comparison to most other agricultural crops,
so runoff and leaching from nutrient applications
are not generally considered to be problems of
apple production in Wisconsin and Iowa. In addition,
fruit quality can deteriorate when too much nitrogen
is applied. Thus, most mature Midwestern orchards
are not fertilized unless tissue testing indicates
a nutrient deficiency.
Visit Michigan State University Extension Fertilizer
Recommendations for Michigan Fruit Crops Growing
Apples in Wisconsin, or Spectrum
Analytic Inc. Fertilizing Apples for information
on nutrient recommendations for apple production.
Including legumes such as clover in the cover
crop for the orchard floor can further reduce
the need for nitrogen fertilizer.
Soil erosion is a possible source of pollution
from apple orchards. Apple trees yield better
when they do not compete with other vegetation
within two or three feet of the tree trunk. Most
Wisconsin and Iowa orchards maintain 5 foot wide
strips of bare ground in the tree rows and 6 to
10 foot wide strips of grass groundcover between
tree rows. If the strips of groundcover are well
maintained and planted along the contours, erosion
can be minimized. Some sustainable growers use
mulch to suppress competing plant growth in the
tree row. Mulching reduces reliance on herbicides
and the potential for soil erosion. Although mulch
can potentially provide cover for rodents and
other pests, a 6 year study of a cherry orchard
in Michigan found that mulching increased yields
over conventional orchard floor management.
How do soils relate to pollution sources and sinks?
- Soil itself can be a pollutant if it is allowed to erode.
- Soil organisms can break down some pollutants, such as disease-causing organisms and some pesticides.
- Other pollutants, such as metals and persistent pesticides, can accumulate in soils and can be taken up by plants that are later eaten by people.
- Pollutants can change the composition of organisms in the soil.
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What
are the interactions of living organisms in the
system?
Typically, sustainable agro-ecosystems will try
to work with species interactions and will favor
species and genetic diversity.
Everything in an ecosystem affects other parts
of the ecosystem. Typically, production agriculture
has focused on the negative impacts of organisms
other than the crop. In this worldview, all non-crop
plants are seen as weeds that compete for water,
nutrients, and sunlight, and all non-crop animals
from insects to birds and mammals are seen as
useless at best and crop-destroying pests or disease
carriers at worst.
There is some truth to this outlook. Weeds do
compete with crop plants, and many types of animals
eat parts of the crop and can cause substantial
yield losses. Agro-ecosystems differ from natural
ecosystems in that we require them to export a
good portion of their production for off-site
human consumption. So farmers cannot afford to
give weeds, crop predators, and diseases a free
hand.
On the other hand, it turns out that many non-crop
organisms benefit crop production in a variety
of ways, such as by improving nutrient cycling
and availability to the crop, pollinating the
crop, eating crop pests, providing habitat for
beneficial species, and reducing disease. Practices
such as heavy use of synthetic fertilizers and
pesticides and mono-cropping may harm beneficial
organisms as much as or more than pests.
The number of different species of plants, animals,
and microorganisms in an ecosystem is referred
to as species diversity. There
is also a different kind of variation, which is
the genetic diversity within
a species or population (the individuals of one
species in an ecosystem).
Let’s take a look at both species
diversity and genetic diversity
in fresh market vegetable production.
A popular model for sustainable vegetable production
is the community supported agriculture
operation or CSA. (Univesity of Iowa Extension
Local
Food Systems for Iowa)
CSAs promote diversity by:
- Planting a wide variety of crops.
Because they try to supply most of the vegetable
needs for shareholders during the growing season,
CSA farms grow a wide variety of vegetables,
herbs, flowers, and fruits. Over the course
of one year, a typical CSA farm will grow more
than 30 different crops on less than 50 acres.
The smallest CSAs have less than 5 acres in
production (Growing
Harmony Farm, Nevada, Iowa) and even the
largest CSAs in the Midwest (Angelic
Organics , Caledonia, Illinois and Harmony
Valley, Viroqua, Wisconsin) have less than
100 acres in crops.
- Rotating crops. CSAs rotate
crops to reduce disease and insect pressures. At least a 4-year rotation is recommended.
- Planting cover crops to
reduce erosion, reduce weed pressure, and manage
nutrient cycling.
- Avoiding use of broad-spectrum pesticides
that can harm non-target species.
- Deliberately encouraging beneficial
organisms, from birds to insects, to
help with pest control.
- Preserving and restoring natural habitats
on the farm. Habitat preservation can help make
the farm more attractive to CSA members, and
it also fits with the values of most CSA farmers.
Natural habitats usually contain far greater
species diversity than crop fields.
- Planting numerous varieties of each
crop, including open-pollinated and heirloom
varieties. CSAs usually value genetic
diversity for several reasons. First and most
important, their customers value diversity and
look for varieties that look interesting and
offer special flavors.
In some cases, using several
different varieties may decrease the chance
that a particular disease or pest will damage
the whole crop. Third, different varieties can
often be harvested at different times,
extending the harvest season
. And finally,
like their customers, many growers take pleasure
in the many different forms and flavors one
species can provide.
For example, in 2005 Harmony Valley CSA
grew 16 varieties of tomatoes: 3 paste (a.k.a.
Roma) tomatoes, 2 standard reds, 4 small
cherry (a.k.a. grape), 2 gold, and 4 heirlooms
out in the field, and Sungold cherry tomatoes.
August
20 newsletter, p. 2.
Activity
6: Tasting Diversity
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Comparison of plant communities
Pre-agricultural / “natural
- Mix of perennial and annual species
- Variety of communities depending on soil
type, climate and micro-climate, and site history
(prairies, savannahs, wetlands, forests, etc)
- Large number of species at one site, typically
little or no exposed soil year-round
- Substantial genetic variation within most
species
Conventional horticulture (vegetable production)
- Sequential mono-cultures (usually 2 or at
most 3 species in rotation, sometimes only one
species)
- Mainly annual row crops, with bare soil between
the rows and completely bare soil exposed for
6 months of the year or more
- Little genetic variety within the species;
often just one or two varieties grown of a crop
and high value placed on consistency of size
and appearance
Sustainable horticulture in Wisconsin and
Iowa
- Sequential mono-cultures (usually 3 to 6 species
in rotation)
- Annual row crops rotated with perennials
and small grains. Use of cover crops to minimize
amount of bare soil exposed in winter
- Interest in increasing genetic variety, including
reintroduction of heirloom varieties as well
as breeding new varieties for diversity of flavor,
appearance, and agronomic traits such as disease
resistance.
- Restoration of complex natural or partly
natural plant communities around crop fields
What are the energy
flows in the system?
Sustainable agro-ecosystems rely more on solar
energy rather than on fossil fuels. Sustainable
systems minimize energy waste.
It is difficult to get recent detailed information
about energy use in modern agriculture, and most
of the work that has been done on energy used
in agricultural production looks at field crops
and livestock production rather than horticultural
farms. Most modern fruit and vegetable production
relies on fossil fuels in a variety of ways, such
as:
- Fuel and electricity to power equipment such
as tractors
- Fertilizers and pesticides made from or with
fossil fuels such as natural gas
- Fuel or electricity to heat greenhouses
- Plastic to cover hoophouses, act as mulch,
and package produce
- Fuel to transport produce to market
(see "Local Foods Could Make for Greener Grocers" by Janet Raloff
- Electricity to keep produce cool
- Fuel and electricity to process produce
Certain farming and food system practices
can provide substantial savings in fossil
energy use.
- Minimize use of nitrogen fertilizer and pesticides
(see strategies recommended in the nutrient
cycling and pollution prevention segments)
- Recycle nutrients and resources on the farm
- Minimize transportation costs by selling
to and buying from local sources
- Use renewable energy sources such as wind,
solar, or biomass-fueled power
More than half of the energy in our food system
is used not on the farm, but in transportation,
processing, storage and packaging, and home cooking. (Center for Sustainable Systems, US Food System Factsheet)
Sustainable practices for the consumer
- Buy local foods, when possible
(see “Food, Fuel, and Freeways”)
- Avoid excess packaging
- Use energy-efficient appliances and techniques
when possible
- Use renewable energy sources, if possible
(solar and wind power)
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