BIO1: SOIL PROPERTIES AND SOIL CONSERVATION

This unit continues to show the properties of soil and how it can be conserved to get better yields.

 

SOIL  PROPERTIES

The physical properties of soils, in order of decreasing importance, are texture, structure, density, porosity, consistency, temperature, colour and resistivity. Most of these determine the aeration of the soil and the ability of water to infiltrate and to be held in the soil. Soil texture is determined by the relative proportion of the three kinds of soil particles, called soil “separates”: sand, silt, and clay.

Larger soil structures called “peds” are created from the separates when iron oxides, carbonates, clay, and silica with the organic constituent humus, coat particles and cause them to adhere into larger, relatively stable secondary structures. Soil density, particularly bulk density, is a measure of soil compaction. Soil porosity consists of the part of the soil volume occupied by gases and water. Soil consistency is the ability of soil to stick together. Soil temperature and colour are self-defining. Resistivity refers to the resistance to conduction of electric currents and affects the rate of corrosion of metal and concrete structures. The properties may vary through the depth of a soil profile.

Generalized Influence of Soil Separates on Some Properties/Behavior of Soils
Property/behaviorSandSiltClay
Water-holding capacityLowMedium to highHigh
AerationGoodMediumPoor
Drainage rateHighSlow to mediumVery slow
Soil organic matter levelLowMedium to highHigh to medium
Decomposition of organic matterRapidMediumSlow
Warm-up in springRapidModerateSlow
CompactabilityLowMediumHigh
Susceptibility to wind erosionModerate (High if fine sand)HighLow
Susceptibility to water erosionLow (unless fine sand)HighLow if aggregated, otherwise high
Shrink/Swell PotentialVery LowLowModerate to very high
Sealing of ponds, dams, and landfillsPoorPoorGood
Suitability for tillage after rainGoodMediumPoor
Pollutant leaching potentialHighMediumLow (unless cracked)
Ability to store plant nutrientsPoorMedium to HighHigh
Resistance to pH changeLowMediumHigh

Texture

 Soil texture

The mineral components of soil are sand, silt and clay, and their relative proportions determine a soil’s texture. Properties that are influenced by soil texture, include porosity, permeability, infiltration, shrink-swell, water-holding capacity, and susceptibility to erosion. In the illustrated USDA textural classification triangle, the only soil in which neither sand, silt nor clay predominates is called “loam”. While even pure sand, silt or clay may be considered a soil, from the perspective of food production a loam soil with a small amount of organic material is considered ideal. The mineral constituents of a loam soil might be 40% sand, 40% silt and the balance 20% clay by weight. Soil texture affects soil behaviour, in particular its retention capacity for nutrients and water.

Sand and silt are the products of physical and chemical weathering; clay, on the other hand, is a product of chemical weathering but often forms as a secondary mineral precipitated from dissolved minerals. It is the specific surface area of soil particles and the unbalanced ionic charges within them that determine their role in the cation exchange capacity of soil, and hence its fertility.

Sand is least active, followed by silt; clay is the most active. Sand’s greatest benefit to soil is that it resists compaction and increases porosity. Silt is mineralogically like sand but with its higher specific surface area it is more chemically active than sand. But it is the clay content, with its very high specific surface area and generally large number of negative charges, that gives a soil its high retention capacity for water and nutrients. Clay soils also resist wind and water erosion better than silty and sandy soils, as the particles are bonded to each other.

Sand is the most stable of the mineral components of soil; it consists of rock fragments, primarily quartz particles, ranging in size from 2.0 to 0.05 mm (0.0787 to 0.0020 in) in diameter. Silt ranges in size from 0.05 to 0.002 mm (0.002 to 0.00008 in). Clay cannot be resolved by optical microscopes as its particles are 0.002 mm (7.9×10−5 in) or less in diameter.In medium-textured soils, clay is often washed downward through the soil profile and accumulates in the subsoil.

Soil components larger than 2.0 mm (0.079 in) are classed as rock and gravel and are removed before determining the percentages of the remaining components and the texture class of the soil, but are included in the name. For example, a sandy loam soil with 20% gravel would be called gravelly sandy loam.

When the organic component of a soil is substantial, the soil is called organic soil rather than mineral soil. A soil is called organic if:

  1. Mineral fraction is 0% clay and organic matter is 20% or more
  2. Mineral fraction is 0% to 50% clay and organic matter is between 20% and 30%
  3. Mineral fraction is 50% or more clay and organic matter 30% or more.

Structure

 

The clumping of the soil textural components of sand, silt and clay forms aggregates and the further association of those aggregates into larger units forms soil structures called peds. The adhesion of the soil textural components by organic substances, iron oxides, carbonates, clays, and silica, and the breakage of those aggregates due to expansion-contraction, freezing-thawing, and wetting-drying cycles, shape soil into distinct geometric forms.

soil structure

These peds evolve into units which may have various shapes, sizes and degrees of development. A soil clod, however, is not a ped but rather a mass of soil that results from mechanical disturbance. The soil structure affects aeration, water movement, conduction of heat, plant root growth and resistance to erosion. Water has the strongest effect on soil structure due to its solution and precipitation of minerals and its effect on plant growth.

Soil structure often gives clues to its texture, organic matter content, biological activity, past soil evolution, human use, and the chemical and mineralogical conditions under which the soil formed. While texture is defined by the mineral component of a soil and is an innate property of the soil that does not change with agricultural activities, soil structure can be improved or destroyed by the choice and timing of farming practices.

At the largest scale, the forces that shape a soil’s structure result from swelling and shrinkage that initially tend to act horizontally, causing vertically oriented prismatic peds. Clayey soil, due to its differential drying rate with respect to the surface, will induce horizontal cracks, reducing columns to blocky peds. Roots, rodents, worms, and freezing-thawing cycles further break the peds into a spherical shape.

At a smaller scale, plant roots extend into voids and remove water and cause the open spaces to increase, and further decrease physical aggregation size. At the same time roots, fungal hyphae and earthworms create microscopic tunnels that break up peds.

At an even smaller scale, soil aggregation continues as bacteria and fungi exude sticky polysaccharides which bind soil into small peds. The addition of the raw organic matter that bacteria and fungi feed upon encourages the formation of this desirable soil structure.

At the lowest scale, the soil chemistry affects the aggregation or dispersal of soil particles. The clay particles contain polyvalent cations which give the faces of clay layers a net negative charge. At the same time the edges of the clay plates have a slight positive charge, thereby allowing the edges to adhere to the faces of other clay particles or to flocculate (form clumps).

On the other hand, when monovalent ions such as sodium invade and displace the polyvalent cations, they weaken the positive charges on the edges, while the negative surface charges are relatively strengthened. This leaves a net negative charge on the clay, causing the particles to push apart, and so prevents the flocculation of clay particles into larger assemblages. As a result, the clay disperses and settles into voids between peds, causing them to close. In this way the soil aggregation is destroyed and the soil made impenetrable to air and water. Such sodic soil tends to form columnar structures near the surface.

Density

 

Density is the weight per unit volume of an object. Particle density is equal to the mass of solid particles divided by the volume of solid particles – it is the density of only the mineral particles that make up a soil; i.e., it excludes pore space and organic material. Soil particle density is typically 2.60 to 2.75 grams per cm3 and is usually unchanging for a given soil. Soil particle density is lower for soils with high organic matter content, and is higher for soils with high Fe-oxides content.

soil density

Soil bulk density is equal to the dry mass of the soil divided by the volume of the soil; i.e., it includes air space and organic materials of the soil volume. A high bulk density is indicative of either soil compaction or high sand content. The bulk density of cultivated loam is about 1.1 to 1.4 g/cm3 (for comparison water is 1.0 g/cm3).  Soil bulk density is highly variable for a given soil. A lower bulk density by itself does not indicate suitability for plant growth due to the influence of soil texture and structure. Soil bulk density is inherently always less than the soil particle density.

Representative bulk densities of soils. The percentage pore space was calculated using 2.7 g/cm3 for particle density except for the peat soil, which is estimated.
Soil treatment and identificationBulk density g/cm3Pore space %
Tilled surface soil of a cotton field1.351
Trafficked inter-rows where wheels passed surface1.6737
Traffic pan at 25 cm deep1.736
Undisturbed soil below traffic pan, clay loam1.543
Rocky silt loam soil under aspen forest1.6240
Loamy sand surface soil1.543
Decomposed peat0.5565

Porosity

soil porosity

 

 

Pore space is that part of the bulk volume that is not occupied by either mineral or organic matter but is open space occupied by either gases or water. Ideally, the total pore space should be 50% of the soil volume. The gas space is needed to supply oxygen to organisms decomposing organic matter, humus, and plant roots. Pore space also allows the movement and storage of water and dissolved nutrients. This property of soils effectively compartmentalizes the soil pore space such that many organisms are not in direct competition with one another, which may explain not only the large number of species present, but the fact that functionally redundant organisms (organisms with the same ecological niche) can co-exist within the same soil.

There are four categories of pores:

  1. Very fine pores: < 2 µm
  2. Fine pores: 2-20 µm
  3. Medium pores: 20-200 µm
  4. Coarse pores: 200 µm-0.2 mm

In comparison, root hairs are 8 to 12 µm in diameter. When pore space is less than 30 µm, the forces of attraction that hold water in place are greater than the gravitational force acting to drain the water. At that point, soil becomes water-logged and it cannot breathe. For a growing plant, pore size is of greater importance than total pore space. A medium-textured loam provides the ideal balance of pore sizes. Having large pore spaces that allow rapid gas and water movement is superior to smaller pore space but has a greater percentage pore space. Soil texture determines the pore space at the smallest scale, but at a larger scale, soil structure has a strong influence on soil, aeration, water infiltration and drainage.  Tillage has the short-term benefit of temporarily increasing the number of pores of largest size, but in the end those will be degraded by the destruction of soil aggregation.Clay soils have smaller pores, but more total pore space than sand.

Consistency

Consistency is the ability of soil to stick to itself or to other objects (cohesion and adhesion respectively) and its ability to resist deformation and rupture. It is of rough use in predicting cultivation problems and the engineering of foundations. Consistency is measured at three moisture conditions: air-dry, moist and wet; and in those conditions the qualities depend upon the clay content. In the wet state, the two qualities of stickiness and plasticity are assessed. A soil’s resistance to fragmentation and crumbling is assessed in the dry state by rubbing the sample. Its resistance to shearing forces is assessed in the moist state by thumb and finger pressure. Finally, a soil’s plasticity is measured in the wet state by moulding with the hand. Finally, the cemented consistency depends on cementation by substances other than clay, such as calcium carbonate, silica, oxides and salts and moisture content has little effect on its assessment. The measures of consistency border on subjective as they employ the “feel” of the soil in those states.

The terms used to describe the soil consistency in three moisture states and a last consistency not affected by the amount of moisture are as follows:

  1. Consistency of Dry Soil: loose, soft, slightly hard, hard, very hard, extremely hard
  2. Consistency of Moist Soil: loose, very friable, friable, firm, very firm, extremely firm
  3. Consistency of Wet Soil: nonsticky, slightly sticky, sticky, very sticky; nonplastic, slightly plastic, plastic, very plastic
  4. Consistency of Cemented Soil: weakly cemented, strongly cemented, indurated (requires hammer blows to break up)

Soil consistency is useful in estimating the ability of soil to support buildings and roads. More precise measures of soil strength are often made prior to construction.

Temperature

Soil temperature depends on the ratio of the energy absorbed to that lost. Soil has a temperature range between -20 to 60°C. Soil temperature regulates seed germination, plant and root growth and the availability of nutrients. Below 50 cm (20 in), soil temperature seldom changes and can be approximated by adding 1.8°C (2°F) to the mean annual air temperature. Soil temperature has important seasonal, monthly and daily variations. Fluctuations in soil temperature are much lower with increasing soil depth. Heavy mulching (a type of soil cover) can slow the warming of soil, and, at the same time, reduce fluctuations in surface temperature.

Most often, agricultural activities must adapt to soil temperatures by:

  1. maximizing germination and growth by timing of planting
  2. optimizing use of anhydrous ammonia by applying to soil below 10°C (50°F)
  3. preventing heaving and thawing due to frosts from damaging shallow-rooted crops
  4. preventing damage to desirable soil structure by freezing of saturated soils
  5. improving uptake of phosphorus by plants

Otherwise soil temperatures can be raised by drying soils or the use of clear plastic mulches. Organic mulches slow the warming of the soil.

There are various factors that affect soil temperature, such as water content, soil color, and relief (slope, orientation, and elevation), and soil cover (shading and insulation). The color of the ground cover and its insulating properties have a strong influence on soil temperature. Whiter soil tends to have a higher albedo than blacker soil cover, which encourages whiter soils to have cooler soil temperatures. The specific heat of soil is the energy required to raise the temperature of soil by 1°C. The specific heat of soil increases as water content increases, since the heat capacity of water is greater than that of dry soil. The specific heat of pure water is ~ 1 calorie per gram, the specific heat of dry soil is ~ 0.2 calories per gram and the specific heat of wet soil is ~ 0.2 to 1 calories per gram. Also, tremendous energy (~540 cal/g) is required and dissipated to evaporate water (known as the heat of vaporization). As such, wet soil usually warms more slowly than dry soil – wet surface soil is typically 3 to 6°C colder than dry surface soil.

Soil heat flux refers to the conduction (or movement) of energy (or heat) in response to a temperature gradient. The heat flux density is the amount of energy that flows through soil per unit area per unit time.

overrightarrow{q} = - k {nabla} T

where (including the SI units)

overrightarrow{q} is the local heat flux, W·m−2
big.kbig. is the material’s conductivity, W·m−1·K−1,
big.nabla Tbig. is the temperature gradient, K·m−1.

The thermal conductivity, k, is often treated as a constant, though this is not always true. While the thermal conductivity of a material generally varies with temperature, the variation can be small over a significant range of temperatures for some common materials. In anisotropic materials, the thermal conductivity typically varies with orientation; in this case k is represented by a second-order tensor. In nonuniform materials, k varies with spatial location. For soil, thermal conductivity also depends on mineral composition, water content, and bulk density. Compact and wet soils have a higher thermal conductivity than loose and dry soils. For many simple applications, Fourier’s law is used in its one-dimensional, x-direction form:

q_x = - k frac{d T}{d x}

ComponentThermal Conductivity (W.m‐1.K‐1)
Quartz8.8
Clay2.9
Organic Matter0.25
Water0.57
Ice2.4
Air0.025
Dry soil0.2‐0.4
Wet soil1-3

Colour

 

Soil colour is often the first impression one has when viewing soil. Striking colours and contrasting patterns are especially noticeable. The Red River (Mississippi watershed) carries sediment eroded from extensive reddish soils like Port Silt Loam in Oklahoma. The Yellow River in China carries yellow sediment from eroding loess soils. Mollisols in the Great Plains of North America are darkened and enriched by organic matter. Podsols in boreal forests have highly contrasting layers due to acidity and leaching.

soil color

In general, color is determined by the organic matter content, drainage conditions, and degree of oxidation. Soil color, while easily discerned, has little use in predicting soil characteristics.Arizona Master Gardener Manual”. Cooperative Extension, College of Agriculture, University of Arizona. p. Chapter 2, pp 4–8. Retrieved 27 May 2013. It is of use in distinguishing boundaries within a soil profile, determining the origin of a soil’s parent material, as an indication of wetness and waterlogged conditions, and as a qualitative means of measuring organic, salt and carbonate contents of soils. Color is recorded in the Munsell color system as for instance 10YR3/4.

Soil color is primarily influenced by soil mineralogy. Many soil colours are due to various iron minerals. The development and distribution of colour in a soil profile result from chemical and biological weathering, especially redox reactions. As the primary minerals in soil parent material weather, the elements combine into new and colourful compounds. Iron forms secondary minerals of a yellow or red colour, organic matter decomposes into black and brown compounds, and manganese, sulfur and nitrogen can form black mineral deposits. These pigments can produce various colour patterns within a soil. Aerobic conditions produce uniform or gradual colour changes, while reducing environments (anaerobic) result in rapid colour flow with complex, mottled patterns and points of colour concentration.

Resistivity

Soil resistivity is a measure of a soil’s ability to retard the conduction of an electric current. The electrical resistivity of soil can affect the rate of galvanic corrosion of metallic structures in contact with the soil. Higher moisture content or increased electrolyte concentration can lower resistivity and increase conductivity, thereby increasing the rate of corrosion. Soil resistivity values typically range from about 2 to 1000 Ω·m, but more extreme values are not unusual.

 

 SOIL CONSERVATION

Soil Conservation is a combination of practices used to protect the soil from degradation.  First and foremost, soil conservation involves treating the soil as a living ecosystem.  This means returning organic matter to the soil on a continual basis.

Soil conservation can be compared to preventive maintenance on a car.  Changing the oil and filter, and checking the hoses and spark plugs regularly will prevent major repairs or engine failure later.  Similarly, practicing conservation now will preserve the quality of the soil for continued use.

Soil conservation is a “combination” of practices used to protect the soil from degradation.  First and foremost, soil conservation involves treating the soil as a living ecosystem, and recognizing that all the organisms that make the soil their home, play important roles in producing a fertile healthy environment.  They are responsible for breaking down organic matter, releasing nutrients, and opening up spaces for the circulation of air and water.

Because most organisms in the soil depend on dead plant and animal matter for their food and energy, soil conservation requires that organic matter be returned to the soil on a continual basis.  Organic matter is what provides good soil structure and water holding capacity, promotes water infiltration, and protects the soil from erosion and compaction.

In addition to preserving soil life and organic matter, the other principles of soil conservation are to:

  • manage surface runoff,
  • protect bare exposed soil surfaces, and highly susceptible sites (e.g. steep slopes), and
  • protect downstream watercourses from sedimentation and pollution.

Soil conservation is an active ongoing process throughout which the practitioner must maintain his/her commitment.  The first step is to obtain a good basic knowledge of the land resource. This means knowing where the soil is most permeable and susceptible to groundwater contamination from excess pesticides; or where the land is most susceptible to water erosion because of a combination of slope and soil texture.  Without this understanding, it is impossible to plan an appropriate conservation strategy.

The next steps are identifying or predicting problem areas, choosing and implementing soil conservation techniques, and maintaining control structures.  The final step is to continually monitor the effectiveness of the plan and make changes if and when necessary.

IMPORTANCES OF SOIL CONSERVATION

  1. To maintain an adequate amount of organic matter and biological life in the soil. These two components account for 90 to 95 percent of the total soil productivity.
  2. To ensure a secure food supply at reasonable prices. Soil conservation is proven to increase the quality and quantity of crop yields over the long term because it keeps topsoil in its place and preserves the long term productivity of the soil.
  3. To grow enough food not only for ourselves; but also for people in third would countries where there are food shortages.
  4. To save farmers money. Erosion is currently costing farmers over $90 million a year in lost income due to lower crop yields, and the loss of nutrients from the soil.
  5. To save citizens money. Soil erosion costs us an addition $9.1 million each year, and probably much more according to recent research.
  6. To improve water quality. All forms of life need clean water to survive.  Agricultural and urban soil erosion are major sources of sedimentation and contamination of water supplies.
  7. To improve wildlife habitat. Soil conservation practices such as providing buffer strips and windbreaks, or replacing soil organic matter, greatly enhance the quality of the environment for wildlife of all kinds.
  8. For aesthetic reasons. To provide more attractive and picturesque scenery.
  9. To help create an environment free of pollution where we can live safely.
  10. For the future of our children, so that they may have enough soil to support life. It has been said that the land has not so much been given to us by our forefathers, but has been borrowed from our children.

 

SOIL CONSERVATION TECHNIQUES

 soil conservation

 

Conservation Tillage

Conservation tillage consists of a variety of practices used in agriculture to reduce wind and water erosion.  The main principles are:

a) to keep bare soil protected at ll time of the year either with living vegetation, or with residue from the previous crop; and

b) to minimize the number of times the field is tilled.

Bare soil is highly susceptible to erosion.  Excess tillage destroys soil structure and organic matter.

In conservation tillage, at least 20 to 30 percent of the soil surface is covered in the previous year’s crop residue after planting.  The residue reduces wind velocity at the soil surface and breaks the impact of raindrops.  Root systems hold the soil in place.  If practiced across a slope, rows of stubble act as small dams to slow water as it runs down hill.

No-till farming involves planting seeds into the residue of the previous crop, with no tillage between harvest.  No till leaves 60 to 70 percent of a field covered with crop residue.

Another aspect of conservation tillage is the choice of machinery used.  The traditional moldboard plow tends to overturn and throw the soil leaving it bare and exposed to erosion. Chisel plows however, leave 30 to 50 percent of the soil surface covered with residue.

Timing is also important.  For most soil types, it is better to till a field in the spring after the major erosive force of spring melt has passed.  (Clay soils however, respond best to fall tillage).  Soil should not be tilled and traveled on when it is wet.  This practice is the leading cause of soil compaction.

Aside from erosion control, the other advantages of conservation tillage are increased water infiltration, a greater addition of organic matter to the soil, and savings of fuel and time for the farmer.  Conservation tillage also enhances wildlife habitat for soil organisms, birds and small animals like field mice and snakes.

Contour Farming

Contour farming involves tilling and planting along the contour, rather than up and down the slope.  The furrows and rows of plants act as dams which slow down the flow of water moving down the slope.  Unless some type of contour farming is used, particularly on long slopes, serious field erosion can result.

Contour farming also uses less fuel and power for tractors.

Strip Cropping

Strip cropping involves alternating strips of small grain (e.g. rye) or forage crops (e.g. clover) with row crops like corn.  It is used to control erosion by reducing the velocity of wind and water.  The forage and cereal grain rows tend to trap sediment that may otherwise end up in watercourses.

Strip cropping is most effective in controlling erosion on a slope when it is placed along the contour of the land.  To control wind erosion, it works best if the strips are placed at right angles to the direction of the prevailing winds.

Another benefit of strip cropping is the organic matter added from the forage or cereal crop rows.

Windbreaks

A windbreak or shelterbelt is a vegetation barrier designed to reduce or eliminate the velocity of the wind and hence reduce wind erosion.  (Windbreaks are also used for snow control and to provide shelter for buildings and livestock).

A distinction can be made between windbreaks and shelter belts.  Windbreaks consist of one to five rows of trees or shrubs; shelter belts are six or more rows wide.

Windbreaks are generally planted on the west, southwest, or northwest boundary of a field to provide protection from prevailing winds.  The protection is maximized downwind of the barrier, where the wind speed is significantly reduced for a distance of 15 to 20 times the height of the trees.  The windward side of the break receives protection for 3 to 5 times the total height.

The benefits of both windbreaks and shelter belts extend far beyond just erosion control.  Crop quality and livestock performance are improved due to less abrasion from blowing soil.  Tree and shrub rows significantly increase the aesthetics of the landscape which is proven by higher land values.  They also trap snow in the winter and thus provide a higher moisture content for the growing season.

A major benefit of windbreaks is their enhancement of wildlife habitat.  They provide places to roost, nest and seek cover from predators from harsh winter climate.  The damage some animals do to crops is balanced by the role others play in controlling insects or unwanted rodents like mice.

The principles of soil conservation coincide with wildlife habitat preservation.

 

CROP ROTATION

Crop Rotation is an alternative to planting a field in the same crop year after year (referred to as continuous mono-shelterbeltsculture cropping).  Instead, the main crop is rotated, ideally with cereal crops like winter wheat or forages such as clover and alfalfa.

Crop rotation provides several benefits.  Rotation reduces the risk of insect and disease problems, thus decreasing a pesticide dependency.  Because the crop is changed each year, pests do not have enough time to become established in damaging numbers.

Forage crops or legumes such as clover and alfalfa are often used as green fertilizers or plow-down crops, meaning they are planted and later mixed in with the soil as a natural fertilizer and soil builder.  Legumes have the special ability to take in atmospheric nitrogen and convert it to forms usable by other plants. (Atmospheric nitrogen is not in a form available to most plants).  For this reason they are also referred to as nitrogen fixing plants.

When used as a green fertilizer, legumes return a significant amount of organic matter to the soil.  Their deep roots create tunnels for air and water to enter the soil.  All these characteristics in turn guard the surface against water and wind erosion.

Cover Crops

Cover crops are crops planted to reduce the impact of wind and water on bare soil.  They absorb the impact of rain, reduce the speed of runoff, hold the soil in place, and encourage greater infiltration; and hence less runoff.

Sweet clover, alfalfa, rye, and winter wheat are common cover crops.  They are planted on areas susceptible to erosion like steep slopes; stream and river banks, grassed waterways or around wells to protect ground water supplies from contamination.  Winter wheat or rye are often planted to provide cover over the winter and 95 to 100 percent erosion control during the spring runoff.

Intercropping involves mixing plants in a field – for example planting legumes between rows of corn or soybean.  This technique may be used by a farmer who cannot afford to take his or her entire crop of corn out of production.

Buffer Strips

A buffer strip is an area of land adjacent to a watercourse that is vegetated with grasses or bushes.  The plant cover filters sediment out of  runoff, holds the soil in place and prevents washout, slumping, and reductions in water quality due to siltation.  Buffer strips are generally 2 to 5 meters wide. (The width varies according to soil texture and slope).  They should be protected from tillage, machinery and cattle access to work effectively.

Aside from erosion control, buffer strips provide excellent wildlife habitat for small animals and insects.  If forested, they can improve stream habitat by shading the water and making the environment more suitable to fish species like trout.  The leaves that fall into the water provide organic matter for small stream invertebrates which are in turn food for larger stream animals like crayfish.  Stream-side forests are extremely productive habitats for wildlife, and like shelter belts, they also improve the aesthetic quality of the environment.

Grassed Waterways

A grassed waterway is a permanently vegetated saucer-shaped channel designed to carry surface runoff across land without causing erosion.  It is commonly used where gully or rill erosion is taking place due to the concentrated flow of water overland.  The grass slows the flow of water and protects the soil from erosion.  The water is carried safely to a stable outlet such as a drainage ditch or stream.

The advantages of using a grassed waterway are that they don’t alter the natural course of the water and they can be crossed by farm, construction or forestry vehicles once well established.

Terrace

A terrace is a constructed earthen ridge with a water channel along the upper side.  There are several design options, but commonly both the ridge and channel are permanently grassed.  Terraces are designed to intercept runoff on a slope, and reduce its erosive action on the soil down the slope.  Water is channeled at a slower speed, along the vegetated channel to a safe, stable outlet such as a grassed waterway or standpipe or drop inlet.

Drop Inlets and Rock Chutes

A drop inlet consists of a vertical intake pipe and a horizontal underground pipe.  The water enters the vertical pipe at ground surface, and falls below where it is guided safely through a large concrete metal or plastic pipe into a stream or ditch.

A rock chute is a pile of rocks designed to move concentrated water flows over steep slopes.  Drop inlets and rock chutes are often used to “step” water down where there are rapid changes in elevation, and thereby protect soil from erosion.

Natural Fertilizers

Natural fertilizers include live stock manure, mulch, municipal sludge, and legume plants such as alfalfa or clover.  Manure and sludge are applied by spreading it over the land and then working it into the soil.  Strict guidelines must be followed in timing applications, since both sludge and manure can cause major water contamination if handled improperly.  Legumes such as clover or alfalfa are grown and then tilled into the soil as “green fertilizer”.

Like chemical fertilizers, natural fertilizer replenish the soil with essential nutrients like nitrogen, phosphorus and potassium.  However, they have the added benefit of providing the soil with organic matter.

Bank Stabilization

Bank stabilization consists of any measure used to hold soil in place on the bank or a watercourse.  Here waves, stream current, ice and surface runoff can scour away the soil.

The benefits of bank stabilization are reduced soil erosion, better water quality and an increased aesthetic environment.

Three common methods used to control stream bank erosion are rip rap, gabion baskets and re-vegetation.  Ther first two methods use loose rock to break the impact of stream water on the bank, and to protect the underlying loose soil surface. Rip rap is loose rock on a steep bank.  An advantage of rip rap is the rock will give to the pressure of ice and frost, whereas concrete might crack.  Gabion baskets are wire baskets filled with rock.  The wire prevents rock movement.  They are typically used on steeper slopes and where water is flowing at higher speeds.

Stream banks can also be stabilized with shoreline planting.  Natural grasses, shrubs, and trees slow the movement of water over the soil, and trap sediment, preventing it from entering the water.

Red osier dogwood and sweet gale are native shrubs that can quickly establish themselves, control erosion and beautify the water’s edge.  These bushes also provide excellent habitat for wildlife.

Organic or Ecological Growing

Organic or ecological growing involves minimizing or eliminating the use of synthetic fertilizers and pesticides, and nurturing rich, long term balanced soil fertility through techniques such as crop rotation, conservation tillage and adding compost and manure to the soil.

Fertilizers usually only replace macro-nutrients (phosphorus, nitrogen, and potassium) and do not provide the organic matter that natural fertilizers do.  Most insecticides are non selective.  In addition to killing target pests, they can kill insects and microorganisms that are essential to soil fertility.

Organic soil management can be applied on any scale from a small backyard to a large commercial farm, although the techniques for each will vary.  The basic principle is taking into account the needs of organisms that live in the soil – ensuring the natural cycling of nutrients, and the return of organic matter to the soil.  Organisms that are beneficial to the soil, to plants or that will help control pest organisms will all be maintained.

In organic growing the goal is never to completely eliminate pests.  Even pesticides cannot do this.  Rather, the objective is to establish a balanced soil ecology with an acceptable level of damage by pests.

Sediment Control

Despite a developer’s best efforts, water erosion often occurs on urban construction sites.  As a result, efforts must be made to keep the sediment or silt on the site, rather than having it transported by the water to a nearby storm sewer or stream.

A silt fence can be used to contain silt on the property being developed.  It does this by filtering runoff, and trapping the sediment behind a filter cloth.  This structure can also reduce the amount of soil blown from a construction site, by reducing wind velocity.

A sediment trap can have several forms, but the design which is now preferred consists of a filter cloth and crushed stone barrier which is placed over an inlet to the storm sewer system.  The stone prevents the movement of large particles while slowing the velocity; and the cloth prevents the finer particles from entering the storm sewer.

A sedimentation pond is especially important on a construction site if large areas of soil must remain exposed for a long period.  The pond generally consists of a large depression (sized according to the drainage area) which allows the sediment laden runoff waters to be temporarily detained.  This storing of runoff reduces its velocity and allows the soil particles to drop out or fall to the bottom of the pond.  The clean water is then taken off the surface and guided to an appropriate outlet to a stream or ditch.

As with any soil conservation method, the above sediment controls only continue to function properly if they are correctly maintained.  After sediment has been collected through these controls it must be properly removed and stabilized.  This will then allow these controls to remove silt effectively.

Integrated Pest Management

Integrated pest management (IPM) uses a variety of techniques designed to cut the use of chemical pesticides, and hence reduce environmental risks.  The backbone of IPM is crop rotation.  By rotating crop from year to year, pests are starved out and less likely to establish themselves in damaging numbers the following year.  Crop rotation is proven to be an effective method of pest control.

IPM also uses pest resistant crops, and biological controls such as the  release of pest predators or parasites to control pest populations.

Although IPM may require more time, the trade-offs of a safer environment and reduced costs for pesticide purchase are indisputable.

 

 For more information:

http://www.johnstonnc.com/mainpage.cfm?category_level_id=716&content_id=1541

 

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ASSIGNMENT : SOIL PROPERTIES AND SOIL CONSERVATION MARKS : 10  DURATION : 1 week, 3 days

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