Final review
calculate soil total porosity
((1 - BD) / PD) x 100
The amount of limestone needed to increase the pH of soil can be determined solely by measuring the soil pH.
F
The organic matter content for a typical well drained soil is 150-500 cmol/kg.
F
The two forces that dictate water movement in soils are adhesion and cohesion.
F
Organic matter CEC is dependent on soil pH.
False
Organic matter is unaffected by soil pH
False
Soil organic carbon is only a minor component of the Earth's carbon cycle
False
The cation exchange capacity of a soil with 10% organic matter likely is greater at pH 4 than at pH 6.
False
calculate soil water content on a gravimetric and volumetric basis
Gravimetric Method measurement of mass Grams of wet soil ÷ grams of dry soil Volume metric method measurement of volume The volume of wet soil ÷ Volume of dry soil
State the relationship (equation) between soil bulk density and porosity.
Porosity (P)=1-(BP/PD)
Humic organic materials in soils are an important contributor to the overall reactivity of soil surface horizons.
T
Isomorphous substitution of Mg for Al takes place in the octahedral sheet.
T
Some organic constituents in soils are highly resistant to breakdown or decomposition
T
The ability of soils to resist changes in pH is called buffering.
T
The soil examined in the field contained a well-developed Bt horizon
T
To increase the pH of a soil, it would be necessary to add more limestone than would be indicated by a measurement of the active acidity.
T
Would a Btg soil have a higher or lower specific surface area compared to an Ap horizon?
higher
Color variations within the Btg horizon of the field soil were the result of depletion of
iron
Soil hydraulic conductivity is mostly unaffected by soil texture.
F
A soil pH of 4 is considered acidic
T
Ap horizons generally are darkened by organic matter.
T
E horizons are generally lighter in color than the horizon below.
T
You have a 200 cm3 soil core with a moist weight of 150g and a dry wt of 100 g. The bulk density is ________ g/cm3. (enter a numeral)
.5
Identify the two fundamental building blocks of aluminosilicate clays discussed in lab.
1. Si^4+ Silicon tetrahedron 2. Al^3+ Aluminum octahedron
Know the fundamental building blocks for aluminosilicate clays and the dominant ions comprising them.
1. Silicon tetrahedral sheet (Si, O) 2. Aluminum octahedral sheet (Al, OH)
State the range of bulk densities for a typical mineral soil.
1.0 g/ cm^3 - 1.6 g/cm^3
How many soil textural classes are identified on the soil textural triangle?
12 soil textural classes
difference between a 1:1 and a 2:1 mineral.
A 1:1 mineral has 1 tetrahedral sheet and 1 octahedral sheet, whereas a 2:1 mineral has 2 tetrahedral sheets and 1 octahedral sheet.
In which soil horizon would you expect to find a higher hydraulic conductivity? Ap or Btg
AP
Understand the two basic types of soil acidity discussed.
Active Acidity- acidity associated with soil solutionCan be measured with pH meterExchangeable Acidity- associated with cation exchange sites
Give two reasons why a subsurface Bt horizon like that described in lab might have a higher bulk density than the surface horizon?
Aeration, drainage and infiltration.
Understand how aluminum contributes to soil acidity
Aluminum participates in hydrolysis, combines with OH- to create a free H+ ion
Be able to define an acid.
Any substance that increases H+ concentration
State the formula for computing bulk density (provide appropriate units).
BD: Dry soil mass/ volume of solids and pores Units: (g/cm^3 )
How would you expect the soil texture and bulk density of a surface soil to compare with the bulk density and texture of a subsurface Bt horizon?
Bt horizon will have higher clay content than that of loamy sand horizon which means the Bt horizon would have a higher bulk density and a smoother texture.
In terms of soil formation, why might a Btg horizon density differ from an Ap horizon?
Btg horizon density horizon differs because it accumulates silicate clay from the above and low oxygen, and looks gray from iron depletion. Ap soil horizon formation differs because it's accumulated by organic materials that's disturb.
Be able to identify the most common base used to raise pH and how it works to change solution pH.
CaCO 3CO32- + H+= HCO3-
Understand the concept of cation charge density and how it impacts the preference of various cations for negatively charged sites on mineral colloids. *Understand the concept of cation charge density and how it impacts flocculation and dispersion.*
Cation charge density indicates how tightly cations are held at exchange sites (charge/size) and concentration in solution. It impacts which cations are more likely to become attached to a net negative site in a mineral colloid
Know what cation exchange capacity is, the units, and the basic procedure for determining CEC.
Cation exchange capacity is the total quantity of cations a clay can adsorb. The unit is centimoles of charge per kilogram (cmolc/kg), and the range for various mineral colloids is 0-180 cmolc/kg Cation exchange capacity is the total quantity of cations a clay can adsorb. The unit is centimoles of charge per kilogram (cmolc/kg), and the range for various mineral colloids is 0-180 cmolc/kg
A soil with 30% sand, 40% silt and 30% clay is what textural class?
Clay Loam
In our experiment, what cation did we use to displace all the native cations from the soils tested?
Cu2+
Know how to combine hydraulic conductivity and the gradient to determine overall flow of water in soils. Note the units of flow
Darcy's Equation Q=K * gradient cm/hr
In the determination of cation exchange capacity, only cations of high charge density can be used as an index cation
F
You have a 200 cm3 soil core with a moist weight of 150g and a dry wt of 100 g. What is the soil's porosity?
Db=300g/200cm^3=1.5g/cm^3. 1-(1.5g/cm^3/2.65g/cm^3)x100=.43= 43%
Identify the variables in Stokes' Law that are temperature-dependent. How would an increase in each of those variables influence the settling rate of a soil particle in the water?
Density and Viscosity The increase in density and viscosity will reduce particle settling velocity.
Identify 2 ways that organic matter influences soil properties
Depth in soil profile influences porosity and pore radius distribution. Organic matter can influence soil properties as well because organic molecules has functions capable of releasing hydrogen ions into the soil solution.
Indicate 3 factors that can strongly influence soil bulk density and how they influence bulk density.
Depth in the profile and how it influences bulk density is by decreasing porosity and thus, increases bulk density. How Compaction, influences soil bulk density is by when the soil is compacted then a decrease in porosity will occur causing the increase in bulk density. The third factor is Organic matter which will unlike the other increase porosity which will then decrease bulk density.
Which would likely have the higher Munsell color value? An E horizon or an Ap horizon?
E
Understand the meaning of % base saturation with respect to soil acidity.
Exchangeable bases/CEC, shows acidic saturation
A common material used to raise the pH of soils is calcium chloride
F
A soil's saturated hydraulic conductivity varies considerably depending on the length and area of the soil core.
F
All aluminosilicate clays possess an electrical charge.
F
Aluminum is acidic in soils because it removes H+ from solution by hydrolysis
F
At high pH, positive charge is developed on Fe and Al oxides
F
Base saturation refers to the ratio of base cations to the soil acid cations
F
Charge on Fe and Al oxides is generated by isomorphous substitution in the crystalline oxide matrix
F
Exchangeable acidity can be measured with a pH meter.
F
Exchangeable acidity in relatively unimportant to soil pH buffering.
F
Hydraulic conductivity is little affected by soil bulk density.
F
If calcium carbonate is added to soil the increase in pH is accomplished by calcium.
F
Identify the two forces acting on a soil particle settling in water. Based on these two forces, indicate why clay particles settle more slowly than sand particles.
Gravity and Drag/ friction. Gravity is a force which pulls heavier particles and or objects faster which is why it forces sand to travel faster due to its mass in weight and size, thus why clay particles settles a lot slower than sand. Also, because of the size (<0.002 mm) and lesser in weight of clay, drag suspends clay easier than it would sand.
Know the two ions considered acids in soils.
H+ and Aluminum
Understand why many Florida soils tend to be acidic.
High rainfall, H+ in rainfall, then Al3+ is released in weathering
Which component of SOM is most resistant to decomposition?
Humas
Understand ion hydration and how it can impact the size of cations in soil solution
Ions from O.M. breakdown and weathering are very well accommodated in Water. Water molecules have the ability to orient themselves relative to the introduction of a solute. Because of water's ability to orient relative to charged ions, they remain in solution. Water is a good solvent for many ionic compounds HYDRATION SPHERE Some ions in soil solution: Na+, K+, Ca2+, Mg2+, Cu2+, Fe3+Hydration sphere size varies widely depending on the ion. The hydration sphere accounts for the sphere of water molecules that oriented itself due to the charge of the element.
Indicate the cation exchange capacities of the following:
Kaolinite 2-5 cmolc/kg smectite 80-120 cmolc/kg SOM 100- 500 cmolc/kg
Understand the relationship between CEC and % B.S. in terms of the ease or difficulty in raising pH.
Low CEC, high base- easy to raise pH, vice versaHigh CEC- more limingHigh Base concentration- more liming
Know the relationship between pH and H+ ion concentration.
Low pH = high concentrations of H+ ionsHigh pH = low concentrations of H+ ions *pH= -log(H+)*
40 g of soil are suspended in water. A six-hour hydrometer reading taken at 20oC is 8 g/L. What is the mass of clay in the sample? What is the percentage of clay in the sample? If the soil contained 40% sand, what is the silt content.
Mass of clay: 8.5g Percentage of clay: 21.25% Silt content (%) 38.75%
High charge density cations are more strongly attracted to negative sites on aluminosilicates than are low charge density cations.
T
Using the simplified version of Stokes' Law, calculate the settling velocity of a sand particle with a diameter of 0.5 mm, and a silt particle with a diameter of 0.003 mm. Hint: note the units of settling velocity compared to those given for the diameter of soil particles.
Sand particle velocity: 28.1 cm/sec Silt particle velocity: 0.001011 cm/sec
Which soil likely would have the greatest specific surface area?
Silty clay
Know the 3 fundamental features of soil mineral colloids.
Small (< 1 micron), Large surface area,Highly reactive.
Understand soil pH buffering. Be able to identify soil horizons which may have higher or lower buffering capacities
Soil pH buffering is the ability to supply hydrogen ions to the soil solution if needed. Basically, it resists the change of pH in that soil solution
Functions of hydraulic conductivity
Soil texture, Density, and structure Texture= size of particles, pores Density= High density is low porosity and small pore sizes Structure= inter aggregate micropores improve conductivity
Be able to define a base.
Substance that decreases H+ concentration
Understand isomorphous substitution in silicate clay minerals
Substitution of lower-charge cations for higher charge cations during mineral formation. Al3+ for Si4+ in tetrahedral,Mg2+ for Al3+ in octahedral. Results in a negative charge.
A Bt horizon designates an accumulation of silicate clays.
T
A soil horizon with high amounts of oxidized iron generally will have a high Munsell color chroma.
T
Define "cation exchange capacity"
The amount of cations clay can absorb.
Define hydraulic conductivity
The ease with which water moves through soils
Know the type of net charge developed on aluminosilicate minerals and the implications in terms of soil solution cations.
The result is a deficit of positive charge or a surplus of negative charge in the mineral structure. Adsorption to negatively charged clay minerals prevents leaching.
Know that mineral colloids are layered structures made of individual 1:1 or 2:1 layers
These layers are extremely thin and are stacked together to form clay minerals.
A small amount of organic matter can have a profound influence on the exchange capacity of soils.
True
Suggest two practical ways you could change soil bulk density in an agricultural or horticultural setting.
Two practical ways of changing bulk density by adding a plant and plowing the soil. The planting of a plant would increase organic matter materials. This method would decrease bulk density and create macro pores with in the soil. Plowing the soil, would disturb the soil, so that would cause a decrease in bulk density and increasing the macro pore distribution in the soil.
The E1 and E2 (Eg) horizons of the field soil were delineated based on
clay content color none of the above
The soil colloids discussed in lab generally possess ________charge.
negative
Know the relationship between pH and H+ concentration in solution.
pH= -log(H+)
Which soil would hold more water at field capacity, a sandy loam or a sandy clay loam?
sandy clay loam
What is humus?
the organic component of soil, formed by the decomposition of leaves and other plant material by soil microorganisms. Humus is a collection of chemical and physical active form of SOM dark, amorphous material
