Soils Midterm 1 Flashcards
What is soil texture?
Texture is the size distribution of primary soil particles. A basic property of soil and is not readily subject to change in hundreds of years. Sand, silt, clay
Describe particle size distribution. And what is a loam?
Sand: 0.05mm
Silt: 0.002mm
Clay: <0.002mm
A loam is a soil that does not have any extreme proportion of clay, silt and sand.
What is specific surface area?
Specific surface area is the surface area for a given mass of particles. When particle size decreases, specific area increases geometrically. It is important because: maintain water films, chemical attachment and adsorption, weathering at the surfaces, electromagnetic charges as forces of soil aggregation, and microbes tend to grow on particle surfaces.
Describe soil structure.
Soil structure relates to the arrangement of primary soil particles into secondary aggregates of peds.
What is an aggregate?
Aggregates are in the size range of >0.01mm & <5mm. They can be divided into categories: macroaggregate (roots, hyphae) microaggregate (root hairs, hyphae, and polysaccharides) and submicroaggregate (mineral grains encrusted with plant microbial debris, plant debris coated with clay)
What is particle density?
The mass per unit of volume of soil solids without any pore space. For most soils the range (Dp) is 2.6-2.75 Mg/M3
What is bulk density?
The mass of a unit volume of dry soil with preserved pore space. This tells us, indirectly, the total pore space in soil because %pore space = 100 (Db/Dp x 100)
What is soil pore space?
the space between soil solids which is mainly comprised of water and oxygen.
What is soil tilth?
Tilth refers to the physical condition of the soil in relation to plant growth, and is an integration of the following properties: texture, aggregate formation and stability, bulk density, moisture, aeration, water infiltration rate, drainage, and capillary water capacity.
What processes influence the formation and stability of aggregates?
Biological and physical-chemical processes. Biological processes include: microorganisms, roots, and fauna. Soil organic matter is large because SOM is much more stable.
What are some factors that affect bulk density?
Texture, clay types, structure, SOM content, depth in a profile, compaction and tillage.
What are 6 purposes of tillage?
Accelerating SOM decomposition, preparing seedbeds, controlling weeds, plant residue incorporation, and reduce soil pests.
What properties of water are important for soil?
Cohesion and adhesion. Cohesion is the attraction of water molecules to each other primarily due to hydrogen bonding. Adhesion or adsorption is the attraction of water molecules for solid surfaces.
What is surface tension?
The greater attraction of water molecules to each other than the air above at liquid-air interfaces primarily due to cohesion.
What is the phenomenon: capillarity?
The product of adhesion and surface tension together which cause the movement.
What is soil water potential?
The soil water status that is expressed in energy forms and compared to pure water under standard temperature and pressure, the difference in energy level between this pure water and that of soil water is the soil water potential.
What is the gravimetric water content and volumetric water content?
Gravimetric water content is the weight of water in a soil sample per unit weight of dry soil. Volumetric water content is the volume of water in a soil sample per unit of total soil volume.
What is the water potential equation?
Yt = Ym + Ys + Yp + Yg Ym : Matric potential Ys : Solute potential or osmotic potential Yp : Pressure potential or submergence Yg : Gravitational or elevation gradient
Why does clay soil hold more water?
Because of its surface area and texture.
In what ways does water move in soils?
Saturated flow, unsaturated flow, infiltration, percolation and water vapor movement. Saturated flow driven by soil texture, structure, organic matter, hardpans, water content and temperature. Unsaturated flow is driven by the matrix potential gradient, or the thickness of water films on particles. Infiltration is the process by which water enters the soil pore space and percolation is the movement of water downward into the profile after infiltration, involving both saturated flow and unsaturated flow. Water vapor moves from moist -> and cool -> warm
What is the relationship between hydraulic conductivity and matric potential?
The higher the matric potential the higher the hydraulic conductivity
What is the maximum retentive capacity?
Soil water content when all pores are filled with water or totally saturated.
What is field capacity?
The maximum soil water content after the drain of micropore water due to gravity gradient (~-33 Kpa)
What is the permanent wilting point?
The soil potential (content) at or below which plant roots cannot absorb water any more ( ~ -1500 Kpa or 15 bars)
What is hygroscopic coefficient?
The soil water content of air-dried soil after reaching equilibrium with water vapor in the air of 98% relative humidity. Clay colloids are the main agents for holding this water.
What are some factors that can influence field capacity, wilting point, hyroscopic coefficient and max retentive capacity?
Texture, SOM, bulk density, and salinity
How is the bulk of plant water uptake done?
by passive absorption (90%)
What is water loss in plants?
Evapotranspiration
What is water use efficiency?
The amount of water required to produce a unit weight of dry plant material.
What is diffusion?
Diffusion is the exchange of gas based on the gas’s particular partial pressure along a pressure gradient to where it exists in a less concentration.
What is mass flow?
Mass flow is influenced by the amount of water in the soil that force air in and out of the soil and also by wind or changes in barometric pressure.
What is the redox potential?
A measure of the tendency of an environment to accept or donate electrons in an oxidation-reduction reaction
What are some factors that influence soil aeration?
Soil air-filled pore space and the distribution of the pore sizes determine soil air exchange rate. Rate of soil respiration is responsible for the depletion of oxygen and other electron acceptors in the soil.
Why is soil aeration important?
Biological and chemical processes are influenced or controlled by aeration.
What is the link between energy balance and soil temperature?
Soil temperature is responsive to many processes such as solar radiation, the heat exchange with air mass above the soil surface, and heat exchange with soil layers down below.
Why is Q10 = 2 assumed and what does it mean?
As commonly recognized, understanding of temperature sensitivity stems from enzyme kinetics. Most enzymatic reaction rates double with a 10 oC increase in temperature. when substrate concentration is at or near saturation level and the temperature is within the optimal range of the enzyme. This why Q10=2 is often assumed
Define specific heat.
The amount of energy required to raise the temperature of 1 gram of the substance by 1 degree C
What is thermal conductivity?
A measure of soil’s property to transmit heat. This property is best described by fourier’s law: Heat flow per unit area = K (ΔT/x)
1:1 lattice clay and 2:1 lattice clay
(1) 1:1 lattice such as kaolinite and serpentine (no expansion, low CEC);
(2) 2:1 lattice, the smectite group such as montmorillonite (large expansion, high CEC), illite (or hydrous mica, K+ holding, moderate expansion and CEC), vermiculite (mg+ holding illite, moderate swelling, high CEC)
What is a colloidal fraction?
Most clay fraction and humus are referred to collectively as the colloidal fraction, because of their small size and colloid-like properties. Some of the important functional properties of these colloids may include high specific surface area, electrostatic charges (cations and anions); adsorption of water, thereby the overall chemical and physical properties of the soil.
What is the origin of mineral colloids?
Mineral colloids came from various stages of weathering of primary materials or inherited from sediments of old clay formation; whereas humus came from various stages of organic residue decomposition.
What is isomorphous substitution?
In mineralogy, the process by which one element fills a structural position “usually” filled by another of similar size is called isomorphous substitution
When does the strength of cation adsorption increase?
(1) the charge of the cation increases; and as (2) the radius of the hydrated cation decreases:
Na+< NH4+=K+< Mg2+< Fe3+< Al3+
What is CEC?
total quantity of exchangeable cation sites (mole of charge) per unit weight of dry soil, often expressed in cmolc/kg
(1 cmolc/kg = 1 milliequivalent /100g)
What are anion exchange sites?
Positively charged particles or organic ligands
Which soils have higher CEC 2:1 clays or 1:1 clays?
2:1 with soils of mostly Fe and Al oxide as anion exchange capacity high
What is soil pH controlled by?
Soil pH is controlled and influenced by (1) the kind of parent material, the amount and the chemical nature of the precipitation (leaching of base cations), land management practices, the kind of organic input, and the activities of organisms (e.g., roots)
What are some sources of hydrogen ions?
Carbonic acid, organic acid, Oxidation of: N, S, Fe, etc., Acid deposition from the air, and Cation uptake by plants
How is alkalinity caused?
Mainly caused by the accumulation of CaCO3 and NaCO3 in arid regions or some irrigated land.
What is the buffering capacity of the soil?
When acidic or basic materials are added, a soil’s ability to resist an appreciable change in pH is the buffering capacity of the soil
How could one change the pH purposefully?
Add CaCO3 (Calcium carbonate), organic matter, and/or FeSO4 (ferrous sulfate)
What are some environmental factors of pH change?
- Acid deposition
- Acidifying chemical fertilizers •Global change
- Acidic waste or pollutants •Irrigation
- Draining of wet lands
What are the key processes responsible for salt accumulation?
(1) evaporation to precipitation ratio; (2) terminal lake effect; (3) saline seepage; (4) change of water table and other hydrological regimes
Why does SOM tend to acidify soil?
First, organic matter forms soluble complexes with
nonacid nutrient cations such as Ca2+ and Mg2+, facilitating their loss by leaching.
Second, organic matter contains numerous acid functional groups from which H+ ions can dissociate
