Flashcards in Soils Management Deck (81)
What is an anion
A negatively charged atom/molecule
Ex found in soils: phosphate, sulfate, nitrate, and chloride
What is a cation
A positively charged atom/ molecule
Ex in soils: Calcium, magnesium, sodium, potassium, and ammonium
Define cation exchange capacity
The amount of positively charged anions that can be held by a given weight of soil
Define anion exchange capacity
The amount of negatively charged anions which can be held by a given weight of soil
What is the unit for CEC/AEC
Centimole charge per kg soil (cmol/kg soil) or meq/100g of soil
What factors determine the CEC of soils
Type of clay
Amount of OM
What is the order of CEC for OM and various types of clay
OM 200 meq/100 g
What affect does pH have on CEC and AEC?
As pH increases, CEC increases, AEC decreases
What ions can become fixed to clay surfaces? How do they become fixed?
“Holes” in the clay surface allow K and NH4 to enter the space, which they clay then collapses around. This makes nutrients unavailable for plant uptake
Differentiate saline, sodic/matrix, calcareous, acidic and alkaline soils
A saline soil contained sufficient soluble salt to impair plant growth (soils that have an electrical conductivity greater than or equal to 0.4 Siemens per meter in the saturation extract are considered saline)
A sodic/natric soil has from 13-15% or more of the CEC occupied by Na. (These soils have poor structure and accompanying poor plant growth)
A saline-sodic soil has ECs greater than or equal to 0.4 Siemens per meter and from 13-15% or more of the CEC occupied by Na. These soils have good physical properties until the salty is removed and they revert to sodic
Calcareous soils contain free calcium carbonate
Acidic soils have a pH less than 7
Alkaline soils have a pH more than 7
What properties change as sand and silt decrease and clay increases?
Bulk density, particle size, and pore size decrease
Pore volume and surface area increase
What does higher surface area create?
Higher surface area = more clay and/or OM
Describe how soil texture affects water holding capacity, amount of available water, and wilting point of soils
The distribution of pore size in soil impacts drainage and plant available water
Plant available water is that water which can be extracted by plants. The max value of available water is the difference between the amount of water a soil can hold after most free drainage has occurred (field capacity) and the amount of water in the soil when plants will wilt and not recover even if water is added (wilting or permanent wilting point)
How is water in the soil measured?
By weight (dry soil basis) percentage
Height of water (cm/in)
Energy of retention (units are bars, atmospheres or pascals)
Define soil structure
The arrangement of soil particles into larger units of varying degrees of coherence called peds
How do Soil microorganisms affect soil structure
Soil microorganisms decompose organic matter, crop residues, and other organic amendments added to the soil.
Short term: increase aggregation through production of decomp products that “glue” soil particles together
Long term: can decrease OM levels and aggregation in conditions that favor decomp (frequent tillage, optimum temp, moisture, oxygen, and limited return of crop residues)
What are macroorganisms and how do they affect soil structure?
Termites, ants, earthworms, moles
Mix the soil and create large channels that improve aeration and drainage
Define bulk density and what determines it
The mass of oven dry soil per unit volume
Soil texture and structure determine bulk density of a soil
How is bulk density impacted by organic matter and farm equipment traffic?
Bulk density increases as soil organic matter decreases (due to incorporation, burning, or removal of crop residues)
Continuous tillage can increase bulk density and cause the creation of tillage pan
Compaction from implement wheel traffic and animal traffic, especially on wet soils, increases bulk density
What are sources of organic matter?
Pant/crop residues (top/root)
Green manures/ cover crops
Composts from ag, industry, municipalities
Biosolids (municipal sewage sludge) and industrial wastes (paper mill sludge)
Soil animals (micro/macroorganisms)
What are the physical properties of biomass?
There are pools of organic matter in soils including organic material decomposing, biomass, and organic material that has undergone various degrees of decomp.
Humus is the most stable form of OM and decomposes very slowly
When organic materials decompose, about 80% is converted to CO2 and lost from the soil. About 15% becomes humus, and 5% is biomass
What are the chemical properties of organic matter
OM retains nutrients by adsorption. Cations are held by caution exchange sites on OM. Negative charges on OM are pH dependent, as pH increases CEC (pH depended negative charge on OM) increases
Anions are held by anion exchange sites on OM. The positive charges on OM is pH dependent, as pH decreases he AEC (pH dependent positive charge on OM) increases
At normal pHs, the CEC is much larger than AEC of OM
OM increases buffering against pH changes
What are the beneficial effects of OM?
Soil particles can be linked together into aggregates by OM. As OM content increases, aggregates tend to become more water stable (resistant to breakdown by water)
Source of nutrients (primarily N, also P and S) when decomposed. Retains cations that are macro/micronutrients for plants.
Increases water holding capacity, plant available water and infiltration
Decreases crusting of soil
Increases pore size, increases aeration
How does temperature influence soil microbial activity?
Microbial activity increases as temp rises above freezing until optimum temp, once above optimum temp activity declines back to zero
For many microbial conversions, there is a two-fold increase in activity for each 10EC increase in temp from about 15-35c. Above 35c, a decline commonly occurs
How does moisture influence soil microbial activity
Aerobic microbial activity is usually optimum at 40-60% of a soil’s water holding capacity (slightly drier than field capacity)
Decline in activity typically occurs as soils become more dry, rapidly declining at a critical soil water content. Critical level varies with microbial reaction being considered
Anaerobic activity (-Oxygen) occurs in saturated souls and ceases in the presence of oxygen.
Facultative anaerobes can function in the presence or absence of oxygen
How does soil pH influence soil microbial activity?
Fungi are more active in acidic soils
Bacteria are more active in neutral to basic soils
Rates of reaction carried out by specific microorganisms change as pH changes
How does OM influence soil microbial activity
Additions of organic C to soil stimulates microbial activity
The more decomposable the organic C, the more rapid the increase in microbial activity
How does salinity influence soil microbial activity?
As salinity increases, microbial activity can decline depending on the microorganism and amount of salinity.
Microbes expend more energy to absorb water as salinity increases
Various enzymatic processes within the microbial cell can be impaired by soluble salts
How do nitrogen applications influence soil microbial activity ?
May have a wide range of impacts on soil microbial activity
In general, better fertility improves microbial activity
Can increase activity if Low N is limiting decomp
Can affect specific groups of microbes