Exam Flashcards
Describe features of good soil structure
- Stable (resistant to compression maintaining below attributes)
- Many small aggregates (good aeration, drainage, large SA)
What makes a clay soil “reactive”
Clay has an electrical charge, mostly negative, which means that it can attract cations such as K+ and NH4+.
It also has a very small size so a huge surface area to volume ratio
Why do coarse textured soils have good drainage but smaller total pore volumes
Coarse-textured soils have smaller total pore volume as coarse-textured soils cannot pack as close together so form more macropores. Macropres are important for drainage as the capillary action is too weak to hold water within them.
What is permanent wilting point
The point where the soil water content decreases so low that the plant dies.
What is plastic limit
Plastic limit is the moisture content at which soil begins to behave as a plastic material.
What is mottling
Mottling is when iron and aluminium oxides become insoluble again, precipitating in the soil profile.
Describe a method for determining soil structure within the field
You need to see the number and stability of peds. Working aggregates within fingers to determine the resistance to compression. The distinctiveness of aggregates and their stability under stress indicates the degree of development.
What is wilting point
Wilting point is the water content in the soil when the plants have extracted all the water that they are capable of taking from the soil.
What is soil saturation
A soil is said to be saturated when all the pores are full of water (i.e all the air has been displaced).
What is field capacity
Field capacity is the water content of the soil after all the excess water has drained away
What is stress point
The drier a soil is relative to field capacity, the greater the force that plants must exert to extract water. If a soil dries to a point where the roots cannot extract water, at the rate demanded by the leaves then the stomata will close and the plant will be “under stress”.
What is infiltration rate
The speed that water enters the profile
What is hydraulic conductivity
The speed that water moves through the soil
What is drainage
The speed that water exits the soil
What are some factors that affect available water storage?
- Pore size
- Texture
- Structure
- Organic matter content
- Depth to which plant roots can penetrate
What is CEC?
CEC is the amount of negative charge which is accessible for the retention of positively charged cations.
What is PZNZ?
The pH are at which the amounts of negative and positive charges on the soil particle surface are the same resulting in zero net charge.
What happens to the soil when the pH is above PZNC?
When the pH is above PZNC, the soil is net negatively charged, resulting in more cation retention.
What happens to the soil when the pH is below PZNC?
When the pH is below PZNC, the soil is postively charges, resulting in anion retention. This is impractical in most nz soils
What does isomorphous substitution create?
It creates a permanent charge that is not affected by the pH of the soil.
What does dissociation of H+/OH- create?
It creates a variable charge that is pH dependent.
How does isomorphous substitution create negative charge?
Permanent negative charges are created by substitution of higher valency cations with lower valency cations.
What are some examples of normal siliceous rocks in NZ?
- Greywacke
- Argillite
- Schist
- Granite
What soils are classified as azonal soils?
Recent soils from alluvium, recent soils from volcanic ash and skeletal soils in steep land.
The principal effect of biological activity in the soil is what?
The respiration of CO2 by plant roots and microorganisms increasing the acidity of the soil solution by producing carbonic acid.
Name 3 ways topography affects soil formation
- The effect of the slope on soil depth
- Modifying effects of climate
- Influencing soil moisture regimes
Chemical formula of phosphate in soil solution?
H2PO4-
Chemical formula of Sulphate in soil solution?
SO4-2
Chemical formula of Nitrate in soil solution ?
NO3-
Chemical formula of Sodium in soil solution?
Na+
Chemical formula of Magnesium in soil solution?
Mg2+
Chemical formula of Potassium in soil solution?
K+
3 types of ion mechanisms?
Root interception
Mass Flow
Diffusion
What mechanism does phosphorus adopt to reach the plant root?
Diffusion
How does phosphorus behave in soil?
As phosphate isn’t very mobile (is adsorbed by soil), fertiliser placement is critical but the potential for nutrient loss is small. Phosphorus doesn’t move more than 6mm to get to a root. Phosphorus is only lost if the soil itself is lost through erosion. Phosphorus accumulates in the soil.
How does nitrogen behave in soil?
As nutrient mobility increases, fertiliser placement becomes less critical, but the potential for nutrient loss becomes greater. Nitrogen placement relative to the plant isn’t very critical for uptake, but the potential for loss of nitrogen once it is applied to the soil is generally very high, and little available nitrogen will accumulate in the soil.
how do micronutrients behave in soil?
The behaviour of most micronutrients is similar to that of phosphorus. The other nutrients fall somewhere between these two extremes in mobility behaviour (phosphorus and nitrogen)
proportion of nutrient uptake by every mechanism
nitrogen - 99% mass flow phosphorus - 90% diffusion potassium - 78% diffusion calcium - 72% mass flow magnesium - 87% mass flow sulphur - 95% mass flow all consist of some root interception
what is pH buffering?
When a solution resists pH change of soil when acidic or alkaline materials are added.
what happens when lime is added to an acidic soil?
CaCO3 is a basic salt - it dissolves in water to form a strong base (Ca(OH)2) and weak acid (H2CO3), so the net effect is a basic solution.
For every mole of CaCO3 that reacts with water, 2 moles of OH- ions are generated. These two moles of OH- ions neutralise excess H+ in soil solution. H+ then moves into soil solution from the soil surface, buffering the change in pH.
PNZC related to the dissociation of H+/OH- ions?
When the pH is above PZNC, the soil is net negatively charged, resulting in more cation (H+) retention; when the pH < PZNC the soil is net positively charged, resulting in more anion (OH-) retention.
what causes a soil to have a high CEC value?
high CEC is when cations are held by the negatively charged clay and organic matter particles in the soil through electrostatic forces. (negative soil particles attract the positive cations). High CEC is caused by high negative charge.
what is the benefit of high CEC?
high CEC means more plant available cations. It represents the total amount of exchangeable cations that the soil can adsorb. The cations used by plants in the largest amounts are calcium, magnesium, and potassium.
what is flocculation?
the process in which particles disperse in a solution contact and adhere each another, forming clusters, flocks, flakes, or clumps of a larger size.
what is a diffuse layer?
the DL is the specific distance from the clay surface that the concentration of the bulk soil solution is equal, the anionic and cationic charges are balanced in solution.
Below the PZNC?
the surface bears a positive charge and can attract anions
Above the PZNC?
the surface bears a negative charge and can attract cations
increase of K+ from effluent…
K+ over time can displace adsorbed Ca2+ and Mg2+, releasing these cations into soil solution and thereby increasing their risk of leaching. Over time the Diffuse Layer may increase leading to soil deflocculation
Increase in H+ from increases in acidity
H+ over time can displace adsorbed Ca2+ and Mg2+, releasing these cations into soil solution and thereby increasing their risk of leaching. Over time the Diffuse Layer increases leading to soil deflocculation
Why is water at the permanent wilting point unavailable?
It is bonded too tightly inside molecules or in micro-pores whose capillary action is too strong for plants to overcome
Why should you not apply force to a soil when it is wetter than the plastic limit?
It will compact, forming pans, ruining aggregates impeding drainage and aeration
How do wetting and drying cycles create good soil structure?
Wetting breaks up as outside expands from absorbing water, creating a pressure difference that causes it to explode
Drying brings soil particles together in greater alignment, bonding and stablising the peds
How does soil structure develop?
Bigger aggregates may be broken up and stabilized Or substituents (sand,silt,clay) are brought into close proximity to form new aggregates
Difference between soil texture and soil structure?
Soil texture is the proportion of sand, silt, and clay in a soil whereas soil structure is the arrangement of soil particles into aggregates and the stability of these aggregates to stress
What is the phase lag of soil temperatures?
Energy is absorbed by the soil more while moving down the profile and the difference between the max and min temperatures will be smaller at depth. This delay in
reaching max and mins is the phase lag
Explain why it is harder to work a clay soil than sandy soil?
Because clay soils are more likely to become waterlogged which makes them very hard to cultivate
Why do coarse-textured soils have smaller total pore volumes but good drainage?
Coarse-textured have more macropores than fine-textured soils, however, fine-textured soils have many micropores which means they have a larger surface area of pores and therefore a larger total pore volume. Coarse-textured soils have more macropores meaning they have better drainage
What is porosity?
The fraction of the soil volume that consists of holes
What is mottling?
The switching between saturated and aerated conditions gives rise to mottling, a speckled pattern of orange or rusty colored flecks
What is the stress point?
When the soil dries to a point where the roots cannot extract water at the rate required by the plant
What is the permanent wilting point?
When the plants have extracted all the water they are capable of taking from the soi
What is the minimum value of macroporosity for good aeration?
10%
Discuss the features of clay soil that make it so reactive
Clay particles are shaped like thin and flat platelets, they are also very small which means they have a very large surface area. Clay also has a charge which is mostly negative which can attract positively charged ions
What do pedogenic processes result in the formation of?
Distinct soil horizons.
What are the processes of soil formation?
- Freezing/thawing
- Mechanical abrasion
- Wetting and drying
- Translocation
- Hydrolysis
- Leaching
- Chelation
- Gleying
What are igneous rocks formed from, and what are they the source of?
The cooling of molten magma, source of primary minerals.
What are sedimentary rocks formed from?
The burial and compaction of weathering products and resistant primary minerals.
What are metamorphic rocks formed from?
Deeper burial and heating of weathering products and resistant primary minerals.
How are minerals formed?
When primary rocks or tephras weather, both physically and chemically, secondary minerals are formed. The rate of weathering is dependent upon the chemical reactivity of the mineral.
How are sedimentary rocks formed?
By the accumulation of sediment in air or water. The sediment becomes hardened or lithified with time and is turned into rocks.
What are the four major physical properties that are used to describe each soil horizon?
- Colour
- Texture
- Consistence
- Structure
What is consistence influenced by?
Texture, water content, clay mineral composition and organic matter content.
What is plasticity?
The ability to change shape continuously under the in fluency of an applied stress and to retain the impressed shape on removal of the stress.
What is soil structure?
Naturally occurring arrangement of soil particles into aggregates that result from pedogenic processes.
What is a soil horizon?
A soil horizon may be defined as a layer of soil, approximately parallel to the soil surface, with characteristics produced by soil-forming processes.
What are non-mineral elements derived from the atmosphere or water are:
- Carbon
- Hydrogen
- Oxygen
What is root interception?
The interception of nutrients half on soil surfaces by roots, as they grow through soils.
What percentage of the soil available nutrient content could be taken up by roots without the need for transport of the nutrient from the soil surface?
1%
What is mass flow?
Mass flow is the movement of nutrient ions in soil water that is absorbed by the plant during transpiration.
What is diffusion?
The mechanism by which nutrients are transported to roots - it is responsible for the majority of phosphate transport to roots.
Why does nutrient diffusion occur?
Thermal motion of molecules/ions. Ions in solution are continually colliding with each other. The greater the amount of energy supplied to the solution, the faster the movement, the higher the number of collisions, the greater the amount of heat that is generated and the faster the diffusion.
What are the 2 main mechanisms for inducing negative charge on soil surfaces?
Isomorphous substitution
H+/OH- dissociation
What is isomorphous substitution?
During the formation of clay minerals, an ion is replaced by another ion of a similar size, but different charge. The substitution of Si4+ with Al3+ causes a permanent negative charge.
What is H+/OH- dissociation?
As the pH increases, H+ ions are removed from the organic functional groups, while at low pH, OH- ions are added. This means that charge can vary with pH, with soils becoming more negative at higher pH’s, as the positive H+ ions are removed from the soil surfaces.
What is diffusion?
Diffusion is the movement of molecules/ions down a concentration gradient - slowly from a zone of higher conc. to a zone of lower conc.
Why does phosphate use diffusion to be transported to the roots?
Because phosphate is specifically adsorbed, so is very concentrated in aggregates (broken peds), has to diffuse through the soil to get down to the roots.
Example of a brown soil
Ashurst
Example of pallic soil
Tokomaru silt
Pallic soil features
Badly drained
Bakes and drys hard in summer - Has a fragipan
Brown soil features
Better drainage
Ages into ultic
Ultic soil features
Forms from weathered brown soil
Drainage not as good as brown
Acidified and weathered (bad nutrient capacities)
E horizon but no Bh +Bs (difference from podzols)
Podzol soil features
Form on more silica rich soils
Very humid, high leaching under ‘mor’ vegetation
E+Bh+Bs horizons
Pumice soil features
Entirely from rhylotic eruptions, generally around taupo
Very erosion prone
Dominated by glass, low CEC
Allophanic soil features
Intermediate age between pumice and granular soils Allophane has high anion retention Very biologically active High P retention EXCELLENT PROPERTIES
Granular soil features
Weathered allophane
1:1 clay, low charge and nutrient retention
Impeded drainage
Why is nitrogen fertiliser added as side dressings for growing crops?
Nitrogen is an anion so is not attracted to NZ’s predominantly negative soil, instead it can be only be retained by non-specific adsorption. This is still not very effective so the majority of nitrogen remains in solution where it quickly leaches out. This means it must be added often to crops so that they have a constant source of nitrogen.
What conditions will promote deflocculation?
Acidity, and increases of cations with less charge eg Na + or K+. Dispersion increases as the diffuse layer increases, the diffuse layer increases as the cations have smaller charges so more are needed to bond with the negative so the concentration of H+, Na+ or K+ increases, so does deflocculation.
List the factors that are used to assess soil versatility
- Presence of stones
- Slope
- Few days of water logging
- Few days of water deficit
- Ease of tillage
- Rooting depth (0.75m)
- Little resistance to root growth
- Large, interconnected pores to ensure good drainage
- High structural stability
- No erosion, salt or flooding
What is colluvium and give an example of a soil name.
Colluvium is crushed rock debris generally found at the foot of a slope. An example is Ohakea soils.
Explain the changes in microbial density detected at the different soil regions
Bacteria rely on water to remain moist and not desiccate, which means they will be near water within the soil. Fungi need water, but their hyphae will not desiccate so can travel through air-filled pores.
Is there more microbial activity at the roots or topsoil region
There is much higher microbial density at the roots than the topsoils because root tip surfaces are the major sites of interaction, growth and attachment for microbial organisms.
Why do #’s of microbes decrease with depth
Microbe #’s decrease as there is less OM within the soil as a food source, anaerobic bacteria are also less efficient so they will be in small quantities.
Would you expect to find both aerobic and anaerobic bacteria at (T), (S) and (W)? Why or why not?
You would expect to find both aerobic or anaerobic bacteria because in each of these areas there will be pores that are either large enough to be aerated or small enough to contain water.
Where would you expect nitrification to occur (top soil or saturated conditions) and what is needed?
Nitrification will occur in shallow parts (T) of the soil as it will occur where a source of ammonia, oxygen and nitrifying micro-organisms exist.
Where would you expect denitrification to occur (top soil or saturated conditions) and what is needed?
Denitrification will occur in (W), the saturated soils because there will be an absence of oxygen. In the absence of oxygen, soil micro-organisms use nitrate as the terminal electron acceptor in respiration, acidity is consumed.
What happens when ammonium fertilisers are added to the soil
They undergo reactions in the soil which tend to increase acidity.
Explain the term “acidity” in terms of hydronium and hydrogen ions
Acidity is the formation of hydronium ions. It occurs through the release of hydrogen ions and its subsequent reactions with water to form hydronium ions.
Why do legumes tend to acidify the soil
Legumes use the neutral molecule of N during N fixation, and they take up large quantities of cations. This will result in excess cations over anions, resulting in the release of acidic hydrogen ions to maintain electrical charge.
Name the 2 groups of surface charge in soils
Permanent and variable surface charges
Identify the most important factors which influence surface charge
Soil components (clay minerals and OM) and soil solution characteristics (pH, ionic concentration).
Identify the components of permanent and variable charge surfaces in the soil
Permanent - 2:1 silicate clay minerals
Variable - Organic matter and Fe and Al oxides
Explain the importance of surface charge in relation to flocculation/deflocculation of soil particles
When the net charge is high, the soil particles repel each other causing dispersion. When the net charge is less(pH close to PZNC), the particles attract each other resulting in flocculation.
Explain the effects of cations in flocculation
Cations with low valency and higher hydrated shell, increase the thickness of the diffuse layer, resulting in dispersion. Ions with higher charge and smaller size decrease the thickness of the diffuse layer, resulting in flocculation.
What is cation exchange
The process of adsorption of positively charged ions (cations) onto the negative charges on the soil colloids and the subsequent release or exchange of other cations to soil solution.
How do earthworms affect soil fertility?
Earthworm activity within the soil profile stabilises and makes aggregates; making good structure and increasing drainage. The aggregates increase reactivity within the soil by providing a large SA:V ratio as there are more surfaces for reacting. This increased reactivity increases the fertility of the soil.
What is the abundance of earthworms affected by?
The pH of the soil (earthworms don’t like acidic soils (<4.5) they prefer neutral soils.
The moisture content of the soil also affects earthworms. Too dry and they desiccate, too wet and they drown. Need moisture but not too much.
More organic matter within a soil - the more abundant they will be.
Hydrolysis is
Using water to split, this creates an equilibrium reaction with hydrogen and hydonium ions (H+ and OH-). This increases acidity.
Hydrolysis increases as leaching increases as this allows the reaction to progress further. Leaf litter also increases acidity, particularly mor vegetation
Three benefits of mycorrhizal fungi
- Increase area of root interception/reach
- Modify root environment so it is easier to absorb phosphorous
- Stores phosphorous
Lignin decomposition
Brown rot degrades polysaccharides associated with lignin
Soft rot mainly after carbohydrates modifying lignin in the process
White rot can decompose lignin CO2 and water
How do wetting and drying cycles promote good structures
Wetting will expand the outside of aggregates but not inside, this makes a pressure difference that causes and explosion, breaking them up.
Drying brings soil particles closer into alignment improving bonding, stabilizing the aggregates
What is the state function of the soil
Climate, organism activity, relief, parent material and time.
What is climate
Climate is a combination of rainfall, temperature and wind, it affects the rate of weathering.
Name some primary minerals
K-Feldspar (orthoclase, sandinine and microcline)
Micas (muscovite, biotite and phlogopite)
Clay-size micas (Illite)
What minerals are found in sedimentary rocks
Quartz, muscovite and K-Feldspar.
What is degradation
Lowering of fluvial surface
What is aggradation
Increase in land elevation, due to deposition of sediment.
What are compound units on soil maps?
Soil complex; mixture of 2 or more soil types which are too intimately associated in the soil pattern to be indicated (e.g hill soils).
Soil association; mixture of 2 or more soil types but the soil pattern is sufficiently coarse to be resolved on ordinary detailed maps.
How does phosphate behave in soils
Phosphate is very strongly retained onto soil sites.
What happens when lime is added to a soil
It increases the pH of an acidic soil, soil acidity is reduced and alkalinity increases.
How do soil aggregates protect organic matter from decomposition
Through occlusion in aggregates.
What are the implications of incorporating residues with a low C/N ratio in farming practises?
Residues with low C:N ratios (high N contents) not only result in net N mineralization, but these residues tend to decompose faster.
Describe the different types of N2 fixation systems
Nitrogen fixation is carried out in the soil by micro-organisms that include bacteria.
Between termites and fungi.
All biological nitrogen fixation is affected by enzymes called nitrogenases.
Describe 3 mechanisms by which mycorrhizal associations with plants increase the supply of p to plants
- Increasing exploration of soil volume (decreasing the distance of diffusion of phosphate ions - increasing SA for absorption).
- Mycorrhizal fungus hype have higher affinity for phosphate ions and lower threshold conc. for absorption than plant roots do.
- Modification of root environment (active or passive mechanisms).
What is a debris avalanche
A mass of rock fragments and soil that has moved down a slope or hillside.
What are the main sources of organic matter in soils
Animal manure and plant litter
