TO SCREW UP (AND FIX) THE SOILMrs. D

http://www.nbclearn.com/portal/site/learn/changing-planet

Withering Crops

Bad results: 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 particles• Water- and nutrient-holding capacities, infiltration, and aeration decline

• Mineralization: loss of humus and collapse of topsoil• All that remains are the minerals (sand, silt, clay)

• Topsoil results from balancing detritus and humus additions and breakdown

The importance of humus to topsoil

Bad results: Soil degradation• Turnover of plant material produces detritus

• When humans cut forests, graze livestock, or plant crops, the soil is managed or mismanaged

• Soil degradation: occurs when key soil attributes required for plant growth or other ecosystem services deteriorate

• Some reports on soil degradation are incorrect or outdated• 75% of the land in Burkina Faso was said to be degraded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 conservation

Bad Soil: Erosion• Erosion: the process of soil and humus particles being picked up and carried away by water and wind• Occurs any time soil is bared and exposed

• Soil removal may be slow and gradual (e.g., by wind) or dramatic (e.g., gullies formed by a single storm)

• Vegetative cover prevents erosion from water• Reducing the energy of raindrops• Allowing slow infiltration

• Grass is excellent for erosion control• Vegetation 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 particles wash between other aggregates• Decreases infiltration and aeration

• Sheet erosion: the result of decreased infiltration• More water runs off, carrying away fine particles

• 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 vegetation exposes soil to more erosion

Causes of erosion: overcultivation• Plowing to grow crops exposes soil to wind and water erosion• Soil remains bare before planting and after harvest

• Plowing causes splash erosion • Destroying soil’s aggregate structure• Decreasing aeration and infiltration

• Tractors compact soil• Reducing aeration and infiltration• Increasing evaporative water loss and humus oxidation

• Rotating cash crops with hay and clover is sustainable

Erosion

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 pollution problem in many areas

• Groundwater is depleted• Rainfall runs off and does not refill soil or ground water

Desert pavement• Another devastating feature of wind and water erosion: differential removal of soil particles• Lighter humus and clay are the first to be carried away• Rocks, stones, coarse sand remain• The remaining soil becomes coarser

• Deserts are sandy because wind removes fine material• Desert pavement: occurs in some deserts

• Removal of fine material leaves a thin surface layer of stones and gravel

• This protective layer is easily damaged (e.g., by vehicles)

Formation of desert pavement

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

• Desertification: a permanent reduction in the productivity of arid, semiarid, and seasonally dry areas (drylands)• Does not mean advancing deserts

Desertification

Drylands• Desertification is a process of land degradation

• Due to droughts, overgrazing, erosion, deforestation, overcultivation

• It is extremely serious because it is permanent

• Dryland ecosystems cover 41% of Earth’s surface• They are defined by precipitation, not temperature• They receive minimal rainfall• Droughts are common—they can last for years

• Rainfall causes vegetation to return so drylands are not desertified

Preventing soil degradation

•Several farming strategies to prevent soil degradation:• • Crop rotation• • Contour farming• • Intercropping• • Terracing• • Shelterbelts• • Conservation tillage

Reducing soil erosion• Contour strip cropping: plowing and cultivating at right angles to contour slopes• Shelterbelts: protective belts of trees and shrubs planted along plowed fields

• The U.S. Natural Resource Conservation Service (NRCS)• Established in response to the Dust Bowl• Regional offices provide information to farmers and others regarding soil and water conservation practices

• U.S. soil erosion has decreased through conservation• Windbreaks, grassed waterways, vegetation to filter runoff

Recent soil conservation laws• The U.S. has continued to pass soil conservation legislation in recent years:• • Food Security Act of 1985

• • Conservation Reserve Program, 1985

• • Freedom to Farm Act, 1996

• • Low-Input Sustainable Agriculture Program, 1998

• Internationally, there is the UN’s “FAR” program in Asia.

Crop rotation• Alternating the crop planted (e.g., between corn and

soybeans) can restore nutrients to soil and fight pests and disease.

Figure 8.16a

Contour farming• Planting along contour lines of slopes helps reduce erosion on

hillsides.

Figure 8.16b

Intercropping• Mixing crops such as in strip cropping can provide nutrients

and reduce erosion.

Figure 8.16c

Terracing• Cutting stairsteps or terraces is the only way to farm

extremely steep hillsides without causing massive erosion. It is labor-intensive to create, but has been a mainstay for centuries in the Himalayas and the Andes.

Figure 8.16d

Shelterbelts• Rows of fast-growing trees around crop plantings provide

windbreaks, reducing erosion by wind.

Figure 8.16e

Tillage

• Plowing has multiple desirable effects:

–Weeds and weed seeds are buried / destroyed.–Crop residue is turned under - decays faster and helps build soil structure.–Leached nutrients brought closer to surface.–Cooler, darker soil is brought to top and warmed.

Problem:

• Each trip over the field is an added expense to the farmer, and at the same time increases the amount of time the soil is open to erosion via wind or water.

No-till planting• No-till agriculture: a technique allowing continuous cropping while minimizing erosion• Routinely practiced in the U.S.

• After spraying a field with herbicide to kill weeds• A planting apparatus cuts a furrow through the mulch• Drops seeds and fertilizer• Closes the furrow

• The waste from the previous crop becomes detritus• So the soil is never exposed

• Low-till farming uses one pass (not 6–12) over a field• http://www.youtube.com/watch?v=I_7d0h2bSoY&feature=related

Apparatus for no-till planting

Conservation tillage• No-till and reduced-tillage farming leaves old crop residue on

the ground instead of plowing it into soil. This covers the soil, keeping it in place.

• Here, corn grows up out of a “cover crop.”

Figure 8.16f

Conservation tillage• Conservation tillage is not a panacea for all crops everywhere.

• It often requires more chemical herbicides (because weeds are not plowed under).

• It often requires more fertilizer (because other plants compete with crops for nutrients).

• But legume cover crops can keep weeds at bay while nourishing soil, and green manures can be used as organic fertilizers.

Reduces erosion

Saves fuel

Cuts costs

Holds more soil water

Reduces soil compaction

Allows several crops per season

Does not reduce crop yields

Reduces CO2

release from soil

Can increase herbicide use for some crops

Leaves stalks that can harbor crop pests and fungal diseases and increase pesticide use

Requires investment in expensive equipment

DisadvantagesAdvantages

Trade-Offs

Conservation Tillage

Waterlogging and salinization

• Overirrigation can raise the water table high enough to suffocate plant roots with waterlogging.

• Salinization (buildup of salts in surface soil layers) is a more widespread problem.

• Evaporation in arid areas draws water up through the soil, bringing salts with it. Irrigation causes repeated evaporation, bringing more salts up.

Fertilizers• Supply nutrients to

crops

• Inorganic fertilizers = mined or synthetically manufactured mineral supplements

• Organic fertilizers = animal manure, crop residues, compost, etc.

Figure 8.18

Global fertilizer usages• Fertilizer use has risen dramatically in the past 50 years.

Figure 8.19b

Inorganic fertilizer• Can provide optimal amounts of nutrients efficiently

• But it lacks organic matter to support organisms and build soil structure

• It can keep nutrient content high under intensive cultivation (two or more cash crops/year) • But mineralization and soil degradation proceed• Additional fertilizer leaches into waterways

• Chemical fertilizers have a valuable place in agriculture• Organic fertilizers may not have enough nutrients

• Growers must use each fertilizer as necessary

Soil conservation & Sustainable Ag• Sustainability means doing all we can to reduce erosion• Soil conservation must be practiced at two levels

• Individual landholders can best preserve soil through traditional knowledge and practices

• Public policies can lead to conservation or disaster

• Goals: • Maintain productive topsoil• Keep food safe and wholesome• Reduce chemical fertilizers and pesticides• Keep farms economically viable• Sustainable options mimic past practices

• Contouring, crop rotation, terracing, little or no chemicals

• The U.S. Sustainable Agriculture Research and Education program (SARE) (1988)• Provides $5–12 million/year for building and disseminating knowledge

about sustainable agriculture