EESC456-CHAPTER4

Question 1

Land-use potentials or limitations (from production pov)

Answer 1

–

Question 2

8 soil physical properties

Answer 2

1. colour
2. texture
3. structure
4. density
5. porosity
6. aggregates/aggregation
7. tillage/soil mgmt
8. engineering considerations

Question 3

1. soil colour

Answer 3

-measured using munsell colour chart (objective)

-clues about soil properties + conditions. used as a diagnostic criterion for classifying soils

-varies in landscape, depth, horizons

-changes with soil moisture (moist=darker)

-3 components: Hue, value, chroma

Question 4

Hue

Answer 4

basic colour (page) Ex: 10YR

–>redness or yellowness

Question 5

Value

Answer 5

darkness/lightness, purity score 0 to 8 (0-black), on side of page increasing bottom to top

Question 6

Chroma

Answer 6

intensity/brightness, score from 0 to 8 (0= dull gray), bottom of page, increasing from left to right

Question 7

Cause of color

Answer 7

a. organic matter content
b. water content (wetness + drainage)
c. presence and oxidation states of iron/manganese oxides in various metals

Question 8

Interpretation of colours

Answer 8

Oxidation conditions; brigth colours (high chroma)
–> ferric irons (Fe3+) = red, yellow, brown

–> manganese (Mn) = black

Reduction conditions; paller, low chromas
–> Ferrous iron (Fe2+) = gray, blue, green

Long-term saturation = gleyed colours
vs.
seasonal/periodic saturation = mottling (mark with spots or smears of color)
–> delination of wetlands

Question 9

Organic matter tends to … mineral particles which … and … colors of minerals themselves

Answer 9

coat
darkening and masking

Question 10

Water content has more profound… effect on soils colours. it influences the level of …. in the soil, which influences the rate of … (darkening the soil). Water also affects the oxidation state of … and…

Answer 10

indirect
oxygen
organic matter accumulation
iron and manganese

Question 11

Well-drained uplands, warm climates, well-oxidized iron compounds =

Answer 11

bright (high chromas) reds + browns

reddish colours associated with concentrations of oxidized irons

Question 12

Manganese oxide =
Glauconite =

Answer 12

black
green

Question 13

Dry regions =

Answer 13

whitish

(calcite + soluble salts)

Question 14

Poorly drained soil profiles =

Answer 14

gray, blue (low chromas) due to reduced iron compounds

gray colors associated with reduced irons

Question 15

gleyed soils:

Answer 15

Soil exhibiting gray colors from reduced iron and iron depletion

Under prolonged anaerobic conditions, reduced iron (which is far more soluble than oxidized iron) is removed from particle coatings, often exposing the light gray colors of the underlying silicate minerals.

Question 16

If sulfur is present under anaerobic conditions, … may color the soil black regardless of the organic matter level

Answer 16

iron sulfides

Question 17

Tropical/subtropical landscapes=

temperate landscapes=

Answer 17

Reddish colours (rhodic B horizons)

Dark greys/browns (mollic epipedon)

Question 18

How colour used to delineate wetlands

Answer 18

The presence in upper horizons of gley-low-chroma colors (either alone or mixed in a mottled pattern with brighter colors) is used in delineate wetlands, for it is indicative of waterlogged conditions during at least a major part of the plant growing season.

The depth in the profile at which gley colors are found helps to define the drainage class of the soil.

Question 19

2. Soil texture (most imp.)

Answer 19

-only refers to mineral fraction (fine Earth fraction)
–>gravels, cobbles, boulders, coarse fragment > 2mm diameter NOT

-NOT artificial media (perlite, peat, styrofoam, nonsoil materials)

• proportions of different-sized particles in a soil

-size class boundaries reflect physical/chemical behaviours + influence mineralogy and nutrient content

-specific surface area is a function of particle size distribution

-specific surface area is a function of clay % + clay type

Question 20

Specific surface area

Answer 20

Area of mineral soil per unit of soil mass or volume. surface area for a given mass of particles

influenced by particle size (smaller particles = higher surface area)

influences: water retetion, absorbed gas/chemicals, nutrient release, cohesion/aggregation, micorbial activity/habitat
–> higher surface area = higher influence

Question 21

The texture of a soil in the field is not readily …, so it is considered a … property of a soil.

Answer 21

subject to change

basic permanent

Question 22

Soil separates

Answer 22

Groups (6) of individual soil particles

1. Clay
2. Silt
3. Fine-sand
4. Medium-sand
5. Coarse-sand
6. Gravel

Question 23

Sand

Answer 23

0.05 - 2 mm
Gritty, visible naked eye
Rounded or angular (depends weathering/abrasion degree)

Coarse: may be rock fragments containing several minerals, but

most sand grains consist of a single mineral, usually quartz (SiO2) or other primary silicate minerals.
–>Quartz dominance = the sand separate generally contains few plant nutrients. The large particle size means that whatever nutrients are present will not likely be released for plant uptake.

Large pores
cannot hold water against the pull of gravity =drain rapidly and promote entry of air into the soil.

Large particles = low specific surface areas = little capacity to hold water or nutrients and do not stick together into a coherent mass.

–>most sandy soils are well aer- ated and loose, but also infertile and prone to drought.

Question 24

Silt

Answer 24

0.002 - 0.05 mm
invisible naked eye
smooth/silky

composed of weatherable minerals
–>small size (and large surface area) of the particles allows weathering rapid enough to release significant amounts of plant nutrients.

More, smaller pores = retains water and lets less drain through.

low stickiness and plasticity (malleability) = highly susceptible to erosion by both wind and water. –>easily washed away by flowing water in a process called piping

little plasticity, cohesion, and adsorptive capacity some silt fractions exhibit is largely due to a film of adhering clay.

Question 25

Clay

Answer 25

<0.002 mm
shape; tiny flakes/flat platelets.

Large specific surface areas, = great capacity to adsorb water/substances.

Large adsorptive surface = clay particles to cohere in a hard mass after drying.

Wet: clay is sticky, easily molded (exhibits high plasticity).

Small/convoluted pores, butso many
= slow movement water and air
=allowing the soil to hold a great deal of water; however, much of it may be unavailable to plants

Question 26

colloids (smallest clay particles)

Answer 26

Fine clay–sized particles are so small that they behave as colloids—if suspended in water they do not readily settle out.

Question 27

Each unique clay mineral imparts different properties to the soils in which it is prominent. Therefore, soil properties such as …, (5) depend on the kind of clay present as well as the amount.

Answer 27

shrink–swell behavior
plasticity
water-holding capacity
soil strength
chemical adsorption

Question 28

5 fundamental surface phenomena (influencing soil properties)

Answer 28

1. High surface area = capacity to hold water films
2. High surface area = high capacity to retain nutrients/chemicals
   –>Gases/dissolved chemicals attracted to + adsorbed by mineral particle surfaces.
3. High surface area = high release rate of plant nutrients from weatherable minerals
   –>surface of mineral particles weathered = releasing constituent elements in soil solution.
4. High surface area = high propensity for soil particles sticking together (coherent mass)
   –>The surfaces of mineral particles carry -/+ electromagnetic charges = particle surfaces + water films between them attract each other
5. Microorganisms tend to grow on and colonize particle surfaces. microbial reactions in soils are greatly affected by the specific surface area.

Question 29

Water films

Answer 29

In addition to the tiny pools of water held in the smaller soil pores, water is also retained in soils as thin films on the surfaces of soil particles.

Question 30

Loams (most soils)

Answer 30

mixture of sand, silt, and clay particles

-exhibits properties of those separates in about equal proportions.

3 separates NOT in equal amounts
–>small clay % required to =clayey properties
–>larger sand/silt % to influence soil behaviour

Question 31

Coarse fragment modifiers

Answer 31

if significant proportion of particles larger than sand –> add qualifying adjective as part of the textural class name

Gravels/pebbles: range from 2 -75 mm diameter are termed

Cobbles: round, range from 75 to 250 mm

Channers: flat, range from 75 to 250 mm

Stones/boulders: >250 mm

Question 32

Alteration of soil textural classes (2)

Answer 32

Changing the texture of a given soil would require mixing it with another soil material of a different textural class.

1. Pedologic processes (illuviation, mineral weathering) alter horizon textures
2. Erosion/subsequent deposition downslope can selectively remove or deposit particles of certain sizes.

–> management practices generally do not alter the textural class of a soil on a field scale.

Question 33

Adding peat or compost to a soil while mixing a potting medium … a change in… since the property of texture refers only to the mineral particles. Adding sand to change the physical properties of a clayey soil (for use in greenhouse pots/turf grass) is …

Answer 33

does not constitute a change in texture

considered to change the soil texture.

Question 34

Sand grains embedded in a silty or clayey matrix do not have large pores (like coarse sand grains)

Need for rapid drainage and resistance to compaction even when wet (golf putting greens and athletic fields) =

Need for smooth, hard surface (tennis court) =

Answer 34

artificial soil from carefully selected uniform sands

an artificial clay soil may be needed.

Question 35

Soils are assigned to textural classes solely on the basis of the ….
–>% of sand, silt, and clay always =100%.

The amounts of stone and gravel are rated separately. … is not considered.

Continuum of particle sizes in any soil = …. even though discrete particle sie classes.

Answer 35

mineral particles of sand size and smaller

Organic matter

smooth curves

Question 36

3. Soil structure

Answer 36

spatial arrangement of particles to complex aggregations, pores, and channels
Sand, silt, clay, and organic particles = aggregated
due to various forces and at different scales to form distinct structural units called peds or aggregates.

ped = large-scale structure ev- ident when observing soil profiles and involving structural units which range in size from a few mm to about 1 m.

physical processes (freeze–thaw, wet–dry, shrink–swell, the penetration and swelling of plant roots, the burrowing of soil animals, and the activities of people and machines) influences attraction of particles that define structural units

The networks of pores within and between the aggregates constitute = key aspect of soil structure.
–> influences movement of air and water, the growth of plant roots, and the activities of soil organisms, including the accumulation and breakdown of organic matter.

-Secondary development (peds = structural unit) requires certain mineralogy. particle sizes, time

-Different types by shape (6)

–>Clay skins on outer surface of peds if active clays

Question 37

Soil = complex building analogy

Answer 37

texture = sizes of building blocks soil particles = analogous to bricks.

soil structure = house.

formation of soil structure = arrangement of various sized bricks into a house with associated windows, doors, and hallways and the associated complex pores and channels .

microbial glues, roots, and fungal hyphae (stabilize soil structure) = cement holding bricks

Question 38

When a mass of soil is excavated and gently broken apart, it tends to break into peds along ….. These zones exhibit low tensile strength because particles within a ped or aggregate are more strongly … than to…of the surrounding soil.

Answer 38

natural zones of weakness

attracted to one an- other than to the particles

Question 39

Clods

Answer 39

compressed, cohesive chunks of soil that can form artificially when wet soil is plowed or excavated.

Question 40

Such practices as timber harvest, grazing, tillage, trafficking, and manuring impact soils largely through their effects on … especially in…

Answer 40

soil pores, especially in the surface horizons.

Question 41

2 structural conditions

Answer 41

a. Single-grained structural conditions (particles not aggregated)
–> loose sand (wind-blown dunes), loose dust accumulations (freshly deposited loess)

b. Large, cohesive masses of material, massive structural condition
–> clay sediments

Question 42

4 principal soil ped shapes (types)

Answer 42

1. Spheroidal (aggregates):
   Granular structure = roughly spheroidal aggregates
   - can separated from each other in a loosely packed arrangement – range from <1 -10 mm
   ->surface soils (A horizons), high in organic matter.
   –>affected by management.
2. Platelike (platy):
   - thin horizontal sheetlike peds (plates)
   - plates have developed as a result of soil-forming processes.
   - may be inherited from soil parent materials, especially if laid down by water or ice.
   - heavy machinery = compaction can = platy structure in clayey soils
   –>surface + subsurface horizons.
3. Prismlike (columnar/prismatic):
   vertically oriented prisms/pillarlike peds
   - vary size, 150 mm or +
   –> associated with swelling types clay.

Columnar structure = pillars with distinct, rounded tops
–> subsoils high in sodium

Prismatic structure = tops of the prisms are relatively angular and flat horizontally
–>subsurface horizons in arid/semiarid regions
–> humid regions, in poorly drained soils, in human-made soils forming from structureless sediments deposited on land, and in fragipans

4. Blocklike (blocky):
   irregular, roughly cubelike
   - range 5 - 50 mm
   - individual blocks molded by the shapes of the surrounding blocks.
   - subtypes:
   a. angular blocky: sharp edges of blocks, rectangular faces
   b. subangular blocky :
   some rounding
   –> B horizons (promote drainage, aeration, and root penetration)

Question 43

most soils exhibit some type of aggregation and are composed of peds that can be characterized by their …(3)

Answer 43

shape (or type) of the structural peds

relative size (fine, medium, coarse)

degree of development or distinctness of the peds (grades such as strong, moderate, or weak)

Question 44

Generally, the structure of a soil is easier to observe when the soil is relatively …. When …. structural peds may swell and press closer together, making the individual peds ….

Answer 44

dry
wet
less well defined.

Question 45

6. aggregates/aggregation
   The granular aggregation of surface soils is a highly …soil property. generally, smaller aggregates are more … than larger ones,

Answer 45

dynamic
stable

Question 46

Hierarchical organization of soil aggregates

Answer 46

Surface horizons are usually characterized by roundish granular structure that exhibits a hi- erarchy in which relatively large macroaggregates (0.25–5 mm in diameter) are comprised of smaller microaggregates (2–250 μm). The latter, in turn, are composed of tiny packets of clay and organic matter only a few μm in diameter.

characteristic of most soils, with the exception of certain Oxisols and some very young Entisols. Small particles of organic matter are often occluded inside the macro- and microaggregates. At each level in the hierarchy of aggregates, different factors are responsible for binding together the subunits

Question 47

Factors Influencing Aggregate Formation and Stability in Soils

Answer 47

A. Biological processes (most important at the larger end of the scale)
–> sandy soils (little clay, aggregation) very dependent on biological processes.

1. soil organisms activities
2. organic matter
3. tillage
4. iron/aluminum oxides

B. Physical/chemical processes (most important at the smaller end of the scale)
–> associated mainly with clays (greater importance in finer-textured soils)

1. Flocculation
2. Swelling-shrinking

Question 48

2. Swelling-shrinking (physical/chemical processes)

Answer 48

Volume Changes in Clayey Materials.

As a soil dries out and water is withdrawn, the plate- lets in clay domains move closer together, causing the domains and the soil mass to shrink in volume.

As a soil mass shrinks, cracks will open up along zones of weakness.

Over the course of many cycles (as occur between rain or irrigation events in the field) the network of cracks becomes better defined. In one of many ways in which physical and biological soil pro- cesses interact, plant water uptake dries the root zone and accentuates the physical aggregation processes associated with wetting and drying.

Freezing and thawing cycles have a similar effect:
formation of ice crystals = drying process, draws water out of clay domains.

The swelling and shrinking actions that accompany freeze–thaw and wet–dry cycles in soils = fissures and pressures that al- ternately break apart large soil masses and compress soil particles =
defined structural peds.

The aggregating effects of these water and temperature cycles are most pronounced in soils:
–>high content of swelling-type clays (Vertisols, Mollisols, and some Alfisols)

Question 49

1. Flocculation (physical/chemical processes)

Answer 49

mutual attraction among clay and organic molecules

aggregation begins with the flocculation of clay particles into microscopic clumps or floccules

If two clay platelets come close enough to each other, cations compressed in a layer between them will attract the negative charges on both platelets, thus serving as bridges to hold the platelets together.
= clay domains: small “stack” of parallel clay platelets

If positive charges on the edges of the clay platelets attract the negative charges on the planar surfaces. Multivalent cations (Ca2+ , Fe2+ , and Al3+) also
complex with hydrophobic organic molecules, allowing them to bind to clay surfaces.
= clay domains: random in orientation

Clay/humus domains form bridges that bind to each other and to fine silt particles (mainly quartz), creating the smallest size groupings in the hierarchy of soil aggregates. These domains, aided by the flocculating influence of certain polyvalent cations (again, mainly Ca2+, Fe2+, and Al3+) and humus, provide much of the long-term stability for the smaller (60.25 mm) microaggregates. In some highly weathered clayey soils (Ultisols and Oxisols) the cementing action of iron oxides and other inorganic compounds produces very stable small aggregates called pseudosand.

When certain cations (especially Na+, but to a lesser degree K+ and even Mg2+) with less flocculating ability than Ca2+ or Al3+ are prominent, the attractive forces are not able to overcome the natural repulsion of one negatively charged clay platelet by another. The clay platelets cannot approach closely enough to flocculate, so remain dispersed and cause the soil to become gellike, impervious to water and air, and very unde- sirable from the standpoint of plant growth. This dispersed condition is most dramatically stimulated by Na+ ions and is most common in soils of arid and semiarid areas (more de- tails in Section 10.6).

Question 50

1. Activities of Soil Organisms. (biological processes)

Answer 50

1. burrowing/molding activities of soil animals
   - Earthworms (and termites) move soil particles
   - ingesting them and forming them into pellets or casts
   –> forested soils; surface horizon primarily = aggregates formed as earth- worm castings
2. enmeshment of particles by sticky networks of roots and fungal hyphae
   - Plant roots move particles, push their way through the soil. movement forces soil particles to come into close contact with each other,= aggregation.

• Plant roots + fungal hyphae exude sugarlike polysaccharides/organic compounds = sticky networks that bind together individual soil particles and tiny microaggregates into larger macroaggregates. The threadlike fungi that associate with plant roots (mycorrhizal) produce a sticky sugar–protein called glomalin, which is thought to be an effective cementing agent

–>At the same time, the channels created by plant roots and soil animals serve as large conduits for new root growth. The channels also break up clods and help to define larger soil structural units.

3. production of organic glues by microorganisms (bacteria/fungi).
   - bacteria produce organic glues such as the polysaccharides intermixed at a very small scale with clay.

–>Many of these root and microbial organic glues resist dissolution by water and so not only enhance the formation of soil aggregates but also help ensure their stability over a period of months to a few years.

–> in surface soils, where root and animal activities and organic matter accumulation are greatest.

Question 51

2. Influence of organic matter (biological processes)

Answer 51

Organic matter provides the energy substrate that makes possible biological activities.

Aggregation process; soil mineral particles (silts and fine sands) coated/encrusted with bits of decomposed plant residue and other organic materials.

Decay = Organic polymers that chemically interact with particles of silicate clays and iron and aluminum oxides.
–>These compounds help orient the clays into packets (domains), which form bridges between individual soil particles=binding them together in water-stable aggregates

–> temperate zone soils:
formation + stabilization of granular aggregates primarily influenced by soil organic matter

–>bacterial polymers and organomineral domains bind soil particles.

Question 52

3. Influence of tillage (biological processes)

Answer 52

A. Tillage can promote aggregation:

1. break large clods into natural aggregates, = temporarily loose, porous condition conducive to the easy growth of young roots and the emergence of tender seedlings
   –> If the soil is not too wet or too dry
2. incorporate organic amendments into the soil and kill weeds.

B. Tillage can destroy aggregation.

1. can speed up the oxidative loss of soil organic matter = weakening soil aggregates.
2. can crush or smear soil aggregates
   = loss of macroporosity
   = puddled condition.
   –> if wet soil

Question 53

4. influence of iron/aluminum oxides (biological processes)

Answer 53

Tropics:

• highly weathered soils (especially Oxisols)
• large amounts of iron and aluminum sesquioxides (in largely amorphous forms) coating soil particles and cement soil aggregates = preventing their ready breakdown when the soil is tilled or wetted.
• greater aggregate stability
• aggregation is less dependent on soil organic matter

Question 54

7. tillage/soil mgmt. why till?

Answer 54

aggregation and associated desirable soil properties such as water infiltration rate decline under long periods of tilled row-crop cultivation.

• To prepare a seed bed
• Reduce weed competition
• Create desirable tilth (good physical conditions for plant growth)
• (In cold climates) to accelerate drying, & warming in spring

Question 55

soil friability :

Answer 55

Soils are said to be friable if their clods are not sticky or hard, but rather crumble easily, revealing their con- stituent aggregates.

• enhanced when the tensile strength (i.e., the force required to pull apart) of individual aggregates is relatively high compared to the tensile strength of the clods. This condition allows tillage or excavation forces to eas- ily break down the large clods, while the resulting aggregates remain stable.
• changes with changes in soil water content
• Each soil typically has an optimal water content for greatest friability

Question 56

Tilth:

Answer 56

• physical condition of the soil in relation to plant growth.
• highly dynamic soil property
• depends on aggregate formation/stability + factors : bulk density, soil moisture content, degree of aeration, rate of water infiltration, drainage, and capillary water capacity.
• Favourable bulk density, strength (looseness),
  moisture content, aeration, infiltration rate, drainage

Question 57

Tillage problems: Puddling (clayey soils too wet when worked)

Answer 57

Clayey soils are especially prone to puddling and compaction because of their high plasticity and cohesion. When puddled clayey soils dry, they usually become dense and hard.

Proper timing of trafficking is more difficult for clayey than for sandy soils, because the former take much longer to dry to a suitable moisture content and may also become too dry to work easily.

Increased soil organic matter content usually enhances soil friability and can partially alleviate the susceptibility of a clay soil to structural damage during tillage and traffic

Question 58

Tillage: Clayey soils of humid tropical regions:

Answer 58

more easily managed

The clay fraction of these soils is dominated by hydrous oxides of iron and aluminum, which are not as sticky, plastic, and difficult to work.

favorable physical properties, since they hold large amounts of water but have such stable ag- gregates that they respond to tillage after rainfall much like sandy soils

–>rice farmers often purposely till extensively when their soils are saturated with water to destroy aggregation and greatly reduce water permeability, = making their soils more suited for holding water in paddies where rice is grown under flooded conditions.

Question 59

Tillage: Clayey soils in temperate regions:

Answer 59

soils too wet for tillage just prior to planting time (early spring),

Question 60

Tillage: Arid tropical regions (long dry season)

Answer 60

Problem: soils too dry for easy tillage just prior to planting (end of dry season). soil often must be tilled in a very dry state in order to prepare the land for planting with the onset of the first rains. Tillage under such dry con- ditions can be very difficult and can result in hard clods if the soils contain much sticky-type silicate clay.

Question 61

Tillage problems: Plowpan (reaction to upward forces of plough)

Answer 61

the moldboard plow has been the primary tillage implement

purpose : lift, twist, and invert the soil while incorporating crop residues and animal wastes into a 10- to 20-cm-thick plow layer of soil

• temporarily loosen the soil, break up clods, and suppress weeds
• detrimental effects:
  1. tillage speeds the loss of soil organic matter and thereby the weakening of soil structure

2. leave the soil naked without a natural blanket of plant litter to protect the soil surface from sun, rain, and wind.

Question 62

conservation tillage practices

Answer 62

agricultural land-management systems have been developed that min- imize the need for soil tillage and leave the soil surface largely covered by plant residues, thereby maintaining soil biological habitat, stabilizing soil structure, conserving soil or- ganic matter, and physically protecting the soil from drying sun, scouring wind, and beat- ing rain

leaves at least 30% of the soil surface covered by residues.

• no-till operation, where one crop is planted in the residue of another, with virtually no tillage.

-minimum-tillage systems (chisel plowing) permit some stirring of the soil, but still leave a high proportion of the plant resi- dues as a protective cover on the soil surface.

Question 63

Tillage problems: Cloddiness (clayey soils break into large,disconnected lumps)
Soil crusting

Answer 63

Falling drops of water during heavy rain or sprinkler irrigation can beat apart aggregates exposed at the soil surface.

In some soils the dilution of salts by this water stimulates the dispersion of clays.

Once the aggregates become dispersed, small particles and dispersed clay tend to wash into and clog the soil pores. The remaining coarse particles at the soil surface become densely packed with little pore space under the influence of beating raindrops.

= soil surface covered with a thin, partially cemented, low permeability layer material called a surface seal. The surface seal inhibits water infiltration and increases erosion losses.

As the surface seal dries, it forms a hard crust. Seedlings, if they emerge at all, can do so only through cracks in the crust. Formation of a crust soon after a crop is sown may allow so few seeds to emerge that the crop has to be replanted. Once a crust has formed, it may be necessary to rescue a newly planted crop by breaking up the crust with light tillage (as with a rotary hoe), preferably while the soil is still moist.

–>In arid and semiarid regions, soil sealing and crusting can have disastrous consequences because high runoff losses leave little water available to support plant growth

–> Crusting can be minimized by keeping some vegetative or mulch cover on the land to reduce the impact of raindrops.

Question 64

Soil conditioners

Answer 64

Improved management of soil organic matter and use of certain soil amendments can “condition” the soil and help prevent clay dispersion and crust formation

a. gypsum: (calcium sulfate)
- effective to improve the physical condition of highly weathered acid soils, low-salinity, high-sodium soils of semiarid regions.

• soluble gypsum provide enough electrolytes (dissolved cations and anions) to pro- mote flocculation and inhibit the dispersion of aggregates, =preventing surface crusting.
• permit greater water infiltration and are less subject to erosion than untreated soils.
• can reduce the strength of hard subsurface layers, =allowing greater root penetration and plant uptake of water from the subsoil.

B. organic polymers: (polysaccharides)
- can stabilize soil structure

• polyacrylamide (PAM) is effective in stabilizing surface aggregates when applied at low rates in irrigation water

–> combining PAM + gypsum = can nearly eliminate irrigation-induced erosion.

C. Several species of algae that live near the soil surface are known to produce quite effective aggregate-stabilizing compounds.

Question 65

Mgmt soil titlth guidelines

Answer 65

1. Minimizing tillage/ploughing (moldboard plowing, disk harrowing, or rototilling)

• reduces the loss of aggregate-stabilizing organic matter.

2. Timing traffic activities to occur when the soil is as dry as possible + restricting tillage to periods of optimum soil moisture conditions (ideal wetness)

• minimizes destruction of soil structure.

3. Mulching the soil surface with crop residues or plant litter (maintaining organic mulch/crop residues):

• adds organic matter
• encourages earthworm activity
• protects surface + aggregates from beating rain and direct solar radiation.

4. Adding crop residues, composts, and animal manures to the soil

• stimulates microbial supply of the decomposition products that help stabilize soil aggregates.

5. Including sod crops (pastures) in the rotation favors stable aggregation by helping to maintain soil organic matter, providing maximal aggregating influence of fine plant roots, and ensuring a period without tillage.
6. Using cover crops and green manure crops

• provides good source of root action and decomposable organic matter for structural management.

7. Applying gypsum or in combination with synthetic polymers

• stabilizes surface aggregates, (in irrigated soils)

Question 66

A high degree of aggregation helps the soil to perform critical …. be- cause most of these functions are influenced by ….and …., properties.

Answer 66

ecosystem functions
soil porosity and density