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2-‐6-‐2: Agriculture
A"er this lecture you should be able to… Iden5fy and describe the factors that contribute to the suitability of soil
for agriculture.
Explain the integrated role of air, water, fer5lizers and soil in the produc5on of agricultural goods.
Iden5fy and analyze the impacts that large scale agriculture has on soil and the surrounding ecosystems.
Describe various soil conserva5on techniques and evaluate their effec5veness.
Explain what it sustainable agriculture means with respect to soil.
Soil conservation
Healthy soils are essen5al for agricultural produc5on Human ac5vi5es (overcul5va5on, overgrazing, deforesta5on) cause
erosion
Sustainability means doing all we can to reduce erosion Because of the slow process of soil forma5on, it is considered a
nonrenewable resource. When it’s gone, it’s gone!
Soil conserva5on must be prac5ced at two levels Individual landholders can best preserve soil through tradi5onal
knowledge and prac5ces Public policies can lead to conserva5on or disaster
Soil “Fertility”
For best growth, plants need a root environment that supplies Mineral nutrients, water,
oxygen The proper pH and salinity
Soil fer)lity: the soil’s ability to support plant growth The presence of proper
amounts of nutrients and all other needs
Farmers refer to a soil’s ability to support plant growth as )lth
Ini5ally become available through rock weathering Phosphate, potassium, calcium,
etc. Much too slow to support
normal plant growth Breakdown and release
(recycling) of detritus provides most nutrients
Leaching: nutrients are washed from the soil by water Decreases soil fer5lity Contributes to water pollu5on Nutrient-‐holding capacity: the
soil’s capacity to bind and hold nutrient ions un5l they are absorbed by roots
Fertilizer Agriculture removes nutrients
from the soil
Fer)lizer: nutrients added to replace those that are lost
Inorganic fer)lizer: chemical formula5ons of nutrients Lacks organic maTer Much more prone to leaching
Organic fer)lizer: plant or animal wastes or both Manure, compost (roTed
organic material) Leguminous fallow crops
(alfalfa, clover) Food crops (len5ls, peas)
Fertilizer Use
Can provide op5mal amounts of nutrients efficiently But it lacks organic maTer to support organisms and build soil
structure
It can keep nutrient content high under intensive cul5va5on (two or more cash crops/year) But mineraliza5on and soil degrada5on proceed Addi5onal fer5lizer leaches into waterways
Chemical fer5lizers have a valuable place in agriculture Organic fer5lizers may not have enough nutrients
Growers must use each fer5lizer as necessary
Water is crucial for plants Transpira)on: water is absorbed by roots and
exits as water vapor through pores (stomata; singular = stoma) in the leaves Oxygen enters, and carbon dioxide exits,
through stomata Loss of water through stomata can be drama5c
Wil6ng: a plant’s response to lack of water Conserves water Shuts off photosynthesis by closing stomata Severe or prolonged wil5ng can kill plants
High enough salt levels can draw water out of a plant By osmosis Dehydrates and kills plants
Novice gardeners also kill plants by overwatering (drowning) Roots must breathe to obtain oxygen for
energy
Water and water-‐holding capacity Water-‐holding capacity: soil’s ability to hold water
a"er it infiltrates
Poor holding capacity: water percolates below root level Plants must depend on rains or irriga5on Sandy soils
Evapora5ve water loss depletes soil of water The O horizon reduces water loss by covering
the soil
Land plants depend on loose, porous soil Soil aera)on: allows diffusion of oxygen into,
and carbon dioxide out of, the soil Overwatering fills air spaces
Compac)on: packing of the soil Due to excessive foot or vehicular traffic Reduces infiltra5on and runoff Strongly influenced by soil texture
Irrigation Irriga5on: supplying water to croplands ar5ficially
Drama5cally increases produc5on
Is a major contributor to land degrada5on
Flood irriga5on: river water flows into canals to flood fields
Center-‐pivot irriga5on: water is pumped from a well into a giant pivo5ng sprinkler
Drip irriga5on: tubes are run parallel with crop lines and provide slow, constant water supply
The U.S. Bureau of Reclama5on is involved with supplying irriga5on water to the western states
Irriga5ng 4 million hectares (10 million acres)
Worldwide irriga5on is huge and is s5ll rising
The soil community
To support plants, soils must Have nutrients and good nutrient-‐holding capacity
Allow infiltra5on and have good water-‐holding capacity
Resist evapora5ve water loss
Have a porous structure that allows aera5on
Have a near-‐neutral pH
Have low salt content
According to the principle of limi5ng factors, the poorest aTribute is the limi5ng factor
Soil degradation
Turnover of plant material produces detritus When humans cut forests, graze livestock, or plant crops, the soil
is managed or mismanaged
Soil degrada6on: occurs when key soil aTributes required for plant growth or other ecosystem services deteriorate
Some reports on soil degrada5on are incorrect or outdated 75% of the land in Burkina Faso was said to be degraded
But agricultural yields have increased due to soil and water conserva5on
Mineralization
If detritus is lost, soil organisms starve Soil will no longer be kept loose
and nutrient-‐rich Humus decomposes, breaking
down the clumpy aggregate structure of glued soil par5cles Water-‐ and nutrient-‐holding
capaci5es, infiltra5on, and aera5on decline
Mineraliza)on: loss of humus and collapse of topsoil All that remains are the
minerals (sand, silt, clay) Topsoil results from balancing
detritus and humus addi5ons and breakdown
Drylands and desertification
Clay and humus are the most important parts of soil For nutrient-‐ and water-‐holding
capacity Their removal results in nutrients
being removed
Regions with sparse rainfall or long dry seasons support grasses, scrub trees, and crops only if soils have good water-‐ and nutrient-‐holding capacity Erosion causes these areas to become
deserts
Deser)fica)on: a permanent reduc5on in the produc5vity of arid, semiarid, and seasonally dry areas (drylands) Does not mean advancing deserts
Salinization
Saliniza)on: the accumula5on of salts in and on the soil Suppresses plant growth
Even the freshest irriga5on water has some salt Watering dryland soils dissolves
minerals in the soil Evapora5on or transpira5on
leaves salts behind
Saliniza5on is considered a form of deser5fica5on 1.5 million hectares (3.7 million
acres) are lost each year to saliniza5on and waterlogging
160,000 hectares (400,000 acres) in California are unproduc5ve, cos5ng $30 million/year
Enough water must be used to leach salts downward Insufficient drainage results in
waterlogged soils Installing drainage pipes is
expensive Kesterson Na5onal Wildlife Refuge
received drainage from selenium-‐enriched soils Killing birds, fish, insects, and
plants It was declared a toxic waste
dump It has been drained and capped
with soil Over 14 other U.S. loca5ons have
toxic irriga5on water The “Kesterson Effect”
Erosion
Erosion: the process of soil and humus par5cles being picked up and carried away by water and wind Occurs any 5me soil is bared and
exposed
Soil removal may be slow and gradual (e.g., by wind) or drama5c (e.g., gullies formed by a single storm)
Vegeta5ve cover prevents erosion from water Reducing the energy of raindrops Allowing slow infiltra5on
Grass is excellent for erosion control
Vegeta5on also slows wind velocity
Splash, sheet, and gully erosion
Splash erosion: begins the process of erosion Raindrops break up the clumpy structure of topsoil Dislodged par5cles wash between other aggregates Decreases infiltra5on and aera5on
Sheet erosion: the result of decreased infiltra5on More water runs off, carrying away fine par5cles
Gully erosion: water converges into rivulets and streams Water’s greater volume, velocity, energy remove soil
Once started, erosion can turn into a vicious cycle Less vegeta5on exposes soil to more erosion
Causes of erosion: overcultivation
Plowing to grow crops exposes soil to wind and water erosion Soil remains bare before plan5ng and a"er harvest
Plowing causes splash erosion Destroying soil’s aggregate structure
Decreasing aera5on and infiltra5on
Tractors compact soil Reducing aera5on and infiltra5on
Increasing evapora5ve water loss and humus oxida5on
Rota5ng cash crops with hay and clover is sustainable
No-‐till planting
No-‐)ll agriculture: a technique allowing con5nuous cropping while minimizing erosion Rou5nely prac5ced in the U.S.
A"er spraying a field with herbicide to kill weeds A plan5ng apparatus cuts a furrow
through the mulch Drops seeds and fer5lizer Closes the furrow
The waste from the previous crop becomes detritus So the soil is never exposed
Low-‐5ll farming uses one pass (not 6–12) over a field
Reducing soil erosion
Contour strip cropping: plowing and cul5va5ng at right angles to contour slopes Shelterbelts: protec5ve belts of trees
and shrubs planted along plowed fields
The U.S. Natural Resource Conserva)on Service (NRCS) Established in response to the Dust
Bowl Regional offices provide informa5on to
farmers and others regarding soil and water conserva5on prac5ces
U.S. soil erosion has decreased through conserva5on Windbreaks, grassed waterways,
vegeta5on to filter runoff
Overgrazing
Livestock graze on grasslands and cleared forest slopes 65% of drylands are grasslands
Land is o"en overgrazed Barren land is eroded and degraded
In the 1800s American buffalo (bison) were slaughtered Rangelands stocked with caTle were overgrazed Leading to erosion and growth of unpalatable plants
U.S. western rangelands produce less than 50% of the forage they produced before commercial grazing Yet 20% of rangelands remain overstocked
The other end of the erosion problem
Water that does not infiltrate enters streams and rivers Causing flooding
Sediment: eroded soil carried into streams and rivers Clogs channels, intensifies floods, fills reservoirs Kills fish and coral reefs Damages streams, rivers, bays, estuaries
Excess sediments and nutrients from erosion are the greatest pollu5on problem in many areas
Groundwater is depleted Rainfall runs off and does not refill soil or ground water
End-‐of-‐Lecture Objectives
Iden5fy and describe the factors that contribute to the suitability of soil for agriculture.
Explain the integrated role of air, water, fer5lizers and soil in the produc5on of agricultural goods.
Iden5fy and analyze the impacts that large scale agriculture has on soil and the surrounding ecosystems.
Describe various soil conserva5on techniques and evaluate their effec5veness.
Explain what it sustainable agriculture means with respect to soil.
